ORCID Profile
0000-0002-2511-7649
Current Organisation
University of Leeds
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Publisher: American Meteorological Society
Date: 06-2020
Abstract: Weather and climate variations on subseasonal to decadal time scales can have enormous social, economic, and environmental impacts, making skillful predictions on these time scales a valuable tool for decision-makers. As such, there is a growing interest in the scientific, operational, and applications communities in developing forecasts to improve our foreknowledge of extreme events. On subseasonal to seasonal (S2S) time scales, these include high-impact meteorological events such as tropical cyclones, extratropical storms, floods, droughts, and heat and cold waves. On seasonal to decadal (S2D) time scales, while the focus broadly remains similar (e.g., on precipitation, surface and upper-ocean temperatures, and their effects on the probabilities of high-impact meteorological events), understanding the roles of internal variability and externally forced variability such as anthropogenic warming in forecasts also becomes important. The S2S and S2D communities share common scientific and technical challenges. These include forecast initialization and ensemble generation initialization shock and drift understanding the onset of model systematic errors bias correction, calibration, and forecast quality assessment model resolution atmosphere–ocean coupling sources and expectations for predictability and linking research, operational forecasting, and end-user needs. In September 2018 a coordinated pair of international conferences, framed by the above challenges, was organized jointly by the World Climate Research Programme (WCRP) and the World Weather Research Programme (WWRP). These conferences surveyed the state of S2S and S2D prediction, ongoing research, and future needs, providing an ideal basis for synthesizing current and emerging developments in these areas that promise to enhance future operational services. This article provides such a synthesis.
Publisher: American Meteorological Society
Date: 07-2023
Abstract: The Maritime Continent experiences some of the world’s most severe convective rainfall, with an intense diurnal cycle. A key feature is offshore propagation of convection overnight, having peaked over land during the evening. Existing hypotheses suggest this propagation is due to the nocturnal land breeze and environmental wind causing low-level convergence and/or gravity waves triggering convection as they propagate. We use a convection-permitting configuration of the Met Office Unified Model over Sumatra to test these hypotheses, verifying against observations from the Japanese Years of the Maritime Continent field c aign. In selected case studies there is an organized squall line propagating with the land-breeze density current, possibly reinforced by convective cold pools, at ∼3 m s −1 to around 150–300 km offshore. Propagation at these speeds is also seen in a composite mean diurnal cycle. The density current is verified by observations, with offshore low-level wind and virtual potential temperature showing a rapid decrease consistent with a density current front, accompanied by rainfall. Gravity waves are identified in the model with a typical phase speed of 16 m s −1 . They trigger isolated cells of convection, usually farther offshore and with much weaker precipitation than the squall line. Occasionally, the isolated convection may deepen and the rainfall intensify, if the gravity wave interacts with a substantial preexisting perturbation such as shallow cloud. The localized convection triggered by gravity waves does not generally propagate at the wave’s own speed, but this phenomenon may appear as propagation along a wave trajectory in a composite that averages over many days of the diurnal cycle. The intense convection experienced by the Maritime Continent causes high-impact weather in the form of heavy precipitation, which can trigger floods and landslides, endangering human life and infrastructure. The geography of the region, with many islands with complex coastlines and orography, means that the spatial and temporal distributions of convection are difficult to predict. This presents challenges for operational forecasters in the region and introduces biases in weather and climate models, which may propagate globally. A key feature of the convection is its diurnal cycle and associated propagation offshore overnight from the islands. Although this phenomenon has been often investigated, there is no strong consensus in the literature on the mechanism or combination of mechanisms responsible. Improving our knowledge of these mechanisms and how they are represented in a convection-permitting model will assist forecasters in understanding how and when the propagation of intense convective storms occurs, and allow model developers to improve biases in numerical weather prediction and climate models.
Publisher: Wiley
Date: 02-05-2013
DOI: 10.1002/QJ.2161
Publisher: Wiley
Date: 24-06-2022
DOI: 10.1002/WEA.4246
Publisher: American Meteorological Society
Date: 08-2018
Abstract: The Bay of Bengal (BoB) plays a fundamental role in controlling the weather systems that make up the South Asian summer monsoon system. In particular, the southern BoB has cooler sea surface temperatures (SST) that influence ocean–atmosphere interaction and impact the monsoon. Compared to the southeastern BoB, the southwestern BoB is cooler, more saline, receives much less rain, and is influenced by the summer monsoon current (SMC). To examine the impact of these features on the monsoon, the BoB Boundary Layer Experiment (BoBBLE) was jointly undertaken by India and the United Kingdom during June–July 2016. Physical and biogeochemical observations were made using a conductivity–temperature–depth (CTD) profiler, five ocean gliders, an Oceanscience Underway CTD (uCTD), a vertical microstructure profiler (VMP), two acoustic Doppler current profilers (ADCPs), Argo floats, drifting buoys, meteorological sensors, and upper-air radiosonde balloons. The observations were made along a zonal section at 8°N between 85.3° and 89°E with a 10-day time series at 8°N, 89°E. This paper presents the new observed features of the southern BoB from the BoBBLE field program, supported by satellite data. Key results from the BoBBLE field c aign show the Sri Lanka dome and the SMC in different stages of their seasonal evolution and two freshening events during which salinity decreased in the upper layer, leading to the formation of thick barrier layers. BoBBLE observations were taken during a suppressed phase of the intraseasonal oscillation they captured in detail the warming of the ocean mixed layer and the preconditioning of the atmosphere to convection.
Publisher: American Geophysical Union (AGU)
Date: 23-06-2023
DOI: 10.1029/2022JD038196
Abstract: Cold surges are synoptic weather systems that occur over the Maritime Continent during the boreal winter. They are characterized by the strengthening of prevailing low‐level northerly to north‐easterly winds, temperature falls of a few degrees over several days, and in some cases, heavy prolonged rainfall and flooding. We investigate the synoptic structure and development of cold surges through composites of dry, moderate and wet surges. Each surge category is defined by the distribution of precipitation averaged within a specified domain over the equatorial South China Sea. Dry surges are characterized by cross‐equatorial flow and positive mean sea‐level pressure anomalies that reach the Southern Hemisphere, and enhanced descent or weaker ascent. Wet surges are characterized by limited cross‐equatorial flow, low‐pressure anomalies over the equator, and enhanced moisture and ascent. The local diurnal precipitation patterns are consistent with the convection being controlled by the mid‐tropospheric buoyancy of an idealized entraining plume. This buoyancy diagnostic suggests that wet surges are characterized by a moister free troposphere because this reduces the effect of entrainment and allows deep convection to develop. On the large scale, the environment in which wet or dry cold surges form is closely related to the Madden‐Julian Oscillation (MJO) phase. Dry and moderate surges occur predominantly in the suppressed phases of the MJO, while wet surges often herald the eastward propagation of the MJO convective envelope. Finally, convectively active phases in the Australian monsoon are more likely following dry cold surges, and are sometimes associated with the MJO.
Publisher: Springer Science and Business Media LLC
Date: 15-02-2015
Publisher: American Meteorological Society
Date: 13-09-2021
Abstract: The canonical view of the Maritime Continent (MC) diurnal cycle is deep convection occurring over land during the afternoon and evening, tending to propagate offshore overnight. However, there is considerable day-to-day variability in the convection, and the mechanism of the offshore propagation is not well understood. We test the hypothesis that large-scale drivers such as ENSO, the MJO and equatorial waves, through their modification of the local circulation, can modify the direction or strength of the propagation, or prevent the deep convection from triggering in the first place. Taking a local-to-large scale approach we use in situ observations, satellite data and reanalyses for five MC coastal regions, and show that the occurrence of the diurnal convection and its offshore propagation is closely tied to coastal wind regimes we define using the k -means cluster algorithm. Strong prevailing onshore winds are associated with a suppressed diurnal cycle of precipitation while prevailing offshore winds are associated with an active diurnal cycle, offshore propagation of convection and a greater risk of extreme rainfall. ENSO, the MJO, equatorial Rossby waves and westward mixed Rossby-gravity waves have varying levels of control over which coastal wind regime occurs, and therefore on precipitation, depending on the MC coastline in question. The large-scale drivers associated with dry and wet regimes are summarised for each location as a reference for forecasters.
Publisher: American Geophysical Union (AGU)
Date: 10-10-2013
DOI: 10.1002/JGRD.50865
Publisher: American Meteorological Society
Date: 09-2018
Abstract: In the Bay of Bengal (BoB), surface heat fluxes play a key role in monsoon dynamics and prediction. The accurate representation of large-scale surface fluxes is dependent on the quality of gridded reanalysis products. Meteorological and surface flux variables from five reanalysis products are compared and evaluated against in situ data from the Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) in the BoB. The reanalysis products: ERA-Interim (ERA-I), TropFlux, MERRA-2, JRA-55, and CFSR are assessed for their characterization of air–sea fluxes during the southwest monsoon season [June–September (JJAS)]. ERA-I captured radiative fluxes best while TropFlux captured turbulent and net heat fluxes Q net best, and both products outperformed JRA-55, MERRA-2, and CFSR, showing highest correlations and smallest biases when compared to the in situ data. In all five products, the largest errors were in shortwave radiation Q SW and latent heat flux Q LH , with nonnegligible biases up to approximately 75 W m −2 . The Q SW and Q LH are the largest drivers of the observed Q net variability, thus highlighting the importance of the results from the buoy comparison. There are also spatially coherent differences in the mean basinwide fields of surface flux variables from the reanalysis products, indicating that the biases at the buoy position are not localized. Biases of this magnitude have severe implications on reanalysis products’ ability to capture the variability of monsoon processes. Hence, the representation of intraseasonal variability was investigated through the boreal summer intraseasonal oscillation, and we found that TropFlux and ERA-I perform best at capturing intraseasonal climate variability during the southwest monsoon season.
Publisher: Wiley
Date: 05-12-2022
Publisher: Copernicus GmbH
Date: 19-09-2023
DOI: 10.5194/GMD-2023-165
Publisher: American Meteorological Society
Date: 23-03-2016
Abstract: State-of-the-art regional climate model simulations that are able to resolve key mesoscale circulations are used, for the first time, to understand the interaction between the large-scale convective environment of the MJO and processes governing the strong diurnal cycle over the islands of the Maritime Continent (MC). Convection is sustained in the late afternoon just inland of the coasts because of sea breeze convergence. Previous work has shown that the variability in MC rainfall associated with the MJO is manifested in changes to this diurnal cycle land-based rainfall peaks before the active convective envelope of the MJO reaches the MC, whereas oceanic rainfall rates peak while the active envelope resides over the region. The model simulations show that the main controls on oceanic MC rainfall in the early active MJO phases are the large-scale environment and atmospheric stability, followed by high oceanic latent heat flux forced by high near-surface winds in the later active MJO phases. Over land, rainfall peaks before the main convective envelope arrives (in agreement with observations), even though the large-scale convective environment is only moderately favorable for convection. The causes of this early rainfall peak are strong convective triggers from land–sea breeze circulations that result from high surface insolation and surface heating. During the peak MJO phases cloud cover increases and surface insolation decreases, which weakens the strength of the mesoscale circulations and reduces land-based rainfall, even though the large-scale environment remains favorable for convection at this time. Hence, scale interactions are an essential part of the MJO transition across the MC.
Location: United Kingdom of Great Britain and Northern Ireland
Location: United Kingdom of Great Britain and Northern Ireland
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 Simon Peatman.