ORCID Profile
0000-0003-0171-6927
Current Organisations
University of Melbourne
,
ARC Centre of Excellence for Climate Extremes
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Research Topics
In Research Link Australia (RLA), "Research Topics" refer to ANZSRC FOR and SEO codes. These topics are either sourced from ANZSRC FOR and SEO codes listed in researchers' related grants or generated by a large language model (LLM) based on their publications.
Top 5 Research Topics
ANZSRC Field of Research (FoR)
Atmospheric Sciences | Atmospheric Dynamics | Physical Oceanography | Meteorology | Climate Change Processes | Meteorology | Atmospheric Sciences Not Elsewhere Classified | Tropospheric and Stratospheric Physics | Fire Management | Atmospheric sciences | Climatology (Incl. Palaeoclimatology) | Cloud Physics | Physical oceanography | Meteorology | Climate change processes | Atmospheric dynamics | Forestry Fire Management | Atmospheric Sciences not elsewhere classified
ANZSRC Socio-Economic Objective (SEO)
Climate Change Models | Atmospheric Processes and Dynamics | Weather | Atmospheric processes | Weather | Climate Variability (excl. Social Impacts) | Climate variability | Expanding Knowledge in the Information and Computing Sciences | Effects of Climate Change and Variability on Australia (excl. Social Impacts) | Other | Natural Hazards in Mountain and High Country Environments | Natural Hazards in Forest and Woodlands Environments | Climate and Climate Change not elsewhere classified | Expanding Knowledge in the Environmental Sciences |
Related Links
Publications
Coupled Atmosphere‐Fire Simulations of the Black Saturday Kilmore East Wildfires With the Unified Model
Publisher: American Geophysical Union (AGU)
Date: 2019
DOI: 10.1029/2017MS001245
Characterization of Momentum Transport Associated with Organized Moist Convection and Gravity Waves
Publisher: American Meteorological Society
Date: 10-2010
Abstract: Tropical convection is inherently multiscalar, involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters. In addition to being a crucial component of the atmospheric water cycle and the global heat budget, tropical convection induces vertical fluxes of horizontal momentum. There are two main contributions to the momentum transport. The first resides entirely in the troposphere and is due to ascent, descent, and organized circulations associated with precipitating convective systems. The second resides in the troposphere, stratosphere, and farther aloft and is caused by vertically propagating gravity waves. Both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models yet in existing parameterizations, these two processes are treated independently. This paper examines the relationship between the convective momentum transport and convectively generated gravity wave momentum flux by utilizing idealized simulations of multiscale tropical convection in different wind shear conditions. The simulations produce convective systems with a variety of regimes of convective organization and therefore different convective momentum transport properties and gravity wave spectra. A number of important connections are identified, including a consistency in the sign of the momentum transports in the lower troposphere and stratosphere that is linked to the generation of gravity waves by tilted convective structures. These results elucidate important relationships between the convective momentum transport and the gravity wave momentum flux that will be useful for interlinking their parameterization in the future.
The vortical response to penetrative convection and the associated gravity-wave generation
Publisher: Wiley
Date: 07-2008
DOI: 10.1002/ASL.167
Locally forced convection in subkilometre‐scale simulations with the Unified Model and WRF
Publisher: Wiley
Date: 16-08-2020
DOI: 10.1002/QJ.3855
Abstract: This study evaluates the performance and benefits of kilometre‐ and subkilometre‐scale convection‐permitting simulations over tropical Australia. Focusing on an extended monsoon break period, we can directly compare Unified Model (UM) and Weather Research and Forecasting model (WRF) simulations to C‐band polarimetric radar observations and soundings. We show that the two models have different behaviour, and both are different to observations. Whereas WRF produces daily squall lines whether or not they occurred in observations, the UM primarily generates small but intense storms. The UM and WRF produce qualitatively different surface density currents at different times in the diurnal cycle. Once the density currents are present, the models also show different behaviour in relation to convective initiation. While higher resolution helps in the distribution of total precipitation over the domain, most characteristics do not change with higher resolutions, and model differences are always larger than resolution differences. While Convective Available Potential Energy/Convective Inhibition (CAPE/CIN) does not seem to be important to explain model differences, our findings point to the evolution of density currents in the boundary layer as the most important source of model errors and differences.
Atmospheric water vapour transport in ACCESS‐S2 and the potential for enhancing skill of subseasonal forecasts of precipitation
Publisher: Wiley
Date: 22-10-2023
DOI: 10.1002/QJ.4585
Long-term observational characteristics of different severe convective wind types around Australia
Publisher: American Meteorological Society
Date: 03-08-2023
Abstract: Regional understanding of severe surface winds produced by convective processes (severe convective winds: SCWs) is important for decision making in several areas of society, including weather forecasting and engineering design. Meteorological studies have demonstrated that SCWs can occur due to a number of different mesoscale and microscale processes, in a range of large-scale atmospheric environments. However, long-term observational studies of SCW characteristics often have not considered this ersity in physical processes, particularly in Australia. Here, a statistical clustering method is used to separate a large dataset of SCW events, measured by automatic weather stations around Australia, into three types, associated with strong background wind, steep lapse rate, and high moisture environments. These different types of SCWs are shown to have different seasonal and spatial variations in their occurrence, as well as different measured wind gust, lightning, and parent-storm characteristics. In addition, various convective diagnostics are tested in their ability to discriminate between measured SCW events and non-severe events, with significant variations in skill between event types. Differences in environmental conditions and wind gust characteristics between clusters suggests potentially different physical processes for SCW production. These findings are intended to improve regional understanding of severe wind characteristics, as well as environmental prediction of SCWs in weather and climate applications, by considering different event types.
Sources and dynamics of turbulence in the upper troposphere and lower stratosphere: A review
Publisher: American Geophysical Union (AGU)
Date: 23-06-2012
DOI: 10.1029/2012GL051996
Gravity waves above a convective boundary layer: A comparison between wind-profiler observations and numerical simulations
Publisher: Wiley
Date: 2007
DOI: 10.1002/QJ.72
Scatterometer estimates of the tropical sea‐breeze circulation near Darwin, with comparison to regional models
Publisher: Wiley
Date: 10-2017
DOI: 10.1002/QJ.3131
Toward an Understanding of Vertical Momentum Transports in Cloud-System-Resolving Model Simulations of Multiscale Tropical Convection
Publisher: American Meteorological Society
Date: 10-2013
Abstract: This study examines the characteristics of convective momentum transport (CMT) and gravity wave momentum transport (GWMT) in two-dimensional cloud-system-resolving model simulations, including the relationships between the two transports. A linear group velocity criterion is shown to objectively separate CMT and GWMT. The GWMT contribution is mostly consistent with upward-propagating gravity waves and is present in the troposphere and the stratosphere. The CMT contribution forms a large part of the residual (nonupward-propagating contribution) and dominates the fluxes in the troposphere. Additional analysis of the vertical sensible heat flux supports the physical interpretation of the two contributions, further isolating the effects of unstable convection from vertically propagating gravity waves. The role of transient and nonconservative (friction and diabatic heating) processes in generating momentum flux and their dependence on changes in convective organization was assessed using a pseudomomentum budget analysis. Nonconservative effects were found to dominate the transports the GWMT contribution involved a diabatic source region in the troposphere and a dissipative sink region in the stratosphere. The CMT contribution was consistent with transport between the boundary layer and free troposphere via tilted convection. Transient buoyancy–vorticity correlations highlighted wave sources in the region of convective outflow and the boundary layer. These sources were akin to the previously described “mechanical oscillator” mechanism. Fluxes associated with this upper-level source were most sensitive to convective organization, highlighting the mechanism by which changes in organization are communicated to GWMT. The results elucidate important interactions between CMT and GWMT, adding further weight to suggestions that the two transports should be linked in parameterizations.
Ground-based observations of overshooting convection during the Tropical Warm Pool-International Cloud Experiment
Publisher: American Geophysical Union (AGU)
Date: 27-01-2014
DOI: 10.1002/2013JD020673
The Mechanisms Leading to a Stratospheric Hydration by Overshooting Convection
Publisher: American Meteorological Society
Date: 12-2018
Abstract: Overshoots are convective air parcels that rise beyond their level of neutral buoyancy. A giga-large-eddy simulation (100-m cubic resolution) of “Hector the Convector,” a deep convective system that regularly forms in northern Australia, is analyzed to identify overshoots and quantify the effect of hydration of the stratosphere. In the simulation, 1507 in idual overshoots were identified, and 46 of them were tracked over more than 10 min. Hydration of the stratosphere occurs through a sequence of mechanisms: overshoot penetration into the stratosphere, followed by entrainment of stratospheric air and then by efficient turbulent mixing between the air in the overshoot and the entrained warmer air, leaving the subsequent mixed air at about the maximum overshooting altitude. The time scale of these mechanisms is about 1 min. Two categories of overshoots are distinguished: those that significantly hydrate the stratosphere and those that have little direct hydration effect. The former reach higher altitudes and hence entrain and mix with air that has higher potential temperatures. The resulting mixed air has higher temperatures and higher saturation mixing ratios. Therefore, a greater amount of the hydrometeors carried by the original overshoot sublimates to form a persistent vapor-enriched layer. This makes the maximum overshooting altitude the key prognostic for the parameterization of deep convection to represent the correct overshoot transport. One common convection parameterization is tested, and the results suggest that the overshoot downward acceleration due to negative buoyancy is too large relative to that predicted by the numerical simulations and needs to be reduced.
Intensity of thunderstorm-generated turbulence revealed by large-eddy simulation
Publisher: American Geophysical Union (AGU)
Date: 25-03-2014
DOI: 10.1002/2014GL059299
Gravity wave breaking, secondary wave generation, and mixing above deep convection in a three-dimensional cloud model
Publisher: American Geophysical Union (AGU)
Date: 15-12-2006
DOI: 10.1029/2006GL027988
Extreme Water Vapor Transport During the March 2021 Sydney Floods in the Context of Climate Projections
Publisher: American Geophysical Union (AGU)
Date: 19-11-2021
DOI: 10.1029/2021GL095335
Abstract: During March 2021, large regions of Eastern Australia experienced prolonged heavy rainfall and extensive flooding. The maximum daily mean column integrated water vapor transport (IVT) over Sydney during this event was within the top 0.3% of all days since 1980, and the 10‐day mean IVT was in the top 0.2%. In this study, we have examined the change in frequency of extreme IVT events over Sydney in 16 climate models from the Coupled Model Intercomparison Project 6 under two Shared Socioeconomic Pathways (SSP245 and SSP585). Generalized Extreme Value modeling was used to further analyze the change in frequency of extreme IVT events. We found the probability of long duration high IVT events is projected to increase by 80% at the end of the century, but the future change in IVT is correlated to the rate of global and regional warming in each model.
The Role of Cold Pools in Tropical Oceanic Convective Systems
Publisher: American Meteorological Society
Date: 20-07-2018
Abstract: The processes governing organized tropical convective systems are not completely understood despite their important influences on the tropical atmosphere and global circulation. In particular, cold pools are known to influence the structure and maintenance of midlatitude systems via Rotunno–Klemp–Weisman (RKW) theory, but cold pools may interact differently with tropical convection because of differences in cold pool strength and environmental shear. In this study, the role of cold pools in organized oceanic tropical convective systems is investigated, including their influence on system intensity, mesoscale structure, and propagation. To accomplish this goal, high-resolution idealized simulations are performed for two different systems that are embedded within a weakly sheared cloud population approaching radiative–convective equilibrium. The cold pools are altered by changing evaporation rates below cloud base in a series of sensitivity tests. The simulations demonstrate surprising findings: when cold pools are weakened, the convective systems become more intense. However, their propagation speeds and mesoscale structure are largely unaffected by the cold pool changes. Passive tracers introduced into the cold pools indicate that the convection intensifies when cold pools are weakened because cold pool air is entrained into updrafts, thereby reducing updraft intensity via the cold pools’ initial negative buoyancy. Gravity waves, rather than cold pools, appear to be the important modulators of system propagation and mesoscale structure. These results reconfirm that RKW theory does not fully explain the behavior of tropical oceanic convective systems, even those that otherwise appear consistent with RKW thinking.
Evolution of the Diurnal Precipitation Cycle with the Passage of a Madden–Julian Oscillation Event through the Maritime Continent
Publisher: American Meteorological Society
Date: 05-2016
Abstract: Changes in the diurnal precipitation cycle as the Madden–Julian oscillation (MJO) propagates through the Maritime Continent are investigated to explore the processes behind seaward-propagating precipitation northeast of New Guinea. Satellite rainfall estimates from TRMM 3B42 and the Climate Prediction Center morphing technique (CMORPH) are combined with simulations from the Weather Research and Forecasting (WRF) Model with a horizontal resolution of 4 km. Comparison with 24-h rain gauge measurements indicates that both satellite estimates and the WRF Model exhibit systematic biases. Despite these biases, the changing patterns of offshore precipitation with the passage of the MJO show good consistency between satellite estimates and the WRF Model. In the few days prior to the main MJO envelope, light background wind, relatively clear skies, and an increasingly moist environment promote favorable conditions for the diurnal precipitation cycle. Two distinct processes are identified: 100–200 km from the coast, precipitation moves offshore as a squall line with a propagation speed of 3–5 m s −1 . Farther offshore, precipitation propagates with a speed close to 18 m s −1 and is associated with an inertia–gravity wave generated by diurnally oscillating heating from radiative and moist convective processes over the land. A gravity wave signature is evident even after the MJO active period when there is little precipitation. By correcting for the background flow perpendicular to the coast, potential temperature anomalies for the lead-up, active, and follow-on MJO periods are shown to collapse to a remarkably invariant shape for a given time of day.
The meteorology of Black Saturday
Publisher: Wiley
Date: 25-06-2012
DOI: 10.1002/QJ.1986
Selecting regional climate models based on their skill could give more credible precipitation projections over the complex Southeast Asia region
Publisher: Springer Science and Business Media LLC
Date: 20-03-2023
DOI: 10.1007/S00382-023-06751-5
Abstract: This study focuses on future seasonal changes in daily precipitation using Regional Climate Models (RCMs) from the Coordinated Regional Climate Downscaling Experiments-Southeast Asia ensemble (CORDEX-SEA). Projections using this RCM ensemble generally show a larger inter-model spread in winter than in summer, with higher significance and model agreement in summer over most land areas. We evaluate how well the RCMs simulate climatological precipitation using two skill metrics. To extract reliable projections, two sub-ensembles of ‘better’ and ‘worse’ performing models are selected and their respective projections compared. We find projected intensification of summer precipitation over northern SEA, which is robust across RCMs. On the contrary, in the southern part of SEA, the ‘worse’ ensemble projects a significant and widespread decrease in summer rainfall intensity whereas a slight intensification is projected by the ‘better’ ensemble. Further exploration of inter-model differences in future changes reveals that these are mainly explained by changes in moisture supply from large-scale sources (i.e., moisture convergence) with enhanced effects from local sources (i.e., evapotranspiration). The ‘worse’ models project greater changes in atmospheric circulation compared with the ‘better’ models, which can explain part of the uncertainty in projections for daily precipitation over the CORDEX-SEA domain. Hence, our findings might help assess more reliable projections over the SEA region by selecting models based on a two-step model evaluation: the ability of models to simulate historical daily precipitation and their performance in reproducing key physical processes of the regional climate.
A 10-Year Austral Summer Climatology of Observed and Modeled Intraseasonal, Mesoscale, and Diurnal Variations over the Maritime Continent
Publisher: American Meteorological Society
Date: 05-2017
Abstract: The Maritime Continent is one of the wettest regions on the planet and has been shown to be important for global budgets of heat and moisture. Convection in the region, however, varies on several interrelated scales, making it difficult to quantify the precipitation climate and understand the key processes. For ex le, the diurnal cycle in precipitation over the land varies substantially according to the phase of the Madden–Julian oscillation (MJO), and the diurnal precipitation cycle over the water is coupled to that over the land, in some cases for distances of over 1000 km from the coast. Here, a 10-yr austral summer climatology of diurnal and MJO-scale variations in rain rate over the land and sea over the Maritime Continent is presented. The climatology is based on mesoscale model simulations with a horizontal grid length of 4 km and satellite precipitation estimates. The litude of the observed diurnal precipitation cycle is shown to reach a maximum just prior to the MJO active phase, with a weaker secondary maximum after the MJO active phase. Although these two maxima also exist in the modeled diurnal precipitation cycle, there is less difference between the maxima before and after the MJO active phase than in the observations. The modeled sea-breeze circulation is also shown to possess approximately equal maxima just before and just after the MJO active period, suggesting that the asymmetry of the diurnal precipitation cycle about the MJO active period is related more to moisture availability than kinematic forcing.
Cloud, precipitation and radiation responses to large perturbations in global dimethyl sulfide
Publisher: Copernicus GmbH
Date: 17-07-2018
DOI: 10.5194/ACP-18-10177-2018
Abstract: Abstract. Natural aerosol emission represents one of the largest uncertainties in our understanding of the radiation budget. Sulfur emitted by marine organisms, as dimethyl sulfide (DMS), constitutes one-fifth of the global sulfur budget and yet the distribution, fluxes and fate of DMS remain poorly constrained. This study evaluates the Australian Community Climate and Earth System Simulator (ACCESS) United Kingdom Chemistry and Aerosol (UKCA) model in terms of cloud fraction, radiation and precipitation, and then quantifies the role of DMS in the chemistry–climate system. We find that ACCESS-UKCA has similar cloud and radiation biases to other global climate models. By removing all DMS, or alternatively significantly enhancing marine DMS, we find a top of the atmosphere radiative effect of 1.7 and −1.4 W m−2 respectively. The largest responses to these DMS perturbations (removal/enhancement) are in stratiform cloud decks in the Southern Hemisphere's eastern ocean basins. These regions show significant differences in low cloud (-9/+6 %), surface incoming shortwave radiation (+7/-5 W m−2) and large-scale rainfall (+15/-10 %). We demonstrate a precipitation suppression effect of DMS-derived aerosol in stratiform cloud deck regions due to DMS, coupled with an increase in low cloud fraction. The difference in low cloud fraction is an ex le of the aerosol lifetime effect. Globally, we find a sensitivity of temperature to annual DMS flux of 0.027 and 0.019 K per Tg yr−1 of sulfur, respectively. Other areas of low cloud formation, such as the Southern Ocean and stratiform cloud decks in the Northern Hemisphere, have a relatively weak response to DMS perturbations. We highlight the need for greater understanding of the DMS–climate cycle within the context of uncertainties and biases of climate models as well as those of DMS–climate observations.
Tropical, Subtropical, and Extratropical Atmospheric Rivers in the Australian Region
Publisher: American Meteorological Society
Date: 05-2022
Abstract: Studies of atmospheric rivers (ARs) over Australia have, so far, only focused on northwest cloudband–type weather systems. Here we perform a comprehensive analysis of AR climatology and impacts over Australia that includes not only northwesterly systems, but easterly and extratropical ARs also. We quantify the impact of ARs on mean and extreme rainfall including assessing how the origin location of ARs can alter their precipitation outcomes. We found a strong relationship between ARs and extreme rainfall in the agriculturally significant Murray–Daring basin region. We test the hypothesis that the tropical and subtropical originating ARs we observe in Australasia differ from canonical extratropical ARs by examining the vertical structure of ARs grouped by origin location. We found that in the moisture abundant tropics and subtropics, wind speed drives the intensity of ARs, while in the extratropics, the strength of an AR is largely determined by moisture availability. Finally, we examine the modulation of AR frequency by different climate modes. We find weak (but occasionally significant) correlations between ARs frequency and El Niño–Southern Oscillation, the Indian Ocean dipole, and the southern annular mode. However, there is a stronger relationship between the phases of the Madden–Julian oscillation and tropical AR frequency, which is an avenue for potential skill in forecasting ARs on subseasonal time scales.
Automated Smartphone Threshold Audiometry: Validity and Time Efficiency
Publisher: Georg Thieme Verlag KG
Date: 03-2017
DOI: 10.3766/JAAA.16002
Abstract: Smartphone-based threshold audiometry with automated testing has the potential to provide affordable access to audiometry in underserved contexts. To validate the threshold version (hearTest) of the validated hearScreen™ smartphone-based application using inexpensive smartphones (Android operating system) and calibrated supra-aural headphones. A repeated measures within-participant study design was employed to compare air-conduction thresholds (0.5–8 kHz) obtained through automated smartphone audiometry to thresholds obtained through conventional audiometry. A total of 95 participants were included in the study. Of these, 30 were adults, who had known bilateral hearing losses of varying degrees (mean age = 59 yr, standard deviation [SD] = 21.8 56.7% female), and 65 were adolescents (mean age = 16.5 yr, SD = 1.2 70.8% female), of which 61 had normal hearing and the remaining 4 had mild hearing losses. Threshold comparisons were made between the two test procedures. The Wilcoxon signed-ranked test was used for comparison of threshold correspondence between manual and smartphone thresholds and the paired s les t test was used to compare test time. Within the adult s le, 94.4% of thresholds obtained through smartphone and conventional audiometry corresponded within 10 dB or less. There was no significant difference between smartphone (6.75-min average, SD = 1.5) and conventional audiometry test duration (6.65-min average, SD = 2.5). Within the adolescent s le, 84.7% of thresholds obtained at 0.5, 2, and 4 kHz with hearTest and conventional audiometry corresponded within ≤5 dB. At 1 kHz, 79.3% of the thresholds differed by ≤10 dB. There was a significant difference (p 0.01) between smartphone (7.09 min, SD = 1.2) and conventional audiometry test duration (3.23 min, SD = 0.6). The hearTest application with calibrated supra-aural headphones provides a cost-effective option to determine valid air-conduction hearing thresholds.
A method for using weather radar data to test cloud resolving models
Publisher: Wiley
Date: 09-2009
DOI: 10.1002/MET.150
Environment and Mechanisms of Severe Turbulence in a Midlatitude Cyclone
Publisher: American Meteorological Society
Date: 11-2020
Abstract: A large midlatitude cyclone occurred over the central United States from 0000 to 1800 UTC 30 April 2017. During this period, there were more than 1100 reports of moderate-or-greater turbulence at commercial aviation cruising altitudes east of the Rocky Mountains. Much of this turbulence was located above or, otherwise, outside the synoptic-scale cloud shield of the cyclone, thus complicating its avoidance. In this study we use two-way nesting in a numerical model with finest horizontal spacing of 370 m to investigate possible mechanisms producing turbulence in two distinct regions of the cyclone. In both regions, model-parameterized turbulence kinetic energy compares well to observed turbulence reports. Despite being outside of hazardous large radar reflectivity locations in deep convection, both regions experienced strong modification of the turbulence environment as a result of upper-tropospheric/lower-stratospheric (UTLS) convective outflow. For one region, where turbulence was isolated and short lived, simulations revealed breaking of ~100-km horizontal-wavelength lower-stratospheric gravity waves in the exit region of a UTLS jet streak as the most likely mechanism for the observed turbulence. Although similar waves occurred in a simulation without convection, the altitude at which wave breaking occurred in the control simulation was strongly affected by UTLS outflow from distant deep convection. In the other analyzed region, turbulence was more persistent and widespread. There, overturning waves of much shorter 5–10-km horizontal wavelengths occurred within layers of gradient Richardson number 0.25, which promoted Kelvin–Helmholtz instability associated with strong vertical shear in different horizontal locations both above and beneath the convectively enhanced UTLS jet.
Statistics and Dynamics of Aircraft Encounters of Turbulence over Greenland
Publisher: American Meteorological Society
Date: 08-2009
Abstract: Historical records of aviation turbulence encounters above Greenland are examined for the period from 2000 to 2006. These data identify an important flow regime that contributes to the occurrence of aircraft turbulence encounters, associated with the passage of surface cyclones that direct easterly or southeasterly flow over Greenland’s imposing terrain. The result of this incident flow is the generation of mountain waves that may become unstable through interactions with the background directional wind shear. It is shown that this regime accounted for approximately 40% of the significant turbulent events identified in the 7-yr database. In addition, two specific cases from the database are examined in more detail using a high-resolution mesoscale model. The model simulations highlight the important role of three-dimensional gravity wave–critical level interactions and demonstrate the utility of high-resolution forecasts in the prediction of such events.
Rapidly Evolving Cirrus Clouds Modulated by Convectively Generated Gravity Waves
Publisher: American Geophysical Union (AGU)
Date: 12-07-2019
DOI: 10.1029/2019JD030538
Long-Lived Mesoscale Systems in a Low–Convective Inhibition Environment. Part II: Downshear Propagation
Publisher: American Meteorological Society
Date: 11-2015
Abstract: Part II of this study of long-lived convective systems in a tropical environment focuses on forward-tilted, downshear-propagating systems that emerge spontaneously from idealized numerical simulations. These systems differ in important ways from the standard mesoscale convective system that is characterized by a rearward-tilted circulation with a trailing stratiform region, an overturning updraft, and a mesoscale downdraft. In contrast to this standard mesoscale system, the downshear-propagating system considered here does not feature a mesoscale downdraft and, although there is a cold pool it is of secondary importance to the propagation and maintenance of the system. The mesoscale downdraft is replaced by hydraulic-jump-like ascent beneath an elevated, forward-tilted overturning updraft with negligible convective available potential energy. Therefore, the mesoscale circulation is sustained almost entirely by the work done by the horizontal pressure gradient and the kinetic energy available from environmental shear. This category of organization is examined by cloud-system-resolving simulations and approximated by a nonlinear archetypal model of the quasi-steady Lagrangian-mean mesoscale circulation.
Linear Rainfall Features and Their Association with Rainfall Extremes near Melbourne, Australia
Publisher: American Meteorological Society
Date: 10-2021
Abstract: Linear precipitation systems are a prominent contributor to rainfall over Melbourne, Australia, and the surrounding region. These systems are often convective in nature, frequently associated with cold fronts, and in some cases can lead to significant rainfall and flash flooding. Various types of linearly organized systems (e.g., squall lines, quasi-linear convective systems) have been the subject of much research in the United States and elsewhere, but thus far relatively little analysis has been done on linear systems in Australia. To begin to understand rainfall extremes and how they may change in this region in the future, it is useful to explore the contribution of these types of systems and the characteristics that define them. To this end, we have examined the recently developed Australian Radar Archive (AURA), identifying objects that meet a specific set of relevant criteria, and used multiple methods to identify heavy and extreme daily rainfall. We found that on average, days with linear systems contribute over half of the total rainfall and 70%–85% of heavy/extreme rainfall in the Melbourne region. The linear systems that occur on heavy rainfall days tend to be larger, slower-moving, and longer-lived, while those on extreme rainfall days also tend to be more intense and have a greater degree of southward propagation than linear systems on other days.
The consequences of a local approach in statistical models of convection on its large-scale coherence
Publisher: American Geophysical Union (AGU)
Date: 07-02-2015
DOI: 10.1002/2014JD022680
A first evaluation of the contribution of aeolian sand transport to lagoon island accretion in the Maldives
Publisher: Elsevier BV
Date: 08-2019
Cloud-System-Resolving Model Simulations of Tropical Cloud Systems Observed during the Tropical Warm Pool-International Cloud Experiment
Publisher: American Meteorological Society
Date: 2010
Abstract: Nested cloud-system-resolving model simulations of tropical convective clouds observed during the recent Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are conducted using the Weather Research and Forecasting (WRF) model. The WRF model is configured with a highest-resolving domain that uses 1.3-km grid spacing and is centered over Darwin, Australia. The performance of the model in simulating two different convective regimes observed during TWP-ICE is considered. The first regime is characteristic of the active monsoon, which features widespread cloud cover that is similar to maritime convection. The second regime is a monsoon break, which contains intense localized systems that are representative of diurnally forced continental convection. Many aspects of the model performance are considered, including their sensitivity to physical parameterizations and initialization time, and the spatial statistics of rainfall accumulations and the rain-rate distribution. While the simulations highlight many challenges and difficulties in correctly modeling the convection in the two regimes, they show that provided the mesoscale environment is adequately reproduced by the model, the statistics of the simulated rainfall agrees reasonably well with the observations.
Erratum: The vortical response to penetrative convection and the associated gravity-wave generation. Todd P. LaneAtmosperic Science Letters, 2008; 9: 103-110
Publisher: Wiley
Date: 10-2008
DOI: 10.1002/ASL.201
Further Sensitivities of Orographic Precipitation to Terrain Geometry in Idealized Simulations
Publisher: American Meteorological Society
Date: 23-07-2014
Abstract: This study examines how variations to the nondimensional mountain height Ĥ and the horizontal aspect ratio β of a straight ridge and a concave ridge influence orographic precipitation. An idealized three-dimensional model is used to simulate a moist flow impinging upon these two ridges with Ĥ = 0.66–2.0 and β = 1.0–8.0. The concave ridge generates substantially more precipitation than the straight ridge via an established precipitation-enhancing funneling mechanism near the ridge vertex when the flow is unblocked. Based on previous work, it was hypothesized that when the approaching flow becomes blocked, the strength of the precipitation enhancement by the concave ridge relative to the straight ridge becomes negligible. This study reveals that, if Ĥ is sufficiently large to induce flow reversal on the windward slope, then a secondary circulation develops that is strengthened by the concave ridge with a subsequent enhancement of precipitation. It is also shown that the competing effects of the ridge length and width render the strength of the precipitation enhancement largely insensitive to β. A flow regime diagram for the straight ridge and the concave ridge is also constructed to illustrate the sensitivity of the critical Ĥ value for flow regime transition to changes in the terrain geometry variations to the low-level relative humidity are also explored.
Identifying Key Controls on Storm Formation over the Lake Victoria Basin
Publisher: American Meteorological Society
Date: 30-08-2019
Abstract: The Lake Victoria region in East Africa is a hot spot for intense convective storms that are responsible for the deaths of thousands of fishermen each year. The processes responsible for the initiation, development, and propagation of the storms are poorly understood and forecast skill is limited. Key processes for the life cycle of two storms are investigated using Met Office Unified Model convection-permitting simulations with 1.5 km horizontal grid spacing. The two cases are analyzed alongside a simulation of a period with no storms to assess the roles of the lake–land breeze, downslope mountain winds, prevailing large-scale winds, and moisture availability. While seasonal changes in large-scale moisture availability play a key role in storm development, the lake–land-breeze circulation is a major control on the initiation location, timing, and propagation of convection. In the dry season, opposing offshore winds form a bulge of moist air above the lake surface overnight that extends from the surface to ~1.5 km and may trigger storms in high CAPE/low CIN environments. Such a feature has not been explicitly observed or modeled in previous literature. Storms over land on the preceding day are shown to alter the local atmospheric moisture and circulation to promote storm formation over the lake. The variety of initiation processes and differing characteristics of just two storms analyzed here show that the mean diurnal cycle over Lake Victoria alone is inadequate to fully understand storm formation. Knowledge of daily changes in local-scale moisture variability and circulations are keys for skillful forecasts over the lake.
A doppler radar study of convective draft lengths over darwin, australia
Publisher: American Meteorological Society
Date: 07-2021
Abstract: Data from an upward-pointing wind profiler radar pair at Darwin in tropical Australia are used to determine the characteristics of in idual convective up- and downdrafts observed at the site. Drafts are identified as vertically contiguous regions of instantaneous upward or downward motion exceeding 0.2 ms −1 . Most updrafts and downdrafts found are less than 2 km in vertical extent, and updrafts exceeding 5 km vertical length carry no more than 33% of the total upward mass flux. Updraft length correlates positively with rain rates, and on very high rain rates (greater than 20 mm/hr), average updraft lengths are ~5 km. Typical peak updraft velocities increase from ~2.5 ms −1 for the smallest to ~ 4 ms −1 for the largest drafts, while those for downdrafts remain ~ 2 ms −1 regardless of size. These results are broadly consistent with other numerical modeling studies, but contrast with the common view of deep convection as being dominated by continuous, deep drafts.
Gravity Wave Influences On Mesoscale Divergence: An Observational Case Study
Publisher: American Geophysical Union (AGU)
Date: 16-01-2020
DOI: 10.1029/2019GL086539
Abstract: Characteristics of tropospheric low‐frequency gravity waves are diagnosed in radiosonde soundings from the Tropical Warm Pool‐International Cloud Experiment near Darwin, Australia. The waves have typical vertical wavelengths of about 4 km, horizontal wavelengths of about 600 km, and intrinsic periods of about 12 hr. These scales match those of the vertical, horizontal, and temporal variability found in area‐averaged horizontal wind ergence over the same domain. Vertical profiles of ergence show wave‐like structures with variability of the order of 2 10 s in the free troposphere. The results for Darwin are similar to previously reported observed mesoscale patterns of ergence/convergence over the tropical Atlantic. The findings imply that tropical ergence on spatial scales of a few hundred kilometers, which is known to influence the organization of convection, may be forced by gravity waves.
Mesoscale Variation in Diabatic Heating around Sumatra, and Its Modulation with the Madden–Julian Oscillation
Publisher: American Meteorological Society
Date: 26-07-2018
Abstract: Diabatic heating in the Maritime Continent region is controlled by a unique blend of mesoscale variability associated with steep topography and complex coastlines and intraseasonal variability associated with propagating planetary-scale disturbances. In this study, the diabatic heating from a 10-yr austral summer simulation over the Maritime Continent with a 4-km horizontal grid length is analyzed with respect to diurnal, spatial, and intraseasonal variations. Results are compared, where possible, to analogous estimates from the TRMM precipitation radar. We show that the heating budget is largely a balance between latent heating and vertical advection, with rays of heating and cooling extending upward and outward from the coast evident in the advection terms, consistent with the gravity wave representation of the tropical sea breeze. By classifying rainfall into convective and stratiform components, it is shown that simulated convective heating over Sumatra peaks in MJO phases 2 and 3, while simulated stratiform heating peaks in phase 4. Similarly, spectral latent heating estimates from the TRMM Precipitation Radar show that stratiform heating peaks in phases 3 and 4, while convective heating peaks in phases 2 and 3. It is also shown that stratiform precipitation plays a greater role in offshore precipitation during the night, albeit with embedded convective cores, than over the land during the day. These results emphasize the importance of achieving a realistic representation of convective and stratiform processes in high-resolution simulations in the tropics, both for total rainfall estimates and for realistic latent heating.
Gravity waves generated by convection during TWP-ICE: 2. High-frequency gravity waves
Publisher: American Geophysical Union (AGU)
Date: 13-05-2014
DOI: 10.1002/2013JD020726
Convectively generated gravity waves simulated by NAM-SCA
Publisher: American Geophysical Union (AGU)
Date: 06-08-2014
DOI: 10.1002/2013JD021419
An improved diagnostic for summertime rainfall along the eastern seaboard of Australia
Publisher: Wiley
Date: 19-03-2015
DOI: 10.1002/JOC.4300
Gravity waves generated by convection during TWP-ICE: I. Inertia-gravity waves
Publisher: American Geophysical Union (AGU)
Date: 13-05-2021
DOI: 10.1002/2013JD020724
On physical realizability and uncertainty of numerical solutions
Publisher: Wiley
Date: 2005
DOI: 10.1002/ASL.100
The Role of Gravity Wave Breaking in a Case of Upper-Level Near-Cloud Turbulence
Publisher: American Meteorological Society
Date: 12-2019
Abstract: An observed turbulence encounter that occurred outside a mesoscale convective system over the central United States on 3 June 2005 is investigated using observations and high-resolution numerical modeling. Here, the mechanisms associated with the observed moderate-to-severe turbulence during the evolution of this convective system are examined. Comparison between aircraft-observed eddy dissipation rate data with satellite and radar shows that a majority of turbulence reports are located on the south side and outside of a nocturnal mesoscale convective system (MCS), relatively large distances from the active convective regions. Simulations show that ergent storm-induced upper-level outflow reduces the environmental flow on the south side of the MCS, while on the north and northwest side it enhances the environmental flow. This upper-level storm outflow enhances the vertical shear near the flight levels and contributes to mesoscale reductions in Richardson number to values that support turbulence. In addition to the role of the MCS-induced outflow, high-resolution simulations (1.1-km horizontal grid spacing) show that turbulence is largely associated with a large- litude gravity wave generated by the convective system, which propagates away from it. As the wave propagates in the region with enhanced vertical shear caused by the storm-induced upper-level outflow, it lifies, overturns, and breaks down into turbulence. The location of the simulated turbulence relative to the storm agrees with the observations and the analysis herein provides insight into the key processes underlying this event.
Insights into convective momentum transport and its parametrization from idealized simulations of organized convection
Publisher: Wiley
Date: 18-09-2017
DOI: 10.1002/QJ.3118
Statistical Assessment of Tropical Convection-Permitting Model Simulations Using a Cell-Tracking Algorithm
Publisher: American Meteorological Society
Date: 02-2013
Abstract: This study presents a method for comparing convection-permitting model simulations to radar observations using an innovative object-based approach. The method uses the automated cell-tracking algorithm, Thunderstorm Identification Tracking Analysis and Nowcasting (TITAN), to identify in idual convective cells and determine their properties. Cell properties are identified in the same way for model and radar data, facilitating comparison of their statistical distributions. The method is applied to simulations of tropical convection during the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) using the Weather Research and Forecasting Model, and compared to data from a ground-based radar. Simulations with different microphysics and model resolution are also conducted. Among other things, the comparisons between the model and the radar elucidate model errors in the depth and size of convective cells. On average, simulated convective cells reached higher altitudes than the observations. Also, when using a low reflectivity (25 dBZ) threshold to define convective cells, the model underestimates the size of the largest cells in the observed population. Some of these differences are alleviated with a change of microphysics scheme and higher model resolution, demonstrating the utility of this method for assessing model changes.
Influences of Moist Convection on a Cold-Season Outbreak of Clear-Air Turbulence (CAT)
Publisher: American Meteorological Society
Date: 08-2012
Abstract: The 9–10 March 2006 aviation turbulence outbreak over the central United States is examined using observations and numerical simulations. Though the turbulence occurs within a deep synoptic cyclone with widespread precipitation, comparison of reports from commercial aircraft with radar and satellite data reveals the majority of the turbulence to be in clear air. This clear-air turbulence (CAT) is located above a strong upper-level jet, where vertical shear ranged between 20 and 30 m s−1 km−1. Comparison of a moist simulation with a dry simulation reveals that simulated vertical shear and subgrid turbulence kinetic energy is significantly enhanced by the anticyclonic upper-level flow perturbation associated with the organized convection in regions of observed CAT. A higher-resolution simulation is used to examine turbulence mechanisms in two primary clusters of reported moderate and severe turbulence. In the northern cluster where vertical shear is strongest, the simulated turbulence arises from Kelvin–Helmholtz (KH) instability. The turbulence farther south occurs several kilometers above shallow, but vigorous, moist convection. There, the simulated turbulence is influenced by vertically propagating gravity waves initiated when the convection impinges on a lowered tropopause. In some locations these gravity waves lify and break leading directly to turbulence, while in others they aid turbulence development by helping excite KH instability within the layers of strongest vertical shear above them. Although both clusters of turbulence occur either above or laterally displaced from cloud, a shared characteristic is their owed existence to moist convection within the wintertime cyclone, which distinguishes them from traditional CAT.
Convectively generated gravity waves observed from radiosonde data taken during MCTEX
Publisher: Wiley
Date: 04-2003
DOI: 10.1256/QJ.02.196
Coupling between Gravity Waves and Tropical Convection at Mesoscales
Publisher: American Meteorological Society
Date: 11-2011
Abstract: An idealized cloud-system-resolving model simulation is used to examine the coupling between a tropical cloud population and the mesoscale gravity waves that it generates. Spectral analyses of the cloud and gravity wave fields identify a clear signal of coupling between the clouds and a deep tropospheric gravity wave mode with a vertical wavelength that matches the depth of the convection, which is about two-thirds of the tropospheric depth. This vertical wavelength and the period of the waves, defined by a characteristic convective time scale, means that the horizontal wavelength is constrained through the dispersion relation. Indeed, the wave–convection coupling manifests at the appropriate wavelength, with the emergence of quasi-regular cloud-system spacing of order 100 km. It is shown that cloud systems at this spacing achieve a quasi-resonant state, at least for a few convective life cycles. Such regular spacing is a key component of cloud organization and is likely a contributor to the processes controlling the upscale growth of convective systems. Other gravity wave processes are also elucidated, including their apparent role in the maintenance of convective systems by providing a mechanism for renewed convective activity and system longevity.
Aviation Turbulence
Publisher: Springer International Publishing
Date: 2016
On the Robustness of Annual Daily Precipitation Maxima Estimates Over Monsoon Asia
Publisher: Frontiers Media SA
Date: 30-10-2020
The Sensitivity of Atmospheric River Identification to Integrated Water Vapor Transport Threshold, Resolution, and Regridding Method
Publisher: American Geophysical Union (AGU)
Date: 21-10-2020
DOI: 10.1029/2020JD032897
Stratospheric Gravity Waves Generated by Multiscale Tropical Convection
Publisher: American Meteorological Society
Date: 08-2008
Abstract: The generation of gravity waves by multiscale cloud systems evolving in an initially motionless and thermodynamically uniform environment is explored using a two-dimensional cloud-system-resolving model. The simulated convection has similar depth and intensity to observed tropical oceanic systems. The convection self-organizes into preferred horizontal and temporal scales involving weakly organized propagating cloud clusters. The multiscale systems generate a broad spectrum of gravity waves with horizontal scales that range from the cloud-system scale up to the cloud-cluster scale. The gravity waves with the largest horizontal scale play an important role in modifying layered tropospheric inflow and outflow to the cloud systems, which in turn influence the multiscale convective organization. Slower-moving short-scale gravity waves make the strongest in idual contribution to the vertical flux of horizontal momentum and cause a robust peak in the momentum flux spectrum that corresponds to the lifetime and spatial scale of the in idual cloud systems.
5-day-wave interactions with tropical precipitation in CMIP5 models
Publisher: American Meteorological Society
Date: 15-11-2016
Abstract: The 5-day Rossby–Haurwitz wave is unlike other large-scale wave modes that interact with tropical rainfall in that associated rainfall presents as a modulation of localized areas of rainfall instead of propagating with the wave. This form of wave-modulated convective organization in climate models has received little attention. This study investigates the simulation of interactions between the 5-day wave and tropical convection in 30 models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) and compares these with the interaction diagnosed from ERA-Interim and TRMM precipitation data. Models simulate the dry dynamics of the 5-day wave well, with realistic coherences between upper- and lower-tropospheric winds, as well as magnitudes and geographic distribution of wave wind anomalies being close to observations. The models consistently display significant coherences between 5-day-wave zonal winds and precipitation but perform less well at simulating the spatial distribution and magnitude of precipitation anomalies. For ex le, a third of the models do not reproduce significant observed anomalies near the Andes, and the best-performing model simulates only 38% of the observed variance over the tropical Andes and 24% of the observed variance over the Gulf of Guinea. Models with higher resolution perform better in simulating the magnitude of the Andean rainfall anomalies, but there is no similar relationship over the Gulf of Guinea. The evidence therefore suggests that the simulated interaction is mostly one way only, with the wave dynamics forcing the precipitation variations on the 5-day time scale.
The Generation of Near-Cloud Turbulence in Idealized Simulations
Publisher: American Meteorological Society
Date: 20-06-2014
Abstract: This study explores the generation of turbulence in the upper outflow regions of simulated idealized mesoscale convective systems. The simulated storms are shown to generate parameterized turbulence that occurs significant distances (& km) from the main convective regions, in both the clear air surrounding the convection and low simulated reflectivity regions with cloud ice but negligible amounts of graupel and snow. The source of the turbulence is related to Kelvin–Helmholtz instabilities that occur in the shear zones above and below the storm-induced upper-level outflow jet that is centered near the tropopause the model produces resolved-scale billows within regions of low gradient Richardson number. Short-scale gravity waves are also coincident with the regions of turbulence, become trapped within the jet core, and appear to be generated by the shear instability. Additional experiments with different initial upper-level wind shear show similar mechanisms to those simulations with no initial upper-level shear. These results help elucidate the dynamics of turbulence generation near convection, which has important implications for the aviation industry and the fundamental understanding of how convective clouds interact with their environment.
A model study on the influence of overshooting convection on TTL water vapour
Publisher: Copernicus GmbH
Date: 19-10-2010
Abstract: Abstract. Overshooting deep convection that penetrates into the Tropical Tropopause Layer (TTL) is thought to have an important role in regulating the water vapour content of this region. Yet, the net effect of such convection and the dominant mechanisms remain unclear. This study uses two idealised three-dimensional cloud-resolving model simulations to examine the influence of overshooting convection on water vapour when it penetrates into two different TTL environments, one supersaturated and the other subsaturated with respect to ice. These simulations show that the overshooting convection plays a direct role in driving the ambient environment towards ice saturation through either net moistening (subsaturated TTL) or net dehydration (supersaturated TTL). Moreover, in these cases the extent of dehydration in supersaturated conditions is greater than the moistening in subsaturated conditions. With the aid of modelled passive tracers, the relative roles of transport, mixing and ice microphysics are assessed ultimately, ice sublimation and scavenging processes play the most important role in defining the different TTL relative humidity tendencies. In addition, significant moistening in both cases is modelled well into the subsaturated tropical lower stratosphere (up to 450 K), even though the overshooting turrets only reach approximately 420 K. It is shown that this moistening is the result of jumping cirrus, which is induced by the localised upward transport and mixing of TTL air following the collapse of the overshooting turret.
A local index of Maritime Continent intraseasonal variability based on rain rates over the land and sea
Publisher: American Geophysical Union (AGU)
Date: 10-09-2016
DOI: 10.1002/2016GL069987
Sensitivities of Orographic Precipitation to Terrain Geometry and Upstream Conditions in Idealized Simulations
Publisher: American Meteorological Society
Date: 30-03-2012
Abstract: This study examines how variations in relatively simple terrain geometries influence orographic precipitation and its spatial patterns of sensitivity to small changes in upstream conditions. An idealized three-dimensional model is used to simulate a moist flow impinging upon three alpine-scale terrain shapes: a straight ridge, a concave ridge, and a convex ridge. A variety of simulations are conducted to investigate the sensitivity of precipitation patterns to ridge length and upstream thermodynamic and wind conditions for an impinging flow with a nondimensional mountain height of approximately unity. It is found that for the straight and convex ridges, the flow response is mostly linear for the conditions examined here and passes over the obstacles with little lateral deflection. The concave ridge, however, exhibits strengthened flow deceleration, wave breaking in the lee, and flow confluence between the ridge arms. The concave ridge also generates substantially more precipitation than the other two ridge geometries via an established precipitation-enhancing funneling mechanism near the ridge vertex. However, for some concave ridge configurations the results feature dual-precipitation maxima, which is an important difference from previous findings. Finally, results from a simple ensemble of simulations elucidate the sensitivity of precipitation patterns to small variations in upstream conditions and how these vary for the different terrain geometries.
Coral-reef-derived dimethyl sulfide and the climatic impact of the loss of coral reefs
Publisher: Copernicus GmbH
Date: 20-04-2021
Abstract: Abstract. Dimethyl sulfide (DMS) is a naturally occurring aerosol precursor gas which plays an important role in the global sulfur budget, aerosol formation and climate. While DMS is produced predominantly by phytoplankton, recent observational literature has suggested that corals and their symbionts produce a comparable amount of DMS, which is unaccounted for in models. It has further been hypothesised that the coral reef source of DMS may modulate regional climate. This hypothesis presents a particular concern given the current threat to coral reefs under anthropogenic climate change. In this paper, a global climate model with online chemistry and aerosol is used to explore the influence of coral-reef-derived DMS on atmospheric composition and climate. A simple representation of coral-reef-derived DMS is developed and added to a common DMS surface water climatology, resulting in an additional flux of 0.3 Tg yr−1 S, or 1.7 % of the global sulfur flux from DMS. By comparing the differences between both nudged and free-running ensemble simulations with and without coral-reef-derived DMS, the influence of coral-reef-derived DMS on regional climate is quantified. In the Maritime Continent–Australian region, where the highest density of coral reefs exists, a small decrease in nucleation- and Aitken-mode aerosol number concentration and mass is found when coral reef DMS emissions are removed from the system. However, these small responses are found to have no robust effect on regional climate via direct and indirect aerosol effects. This work emphasises the complexities of the aerosol–climate system, and the limitations of current modelling capabilities are highlighted, in particular surrounding convective responses to changes in aerosol. In conclusion, we find no robust evidence that coral-reef-derived DMS influences global and regional climate.
Diurnal Cycle of Surface Winds in the Maritime Continent Observed through Satellite Scatterometry
Publisher: American Meteorological Society
Date: 06-2019
Abstract: The diurnal cycle of surface winds throughout the Maritime Continent plays a significant role in the formation of precipitation over the islands of the region and over the surrounding seas. This study investigates the connection between the diurnal cycles of surface wind and offshore precipitation using data from four satellite scatterometer instruments and two satellite precipitation radar instruments. For the first time, data from three scatterometer instruments are combined to yield a more temporally complete picture of the surface wind diurnal cycles over the Maritime Continent’s surrounding seas. The results indicate that land–sea breezes typically propagate over 400 km offshore, produce mean wind perturbations of between 1 and 5 m s −1 , and propagate as gravity waves at 25–30 m s −1 . Diurnal precipitation cycles are affected through gravity wave propagation processes associated with the land–sea breezes, and through the convergence of land breezes from nearby islands. These observational results are then compared with previous mesoscale modeling results. It is shown that land–sea breezes occur too early, and are too intense in these modeling results, and this may partly explain why these modeling results also exhibit an early, overly intense diurnal precipitation cycle. This study also investigates variations in the diurnal cycle of surface winds at seasonal and intraseasonal time scales. Previous work has suggested that seasonal and intraseasonal variations in surface heating affect the land–sea breeze circulation and diurnal precipitation cycles we argue that variations in background winds also play a defining role in modulating coastally influenced local winds.
A Census of Atmospheric Variability From Seconds to Decades
Publisher: American Geophysical Union (AGU)
Date: 15-11-2017
DOI: 10.1002/2017GL075483
A climatology of atmospheric pressure jumps over southeastern Australia
Publisher: Wiley
Date: 14-12-2016
DOI: 10.1002/QJ.2933
Sub-seasonal to seasonal prediction of rainfall extremes in Australia
Publisher: Wiley
Date: 13-04-2020
DOI: 10.1002/QJ.3789
Processes Underlying Near-Cloud Turbulence
Publisher: Springer International Publishing
Date: 2016
Extreme rainfall in New Zealand and its association with Atmospheric Rivers
Publisher: IOP Publishing
Date: 12-03-2021
Abstract: Atmospheric rivers (ARs) are narrow and elongated regions of enhanced horizontal water vapour transport. Considerable research on understanding Northern Hemisphere ARs and their relationship with extreme precipitation has shown that ARs have a strong association with heavy rainfall and flooding. While there has been very little work on ARs in the Southern Hemisphere, global climatologies suggest that ARs are equally as common in both hemispheres. New Zealand in particular is located in a region of high AR frequency. This study aims to test the hypothesis that ARs play a significant role in heavy precipitation and flooding events in New Zealand. We used a recently developed AR identification method and daily station data across New Zealand to test for the concurrence of ARs and extreme rainfall. We found that, at each of the eleven stations analysed, at least seven to all ten of the top ten heaviest precipitation days between 1980 and 2018 were associated with AR conditions. Nine of the ten most damaging floods in New Zealand between 2007 and 2017 occurred during AR events. These results have important implications for understanding extreme rainfall in New Zealand, and ultimately for predicting some of the most hazardous events in the region. This work also highlights that more research on ARs in New Zealand is needed.
Recent Advances in the Understanding of Near-Cloud Turbulence
Publisher: American Meteorological Society
Date: 04-2012
DOI: 10.1175/BAMS-D-11-00062.1
Abstract: Anyone who has flown in a commercial aircraft is familiar with turbulence. Unexpected encounters with turbulence pose a safety risk to airline passengers and crew, can occasionally damage aircraft, and indirectly increase the cost of air travel. Deep convective clouds are one of the most important sources of turbulence. Cloud-induced turbulence can occur both within clouds and in the surrounding clear air. Turbulence associated with but outside of clouds is of particular concern because it is more difficult to discern using standard hazard identification technologies (e.g., satellite and radar) and thus is often the source of unexpected turbulence encounters. Although operational guidelines for avoiding near-cloud turbulence exist, they are in many ways inadequate because they were developed before the governing dynamical processes were understood. Recently, there have been significant advances in the understanding of the dynamics of near-cloud turbulence. Using ex les, this article demonstrates how these advances have stemmed from improved turbulence observing and reporting systems, the establishment of archives of turbulence encounters, detailed case studies, and high-resolution numerical simulations. Some of the important phenomena that have recently been identified as contributing to near-cloud turbulence include atmospheric wave breaking, unstable upper-level thunderstorm outflows, shearing instabilities, and cirrus cloud bands. The consequences of these phenomena for developing new en route turbulence avoidance guidelines and forecasting methods are discussed, along with outstanding research questions.
On the Identification of the Large-Scale Properties of Tropical Convection Using Cloud Regimes
Publisher: American Meteorological Society
Date: 23-08-2013
DOI: 10.1175/JCLI-D-12-00624.1
Abstract: The use of cloud regimes in identifying tropical convection and the associated large-scale atmospheric properties is investigated. The regimes are derived by applying cluster analysis to satellite retrievals of daytime-averaged frequency distributions of cloud-top pressure and optical thickness within grids of 280 km by 280 km resolution from the International Satellite Cloud Climatology Project between 1983 and 2008. An investigation of atmospheric state variables as a function of cloud regime reveals that the regimes are useful indicators of the archetypal states of the tropical atmosphere ranging from a strongly convecting regime with large stratiform cloudiness to strongly suppressed conditions showing a large coverage with stratocumulus clouds. The convectively active regimes are shown to be moist and unstable with large-scale ascending motion, while convectively suppressed regimes are dry and stable with large-scale descending winds. Importantly, the cloud regimes also represent several transitional states. In particular, the cloud regime approach allows for the identification of the “building blocks” of tropical convection, namely, the regimes dominated by stratiform, deep, and congestus convection. The availability of the daily distribution of these building blocks for more than 20 years opens new avenues for the diagnosis of convective behavior as well as the evaluation of the representation of convection in global and regional models.
The impact of overshooting deep convection on local transport and mixing in the tropical upper troposphere/lower stratosphere (UTLS)
Publisher: Copernicus GmbH
Date: 12-06-2015
Abstract: Abstract. In this study we examine the simulated downward transport and mixing of stratospheric air into the upper tropical troposphere as observed on a research flight during the SCOUT-O3 c aign in connection with a deep convective system. We use the Advanced Research Weather and Research Forecasting (WRF-ARW) model with a horizontal resolution of 333 m to examine this downward transport. The simulation reproduces the deep convective system, its timing and overshooting altitudes reasonably well compared to radar and aircraft observations. Passive tracers initialised at pre-storm times indicate the downward transport of air from the stratosphere to the upper troposphere as well as upward transport from the boundary layer into the cloud anvils and overshooting tops. For ex le, a passive ozone tracer (i.e. a tracer not undergoing chemical processing) shows an enhancement in the upper troposphere of up to about 30 ppbv locally in the cloud, while the in situ measurements show an increase of 50 ppbv. However, the passive carbon monoxide tracer exhibits an increase, while the observations show a decrease of about 10 ppbv, indicative of an erroneous model representation of the transport processes in the tropical tropopause layer. Furthermore, it could point to insufficient entrainment and detrainment in the model. The simulation shows a general moistening of air in the lower stratosphere, but it also exhibits local dehydration features. Here we use the model to explain the processes causing the transport and also expose areas of inconsistencies between the model and observations.
The diurnal cycle of rainfall and cloud properties from Himawari-8 during the austral summer (2016-2020).
Publisher: Wiley
Date: 14-12-2021
A case of offshore convective initiation by interacting land breezes near Darwin, Australia
Publisher: Springer Science and Business Media LLC
Date: 07-01-2012
Shear‐Parallel Tropical Convective Systems: Importance of Cold Pools and Wind Shear
Publisher: American Geophysical Union (AGU)
Date: 24-06-2020
DOI: 10.1029/2020GL087720
Abstract: The impact of cold pools on line‐orientated convective systems is assessed using idealized simulations of tropical oceanic convection under weak, moderate, and strong wind shear regimes. Cold pools are weakened by suppressing evaporation in the shallow subcloud layer. Analysis of objectively identified convective systems reveals that the convection with weaker cold pools is more often oriented parallel, rather than perpendicular, to the wind shear. The cold pool‐induced orientation changes are most pronounced in the strong shear environment. Interactions between convective orientation and the tropical atmosphere are assessed. Simulations with shear‐parallel convection demonstrate more top‐of‐atmosphere upwelling longwave radiation and less reflected shortwave radiation due to changes in convective anvils, faster‐propagating larger‐scale gravity waves, narrower cross‐shear moisture distributions, and differences in convective momentum fluxes. The results highlight critical interactions across convective scales, mesoscales, and climate scales, as well as avenues for parameterizing structural modes of mesoscale‐organized convection in global models.
Some Influences of Background Flow Conditions on the Generation of Turbulence due to Gravity Wave Breaking above Deep Convection
Publisher: American Meteorological Society
Date: 11-2008
Abstract: Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.
Research Needs
Publisher: Springer International Publishing
Date: 2016
Long-Lived Mesoscale Systems in a Low–Convective Inhibition Environment. Part I: Upshear Propagation
Publisher: American Meteorological Society
Date: 11-2015
Abstract: Dynamical models of organized mesoscale convective systems have identified the important features that help maintain their overarching structure and longevity. The standard model is the trailing stratiform archetype, featuring a front-to-rear ascending circulation, a mesoscale downdraft circulation, and a cold pool/density current that affects the propagation speed and the maintenance of the system. However, this model does not represent all types of mesoscale convective systems, especially in moist environments where the evaporation-driven cold pools are weak and the convective inhibition is small. Moreover, questions remain about the role of gravity waves in creating and maintaining organized systems and affecting their propagation speed. This study presents simulations and dynamical models of self-organizing convection in a moist, low–convective inhibition environment and examines the long-lived convective regimes that emerge spontaneously. This paper, which is Part I of this study, specifically examines the structure, kinematics, and maintenance of long-lived, upshear-propagating convective systems that differ in important respects from the standard model of long-lived convective systems. Linear theory demonstrates the role of ducted gravity waves in maintaining the long-lived, upshear-propagating systems. A steady nonlinear model approximates the dynamics of upshear-propagating density currents that are key to the maintenance of the mesoscale convective system.
Global and regional impacts differ between transient and equilibrium warmer worlds
Publisher: Springer Science and Business Media LLC
Date: 23-12-2020
The Link Between the Madden‐Julian Oscillation and Rainfall Trends in Northwest Australia
Publisher: American Geophysical Union (AGU)
Date: 18-04-2023
DOI: 10.1029/2022GL101799
Abstract: Rainfall during the monsoon in northwest Australia has increased since the 1950s. Previous studies have explored possible causes of the rainfall increase however, the trend has not been fully explained. Understanding the cause of this trend is important for interpreting climate projections and local water‐sensitive services. We explore the role of the Madden‐Julian Oscillation (MJO) in explaining the rainfall increase. The MJO, since 1974, has had a longer duration in phases associated with enhanced rainfall in northwest Australia (Phases 5 and 6) during the monsoon. We show that the rainfall trend in northwest Australia is identified only during MJO phases associated with enhanced rainfall, with a large change in daily rainfall distribution in these phases. The increasing occurrence of these MJO phases explains most of the rainfall increase, as opposed to an increase in daily rainfall independent of MJO phase, albeit with some sensitivity to MJO definition.
A Case of an Undular Bore and Prefrontal Precipitation in the Australian Alps
Publisher: American Meteorological Society
Date: 23-06-2016
Abstract: This study explores the mesoscale processes that led to the development of two prefrontal precipitation events in the Australian Alps on 29–30 October 2010. The synoptic setting was characterized by the passage of an interacting front and prefrontal trough across southern Australia. Observations and model simulations revealed that when the prefrontal trough entered southeast Australia it resembled a density current advancing into a stable nocturnal layer, forming a bore at its leading edge. The bore detached from and propagated ahead of the prefrontal trough and became undular, supported by a wave-ducting mechanism. The undular bore was observed in the Doppler wind field of a radar, parts of which were collocated with bands of reflectivity. Strong winds coincident with this band of reflectivity suggest the undular bore triggered convection that eventually led to the bore’s demise. An ensemble of high-resolution model simulations (with perturbed initial and boundary conditions) was used to understand the key processes affecting the undular bore and two prefrontal precipitation events. While no member of the ensemble reproduced the first prefrontal precipitation event, at least six members (20%) reproduced parts of the second prefrontal precipitation event. Despite the low precipitation predictability, analysis of the ensemble suggests the undular bore was both a predictable phenomenon and integral to the initiation and/or evolution of the two prefrontal precipitation events.
Mechanisms Linking Global 5-Day Waves to Tropical Convection
Publisher: American Meteorological Society
Date: 11-2017
Abstract: Reanalysis data and satellite-derived rainfall measurements are examined to determine possible mechanisms linking the “5 day” Rossby–Haurwitz wave to localized variations of tropical convection. The mechanisms in all regions rely on the modulation of zonal winds near the equator by the wave, but the nature of these mechanisms depends strongly on local topography and local climate. In the upper Amazon basin, the wave modulates the strength of prevailing easterlies and thus the upslope flow and associated convection on the eastern edge of the Andes. Similar modulation of upslope flow is involved off the Panamanian and Colombian Pacific coasts, but the deflection and confluence of low-level wind in the presence of the Andes and moisture transports across the Andes from the Amazon basin are also factors. Similar deflection and confluence of winds around and through the Maritime Continent lead to low-level ergence and convection anomalies over the eastern Indian Ocean. Anomalous moisture transports from the Congo basin to the eastern and northeastern Gulf of Guinea due to the wave affect atmospheric moisture over the Gulf of Guinea and thus convection in the region. Over oceanic convergence zones, modulations of the prevailing winds by the wave affect the overall wind magnitude, changing evaporation from the ocean surface and atmospheric moisture. Most of these mechanisms arise from the nonuniform nature of Earth’s surface and suggest that other external Rossby–Haurwitz waves may have similar interactions with convection.
Does Lower‐Stratospheric Shear Influence the Mesoscale Organization of Convection?
Publisher: American Geophysical Union (AGU)
Date: 02-2021
DOI: 10.1029/2020GL091025
Abstract: Organized mesoscale convection is important for many atmospheric phenomena and hazards, however the understanding of its governing mechanisms is incomplete. Theories explaining mesoscale organization rely on the interaction between convection outflows and lower‐tropospheric wind shear. Here a new mechanism is presented, where lower‐stratospheric wind shear is shown to influence mesoscale organization. The mechanism is linked to coupling between convection and gravity waves, with the stratosphere playing a role in shaping the tropospheric wave spectrum. The key result is that lower‐stratospheric shear creates a preference for organized systems propagating in the same direction as the shear vector by weakening the systems propagating in the opposite direction to the shear. This result has important implications for stratosphere‐troposphere interactions, numerical modeling, and understanding of convective organization in general.
Related Organisations
National Center For Atmospheric Research
Location: United States of America
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