ORCID Profile
0000-0003-1101-7249
Current Organisation
Australian Bureau of Meteorology
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: American Geophysical Union (AGU)
Date: 21-04-2018
DOI: 10.1002/2017JD027740
Publisher: Public Library of Science (PLoS)
Date: 19-09-2019
Publisher: American Meteorological Society
Date: 13-10-2015
Abstract: Changes of the Southern Hemisphere Hadley cell over the twentieth century are investigated using the Twentieth Century Reanalysis (20CR) and coupled model simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Trends computed on a 30-yr sliding window on the 20CR dataset reveal a statistically significant expansion of the Hadley cell from 1968 forced by an increasing surface global warming. This expansion is strongly associated with the intensification and poleward shift of the subtropical dry zone, which potentially explain the increasing trends of droughts in the subtropical regions such as southern Australia, South America, and Africa. Coupled models from the CMIP5 do not adequately simulate the observed amount of the Hadley expansion, only showing an average of one-fourth of the expansion as determined from the 20CR and only when simulations include greenhouse gas forcing as opposed to simulations including natural forcing only.
Publisher: American Meteorological Society
Date: 06-2003
Publisher: MDPI AG
Date: 17-09-2021
Abstract: Improved quantitative forecasts of volcanic ash are in great demand by the aviation industry to enable better risk management during disruptive volcanic eruption events. However, poor knowledge of volcanic source parameters and other dispersion and transport modelling uncertainties, such as those due to errors in numerical weather prediction fields, make this problem very challenging. Nonetheless, satellite-based algorithms that retrieve ash properties, such as mass load, effective radius, and cloud top height, combined with inverse modelling techniques, such as ensemble filtering, can significantly ameliorate these problems. The satellite-retrieved data can be used to better constrain the volcanic source parameters, but they can also be used to avoid the description of the volcanic source altogether by direct insertion into the forecasting model. In this study we investigate the utility of the direct insertion approach when employed within an ensemble filtering framework. Ensemble members are formed by initializing dispersion models with data from different timesteps, different values of cloud top height, thickness, and NWP ensemble members. This large ensemble is then filtered with respect to observations to produce a refined forecast. We apply this approach to 14 different eruption case studies in the tropical atmosphere. We demonstrate that the direct insertion of data improves model forecast skill, particularly when it is used in a hybrid ensemble in which some ensemble members are initialized from the volcanic source. Moreover, good forecast skill can be obtained even when detailed satellite retrievals are not available, which is frequently the case for volcanic eruptions in the tropics.
Publisher: American Geophysical Union (AGU)
Date: 10-08-2017
DOI: 10.1002/2017JD026676
Publisher: American Meteorological Society
Date: 09-2002
Publisher: American Meteorological Society
Date: 09-1996
Publisher: Springer Science and Business Media LLC
Date: 03-08-2022
DOI: 10.1007/S00382-021-05903-9
Abstract: We examine the climatology, variability and change in the global mean meridional circulation (MMC) as measured in a dry isentropic coordinate system from 1979–2017 using the ERA-Interim reanalysis. The methodology presents a zonal-mean view of the MMC as a single thermally direct circulation cell in each hemisphere. The circulation is decomposed into 'steady' and 'transient' components which allows us to identify and quantify several MMC features, including the Intertropical Convergence Zone, the descending branches of the Hadley circulation and a 'transient updraft' associated with the extratropical storm track. Large changes were identified in the Southern Hemisphere (SH) in both the Hadley Cell and the extratropical storm track in the late-1990s. These changes intertwine with the Interdecadal Pacific Oscillation that changed from a warm to a cold phase around 2000. Less significant changes were observed in the Northern Hemisphere, although high rates of tropical expansion during boreal summer may have been exacerbated by volcanic eruptions in the 1980s and 1990s. Further to those changes, tropical expansion was observed in autumn, with little change in the extratropical storm track. While potential inhomogeneities in the reanalysis limit the certainty about the magnitude of the identified changes, multiple non-reanalysis-based datasets suggest that large changes did occur in the 1990s in the SH, supporting the presented analysis.
Publisher: American Meteorological Society
Date: 12-1994
Publisher: Wiley
Date: 30-09-2013
DOI: 10.1002/WCC.251
Abstract: This review provides comprehensive coverage of the tropical expansion literature to date. The primary focus is on the annual‐ and zonal‐mean behavior of the phenomenon. An idealized model that identifies the mean meridional circulation as a hemisphere‐wide structure with significant tropical–extratropical interaction is introduced as background for the understanding of the expansion and the methodologies used for detection. A variety of metrics from different data sources have been used to identify an expansion of the global tropics since 1979 by 1°–3° latitude in each hemisphere, an average trend of approximately 0.5°–1.0° decade −1 . The symmetry of this expansion—whether Northern and Southern hemispheres are expanding at the same rate—is unclear. Limitations of observational datasets, including reanalyses, prevent a more precise determination at this time. General circulation models are able to qualitatively reproduce this expansion, but generally underestimate its magnitude. Multiple factors have been identified as potential drivers of the expansion, including increasing greenhouses gases, stratospheric ozone depletion, and anthropogenic aerosols. No single factor by itself appears to explain the full expansion, perhaps a shortcoming of the models or experiment design. It may be that some combination of these forcings is producing the change, but the relative contribution of each forcing to the expansion is currently unknown. The key issues remaining to be resolved are briefly summarized at the end. WIREs Clim Change 2014, 5:89–112. doi: 10.1002/wcc.251 This article is categorized under: Paleoclimates and Current Trends Modern Climate Change Climate Models and Modeling Knowledge Generation with Models
Publisher: Frontiers Media SA
Date: 23-12-2020
DOI: 10.3389/FPSYG.2020.558856
Abstract: Both daily demands as well as training and competition characteristics in sports can result in a psychobiological state of mental fatigue leading to feelings of tiredness, lack of energy, an increased perception of effort, and performance decrements. Moreover, optimal performance will only be achievable if the balance between recovery and stress states is re-established. Consequently, recovery strategies are needed aiming at mental aspects of recovery. The aim of the study was to examine acute effects of potential mental recovery strategies (MR) on subjective-psychological and on cognitive performance outcomes after a mentally fatiguing task. A laboratory-based randomized cross-over study with twenty-four students (22.8 ± 3.6 years) was applied. Participants were run through a powernap intervention (PN), a systematic breathing intervention (SB), a systematic breathing plus mental imagery intervention (SB+), and a control condition (CC) with one trial a week over four consecutive weeks. Mental fatigue was induced by completion of the 60-min version of the AX-continuous performance test (AX-CPT). The Short Recovery and Stress Scale (SRSS) and Visual Analog Scales (VAS) were assessed to measure effects on perceptual outcomes. Cognitive performance was measured with a reaction time test of the Vienna Test System (VTS). During all three recovery interventions and CC portable polysomnography was applied. Results showed a significant increase from pre-AX-CPT to pre-MR on fatigue states and recovery-stress states indicating that the induction of mental fatigue was effective. Moreover, results underlined that analysis yielded no significant differences between recovery interventions and the control condition but they revealed significant time effects for VAS, SRSS items, and cognitive performance. However, it could be derived that the application of a rest break with 20 min of mental recovery strategies appears to enhance recovery on a mainly mental and emotional level and to reduce perceived mental fatigue.
Publisher: Elsevier BV
Date: 06-2019
Publisher: CSIRO Publishing
Date: 2009
DOI: 10.1071/WF08133
Abstract: The probability of large-fire (≥1000 ha) ignition days, in the Sydney region, was examined using historical records. Relative influences of the ambient and drought components of the Forest Fire Danger Index (FFDI) on large fire ignition probability were explored using Bayesian logistic regression. The preferred models for two areas (Blue Mountains and Central Coast) were composed of the sum of FFDI (Drought Factor, DF = 1) (ambient component) and DF as predictors. Both drought and ambient weather positively affected the chance of large fire ignitions, with large fires more probable on the Central Coast than in the Blue Mountains. The preferred, additive combination of drought and ambient weather had a marked threshold effect on large-fire ignition and total area burned in both localities. This may be due to a landscape-scale increase in the connectivity of available fuel at high values of the index. Higher probability of large fires on the Central Coast may be due to more subdued terrain or higher population density and ignitions. Climate scenarios for 2050 yielded predictions of a 20–84% increase in potential large-fire ignitions days, using the preferred model.
Publisher: CSIRO Publishing
Date: 2010
DOI: 10.1071/WF08175
Abstract: Wildland fires or bushfires occurring under very severe weather conditions are likely to be destructive to infrastructure. This paper reports an analysis of the statistical relationship between house loss and the fire weather under which it occurred. A dataset was derived from 54 bushfires that occurred in Australia between 1957 and 2009, which resulted in the destruction of 8256 houses. The dataset was statistically compared with relevant local meteorological conditions, and a standardised calculation of the McArthur Forest Fire Danger Index (FFDI) applied. The analysis highlights how house loss statistics in Australia are dominated by a few iconic events that have occurred during very intense fire weather with the majority of losses occurring on days when the FFDI exceeds 100. Virtually all of the house loss has occurred above the 99.5th percentile level in the distribution of daily FFDI for each of the regions considered. Regulatory tools will need to focus on the most appropriate fire weather potential of a local area in order to ensure that infrastructure is adequately designed. In Australia, little house loss has occurred on days where the FFDI did not exceed 50, suggesting that historic building practices may be maintained in regions where this level is not likely to be exceeded.
Publisher: American Meteorological Society
Date: 08-05-2013
DOI: 10.1175/JCLI-D-12-00224.1
Abstract: Analysis of the annual cycle of intensity, extent, and width of the Hadley circulation across a 31-yr period (1979–2009) from all existent reanalyses reveals a good agreement among the datasets. All datasets show that intensity is at a maximum in the winter hemisphere and at a minimum in the summer hemisphere. Maximum and minimum values of meridional extent are reached in the respective autumn and spring hemispheres. While considering the horizontal momentum balance, where a weakening of the Hadley cell (HC) is expected in association with a widening, it is shown here that there is no direct relationship between intensity and extent on a monthly time scale. All reanalyses show an expansion in both hemispheres, most pronounced and statistically significant during summer and autumn at an average rate of expansion of 0.55° decade−1 in each hemisphere. In contrast, intensity trends are inconsistent among the datasets, although there is a tendency toward intensification, particularly in winter and spring. Correlations between the HC and tropical and extratropical large-scale modes of variability suggest interactions where the extent of the HC is influenced by El Niño–Southern Oscillation (ENSO) and the annular modes. The cells tend to shrink (expand) during the warm (cold) phase of ENSO and during the low (high) phase of the annular modes. Intensity appears to be influenced only by ENSO and only during spring for the southern cell and during winter for the northern cell.
Publisher: Routledge
Date: 17-09-2020
Publisher: Springer Science and Business Media LLC
Date: 27-02-2019
DOI: 10.1038/S41598-019-40034-6
Abstract: A strong relationship between the quasi-biennial oscillation (QBO) of equatorial stratospheric winds and the litude of the Madden-Julian oscillation (MJO) during the boreal winter has recently been uncovered using observational data from the mid-1970s to the present. When the QBO is in its easterly phase in the lower stratosphere, it favors stronger MJO activity during boreal winter, while the MJO tends to be weaker during the westerly phase of the QBO. Here we show using reconstructed indices of the MJO and QBO back to 1905 that the relationship between enhanced boreal winter MJO activity and the easterly phase of the QBO has only emerged since the early 1980s. The emergence of this relationship coincides with the recent cooling trend in the equatorial lower stratosphere and the warming trend in the equatorial upper troposphere, which appears to have sensitized MJO convective activity to QBO-induced changes in static stability near the tropopause. Climate change is thus suggested to have played a role in promoting coupling between the MJO and the QBO.
Publisher: IEEE
Date: 2003
Publisher: Elsevier BV
Date: 12-2013
Publisher: Wiley
Date: 11-04-2013
DOI: 10.1002/JOC.3480
Publisher: American Geophysical Union (AGU)
Date: 27-07-2015
DOI: 10.1002/2015JD023130
Publisher: MDPI AG
Date: 31-03-2020
Abstract: Poor knowledge of dispersion model source parameters related to quantities such as the total fine ash mass emission rate, its effective spatial distribution, and particle size distribution makes the provision of quantitative forecasts of volcanic ash a difficult problem. To ameliorate this problem, we make use of satellite-retrieved mass load data from 14 eruption case studies to estimate fine ash mass emission rates and other source parameters by an inverse modelling procedure, which requires multidimensional s ling of several thousand trial simulations with different values of source parameters. We then estimate the dependence of these optimal source parameters on eruption height. We show that using these empirical relationships in a data assimilation procedure leads to substantial improvements to the forecasts of ash mass loads, with the use of empirical relationships between parameters and eruption height having the added advantage of computational efficiency because of dimensional reduction. In addition, the use of empirical relationships, which encode information in satellite retrievals from past case studies, implies that quantitative forecasts can still be issued even when satellite retrievals of mass load are not available in real time due to cloud cover or other reasons, making it especially useful for operations in the tropics where ice and water clouds are ubiquitous.
Publisher: American Meteorological Society
Date: 08-2000
Publisher: CSIRO Publishing
Date: 2007
DOI: 10.1071/WF06125
Abstract: The area burnt each summer in Tasmania is related to coincident (summer) climate variables, especially the total summer rainfall. The relationship with temperature is weaker and largely reflects the relationship between rainfall and temperature. As the El Niño–Southern Oscillation is known to be related to Australian rainfall and simple indices of this phenomenon form the basis of the operational seasonal climate forecast scheme used in Australia, it is not surprising that indices of the El Niño–Southern Oscillation can also, it appears, provide a potentially useful forecast system for Tasmanian bushfire extent. In particular, sea surface temperatures in the Coral Sea during winter are correlated with the area burnt in the following summer. The effect of summer rainfall on the area burnt each year suggests that global warming may not simply lead to increased burning, contrasting with the situation in other parts of the globe. A weak, long-term decline in area burnt appears to be due to a weak increase in summer rainfall.
Publisher: American Geophysical Union (AGU)
Date: 25-01-2021
DOI: 10.1029/2020JD033094
Abstract: A method of assimilating satellite observations in quantitative ensemble forecasting models of airborne volcanic ash is presented in this study. The method employs many trial dispersion model simulations that are generated by both deterministic and random perturbations of the source term and use of an ensemble of numerical weather prediction model fields. An ensemble filter is then applied to the trial simulations, which are either selected or rejected by the filter based on their degree of agreement with observations within a specified time window. The observations may be in the form of quantitative satellite retrieved mass load fields or qualitative ash detection fields, which means that useful results can be obtained even when retrievals are not available in real time provided that the ash boundaries can be identified. The filtering process is repeated several times with different random realizations of the source term to reduce s ling error and minimize filter degeneracy, a phenomenon that plagues all ensemble filter models. The selected members are then propagated forward in time beyond the observational time window to form the forecast ensemble. We show, using several eruption case studies, that forecast ensembles constructed in this way are generally superior in skill to reference forecasts that do not assimilate observations.
Publisher: American Geophysical Union (AGU)
Date: 07-09-2012
DOI: 10.1029/2011JD017033
Publisher: American Meteorological Society
Date: 08-2000
Publisher: Springer Berlin Heidelberg
Date: 22-12-2018
Publisher: American Meteorological Society
Date: 04-2001
Publisher: CSIRO Publishing
Date: 10-05-2023
DOI: 10.1071/ES22033
Abstract: Volcanic eruptions are significant aviation hazards due to the formation of airborne volcanic ash clouds. Further, deep eruptions that reach the upper troposphere and lower stratosphere may have significant weather and climate impacts. A key variable for both dispersion model forecasting for aviation hazards and understanding climate impacts is the volcanic plume height. This work presents a method to quickly and reliably estimate the maximum plume heights of volcanic eruptions that interact with the tropical tropopause layer in the tropical western Pacific region. The method uses infrared (11.2 μm) data from Himawari-8 to identify ‘stratospheric warm spots’ in optically thick portions of the eruption cloud top by searching for reversals in the local-brightness temperature gradient. The brightness temperature of these warm spots is converted to height using seasonal stratospheric reference temperature profiles derived from 20 years of radiosonde data from 17 stations spread throughout the western Pacific. An approach for estimating the height of cold ‘overshooting tops’ is also adopted. Based on the radiosonde data, estimates of the uncertainty in the plume height depend on the height and range within 0.5–5.0 km. A case study of the 19 December 2021 eruption of Hunga Tonga-Hunga Ha’apai demonstrates the technique. The heights are robustly determined with this simple technique and compare well with height estimates of eruptions in the literature that use more complex satellite techniques.
Publisher: American Geophysical Union (AGU)
Date: 10-08-2017
DOI: 10.1002/2017JD026679
Publisher: American Meteorological Society
Date: 2004
No related grants have been discovered for Christopher Lucas.