ORCID Profile
0000-0003-0108-1220
Current Organisations
University of Reading
,
University of Oxford
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Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-7411
Abstract: & & & & This work considers the sub-seasonal predictability of two sets of weather regimes for South East Asia: a two-tiered assignment, that first considers large-scale patterns and then assigns synoptic-scale regimes, and a flat classification, which only considers the synoptic scale. In the two-tiered approach, the tier 1 large-scale regimes, which capture ENSO and seasonal variations, are each partitioned into South East Asia regional clusters that capture synoptic variability.& & & & & & / & & & & & The sub-seasonal predictability of both the standard and tiered regimes is assessed using UKMO GloSea5 hindcasts and forecasts for lead times of up to 5 weeks. We find that the GloSea5 system presents an accurate representation of the regimes& #8217 climatology and a good level of skill for their assignment. Nonetheless, the predictability depends on the specific regimes and some significant forecast drifts are also identified. Additionally, the predictive skill of high impact precipitation events obtained statistically from the prediction of the regimes is assessed and compared with the probabilistic precipitation forecasts of the GloSea5 ensemble.& & & & & & & / & & & & & A description of the regime classification methodology and their connections to seasonal and synoptic phenomena will be discussed in a separate presentation, titled & #8220 Weather regimes in South East Asia: connections with synoptic phenomena and high impact weather& #8221 and presented by Emma Howard.& & & & / &
Publisher: American Meteorological Society
Date: 06-11-2019
Abstract: Projected rainfall decline in southern Africa is likely to be highly sensitive to subtleties in the local atmospheric circulation. In an effort to understand the regional circulation complexities, a novel algorithm is developed to identify the Congo air boundary (CAB) in ERA-5, a high-resolution reanalysis dataset. The CAB, a forgotten feature of the circulation, is defined in the austral spring and early summer, using surface humidity gradients and near-surface wind convergence lines, and it is found to be an indicator of the location of the southern edge of the African rain belt. A related convergence-line and dryline feature, described in this paper as the Kalahari discontinuity (KD), is also identified. It is established that either a dryline CAB or KD is present in southern Africa for over 95% of days between August and December, with arc lengths typically exceeding 10°. The seasonal and diurnal cycles of the CAB and the KD are presented, and their prevalence in station observational data is confirmed. The interannual variability of the CAB latitude and detection frequency is found to explain at least 55% of interannual spring rainfall variability in southern Africa between 15° to 25°S. Links are established with the Angola and Kalahari heat lows and tropical temperate trough events.
Publisher: American Meteorological Society
Date: 09-2018
Abstract: The Angola low is a key feature of the southern Africa wet season atmosphere that influences precipitation across the continent. This paper uses ERA-Interim to show that the synoptic expression of the Angola low is a combination of dry heat lows and moist tropical low pressure systems. The Angola heat low and Angola tropical low composites are contrasted against similar lows observed in other continental tropical regions and found to be broadly comparable. The implications that the distinction between dry and moist events has for the interannual relationship among the Angola low, precipitation, and ENSO are examined. The tropical lows exhibit unusual semistationary behavior by lingering in the Angola region rather than traveling offshore. This behavior is proposed to be caused by an integrated sea breeze–anabatic wind that enhances (inhibits) cyclonic vorticity stretching and convection inland (near the coast). The combined effect of the heat lows and the anchored tropical lows creates the Angola low in the climatological average. By elucidating the mechanisms of the Angola low, this research improves the foundation of process-based evaluation of southern Africa present and future climate in CMIP and AMIP models.
Publisher: Wiley
Date: 24-12-2021
DOI: 10.1002/QJ.4227
Abstract: Two sets of weather patterns describing variability in 850 hPa winds in Southeast Asia are presented and compared. Patterns are calculated using EOF/ k ‐means clustering with and without imposing a separation between planetary‐scale and regional‐scale circulation features. The former are labelled as tiered patterns while the latter are referred to as flat. The ability of the patterns to distinguish between known modes of tropical circulation variability is examined. This includes climate modes such as the seasonal monsoons, the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) as well as sub‐seasonal modes including cold surges, phases of the MJO and Boreal summer intraseasonal oscillation (BSISO), tropical cyclones, Borneo vortices and equatorial waves. All these modes are well captured by the weather patterns except for the equatorial waves and the IOD. The tiered patterns are shown to better describe large‐scale modes of variability, while the flat patterns better describe the synoptic variability. Both sets of weather patterns are then used to study the likelihood of heavy precipitation depending on synoptic circulation by considering the regime‐conditioned probability of high‐percentile precipitation using the satellite‐derived Global Precipitation Measurement (GPM) dataset. It is shown that the pattern centroids explain up to 10% of the seasonally anomalous precipitation over land, and that a perfect weather pattern forecast would outperform a perfect MJO forecast. These weather patterns show promising potential in extending the useful forecast range for the risk of heavy precipitation, dependent on their forecastability.
Publisher: Copernicus GmbH
Date: 04-03-2021
DOI: 10.5194/EGUSPHERE-EGU21-7472
Abstract: & & & & & & & & / & & & & & A tiered set of weather regimes describing variability in 850& hPa& winds in South East Asia (SEA) is presented and compared to a corresponding non-tiered set of weather regimes. The tiered regimes are calculated in two parts: the first tier computed by applying EOF/K-means clustering on a planetary scale domain which partitioning seasonal variation and ENSO, and the second tier obtained by EOF/K-means clustering on a smaller SE Asia regional domain, partitioning the synoptic variability within each of the& first tier& regimes. This identifies synoptic weather phenomena with multi-day persistence. In contrast, the un-tiered (& #8220 flat& #8221 ) clustering approach uses a standard EOF/K-means classification in the regional domain without conditional dependence on large-scale, with the number of regimes set to match the tiered regimes.& & & & & / & & & & & These regimes are used to study the likelihood of extreme precipitation depending on synoptic circulation. We consider the conditional probability depending on regime type of synoptic weather events including cold surges, phases of the MJO and BSISO, tropical cyclones, Borneo Vortices and equatorial waves. We then study the regime-conditioned probability of high percentile TRMM precipitation. We find that a perfect regime forecast would have greater skill than the GloSEA5 precipitation forecast for lead times longer than approximately one week. The tiered regimes distinguish a greater fraction of considered modes of variability, while the flat regimes better distinguish the precipitation variability.& & & & & / & & & & & The predictability of these regimes will be discussed in a separate presentation, titled & #8220 Weather regimes in South East Asia: Sub-seasonal predictability of the regimes and the associated high impact weather& #8221 and presented by Paula Gonzalez.& & & & / &
Publisher: Wiley
Date: 11-12-2022
DOI: 10.1002/QJ.4378
Abstract: While skilful forecasts of heavy rainfall are highly desirable for weather warnings and mitigating impacts, forecasting such events is notoriously difficult, even with the most advanced numerical weather prediction models, due to the strong dependence on convective‐scale processes. The large‐scale circulation, on the other hand, is typically more predictable. Weather patterns (WPs) are a set of circulation types obtained statistically that can be used to characterize regional weather and harness the predictability of the large‐scale circulation. In this work we produce pattern‐conditioned probabilistic rainfall forecasts by projecting the horizontal winds from the Met Office GloSea5 prediction system on to WPs and then using the observed relationship between each WP and rainfall estimated by satellite. The WPs are derived following a novel two‐tier clustering technique: the WPs in the first tier represent planetary‐scale variability, such as El Niño–Southern Oscillation (ENSO), while the WPs in the second tier capture synoptic‐scale variability. We investigate WP predictability as well as the improvement in skill of subseasonal rainfall forecasts gained by this technique. GloSea5 predicts the WP occurrence with skill extending beyond lead day 10. The pattern‐conditioned rainfall forecasts were evaluated against climatological forecasts and model‐simulated rainfall hindcasts. We show that the pattern‐conditioned forecasts are skilful and outperform the model‐simulated rainfall hindcasts for lead times extending to days 10–20, depending on the specific exceedance criteria and region. Spatial aggregation leads to increased levels of skill, but not to a significant extension of the skilful prediction horizon. These results constitute a fundamental step for the development of subseasonal prediction systems for Southeast Asia.
Publisher: American Meteorological Society
Date: 2020
Publisher: American Meteorological Society
Date: 2020
Publisher: Springer Science and Business Media LLC
Date: 12-03-2020
Publisher: American Geophysical Union (AGU)
Date: 05-11-2019
DOI: 10.1029/2019JD030803
Publisher: American Meteorological Society
Date: 2015
Abstract: An overturning circulation, driven by prescribed buoyancy forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and buoyancy forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing. The zonal momentum budget integrated over each layer is considered in the buoyancy-forced, wind-forced, and combined forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the buoyancy-forced case, the forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom friction to act as the initial sources of eastward momentum, with bottom friction acting over a longer time frame. In the wind-forced and combined forcing cases, the surface wind stress dominates the initial momentum budget, and the time to reach equilibration is shorter in the combined forcing simulation. These results imply that future changes in the rate of formation of Antarctic Bottom Water may alter the volume transport of the Antarctic Circumpolar Current.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Emma Howard.