11 results on '"Pohl, Benjamin"'
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2. Intraseasonal descriptors and extremes in South African rainfall. Part I: Summer climatology and statistical characteristics.
- Author
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Ullah, Asmat, Pohl, Benjamin, Pergaud, Julien, Dieppois, Bastien, and Rouault, Mathieu
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CLIMATOLOGY , *SUMMER , *WATER supply , *RAIN gauges - Abstract
Rainfall extremes are of major and increasing importance in semi‐arid countries and their variability has strong implications for water resource and climate impacts on the local societies and environment. Here, we examine intraseasonal descriptors (ISDs) and wet extremes in austral summer rainfall (November–February) over South Africa (SA). Using daily observations from 225 rain gauges and ERA5 reanalysis between 1979 and 2015, we propose a novel typology of wet extreme events based on their spatial fraction, thus differentiating large‐ and small‐scale extremes. Long‐term variability of both types of extreme rainfall events is then extensively discussed in the context of ISDs. The results demonstrate that using 7% of spatial fraction simultaneously exceeding the local threshold of the 90th percentile produces remarkable results in characterizing rainfall extremes into large‐ and small‐scale extremes. Austral summer total rainfall is found to be primarily shaped by large‐scale extremes which constitute more than half of the rainfall amount under observation, and nearly half in ERA5. Observation (ERA5) shows an average of 8 ± 5 (20 ± 7) days per season associated with large‐scale extremes, which are comprised in 5 ± 3 (10 ± 3) spells with an average persistence of at least 2 days. Overall, we find a strong dependence of total rainfall on the number of wet days and wet spells that are associated with large‐scale extremes. We also find that large‐ and small‐scale extremes are well‐organized and spatially coherent in nature yet extreme conditions during small‐scale events are found sporadic over the region, contrasting with large‐scale events for which extreme conditions are found over a larger and coherent region. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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3. Insights into the Summer Diurnal Cycle over Eastern South Africa.
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Koseki, Shunya, Pohl, Benjamin, Bhatt, Bhuwan Chandra, Keenlyside, Noel, and Nkwinkwa Njouodo, Arielle Stela
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SUMMER , *CIRCADIAN rhythms , *DIURNAL variations in meteorology , *HYDROMETEOROLOGY , *CLIMATOLOGY , *RAINFALL - Abstract
Adopting a state-of-the-art numerical model system, we investigate how the diurnal variations in precipitation and local breeze systems are characterized by lower-boundary conditions related to the Drakensberg highland and warm SST associated with the Agulhas Current. A control simulation can simulate the hydrometeorological climates in the region realistically, but the terrestrial rainfall is overestimated. During daytime, the precipitation is confined to the Drakensberg highland, and there is an onshore local breeze, while during midnight to morning, the rainfall is confined to the Agulhas Current, and the breeze is offshore. These variations are captured by the numerical simulation, although the timing of maximum rainfall is early over the land and delayed over the ocean. The sensitivity experiment in which the Drakensberg is absent shows a drastic modification in the diurnal variations over land and ocean. The terrestrial precipitation is largely decreased around the Drakensberg and is largest along the coast during daytime. The nocturnal marine precipitation along the Agulhas Current is also reduced. Although the daily residual breeze is still pronounced even without the Drakensberg, wind speed is weakened. We attribute this to the reduction of precipitation. In another sensitivity experiment with smoothened warm SST due to the Agulhas Current, the amplitudes of diurnal variations are not modified remarkably, but the coastal rainfall is diminished to some extent due to less evaporation along the Agulhas Current. This study concludes that the Drakensberg plays a crucial role for the diurnal cycle, and the impact of the Agulhas Current is limited on the diurnal cycle of the coastal precipitation in this region. [ABSTRACT FROM AUTHOR]
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- 2018
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4. An original way to evaluate daily rainfall variability simulated by a regional climate model: the case of South African austral summer rainfall.
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Crétat, Julien, Pohl, Benjamin, Chateau Smith, Carmela, Vigaud, Nicolas, and Richard, Yves
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RAINFALL , *GEOGRAPHIC information systems , *EVAPORATION (Meteorology) , *CLIMATOLOGY , *METEOROLOGICAL precipitation - Abstract
ABSTRACT We discuss the value of a clustering approach as a tool for evaluating daily rainfall output from climate models. Ascendant hierarchical clustering is used to evaluate how well South African recurrent daily rainfall patterns are simulated during the austral summer (December to February 1970-1971 to 1998-1999). A set of 35-km regional climate simulations, run with the WRF model and driven by the ERA40 reanalysis, is chosen as a case study. Six recurrent patterns are identified and compared to the observed clusters obtained by applying the same methodology to 5352 daily rain gauge records. Two of the WRF clusters describe either a persistent and widespread dryness (65% of the days) or patterns similar to the seasonal mean rainfall gradient (13% of the days). The four remaining WRF clusters (∼20% of the days) are wetter; they describe the weakening, conservation or strengthening of the average rainfall gradient. The WRF cluster rainfall patterns and their associated circulation match the observed clusters rather well, but their frequency of occurrence is greatly overestimated by WRF during dry events, and underestimated for near-normal rainfall conditions. The weak model biases found at the seasonal timescale conceal strongly biased intraseasonal rainfall variability. The WRF-simulated rainfall patterns are then temporally or spatially projected on to the observed clusters. Spatial projection proves to be the more useful of these two approaches in quantifying model skill by assessing both the temporal co-variability between WRF and observations, and the rainfall biases of the model with or without temporal dephasing. The WRF model simulates transient rainfall activity partially out of phase with observations, which induces large rainfall biases when temporal dephasing is not removed. Rainfall biases are significantly reduced, however, when temporal dephasing is removed. The clustering approach therefore proves its efficiency to highlight climate model strengths and deficiencies. [ABSTRACT FROM AUTHOR]
- Published
- 2015
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5. Regionalizing Rainfall at Very High Resolution over La Réunion Island Using a Regional Climate Model.
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Morel, Béatrice, Pohl, Benjamin, Richard, Yves, Bois, Benjamin, and Bessafi, Miloud
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RAINFALL , *CLIMATOLOGY , *CLIMATE change mathematical models , *KRIGING , *TROPICAL cyclones , *ISLANDS - Abstract
Regional climate models (RCMs) should be evaluated with respect to their ability to downscale large-scale climate information to the local scales, which are sometimes strongly modulated by surface conditions. This is the case for La Réunion (southwest Indian Ocean) because of its island context and its complex topography. Large-scale atmospheric configurations such as tropical cyclones (TCs) may have an amplifying effect on local rainfall patterns that only a very high-resolution RCM, forced by the large scales and resolving finescale processes, may simulate properly. This paper documents the capability of the Weather Research and Forecasting Model (WRF) RCM to regionalize rainfall variability at very high resolution (680 m) over La Réunion island for daily to seasonal time scales and year-to-year differences. Two contrasted wet seasons (November-April) are selected: 2000-01 (abnormally dry) and 2004-05 (abnormally wet). WRF rainfall is compared to a dense network of rain gauge records interpolated onto the WRF grid through the regression-kriging (RK) technique. RK avoids the point-to-grid comparison issue, but produces imperfect estimates due to sampling, so its quality also needs to be tested. Seasonal rainfall amounts and contrasts produced by WRF are fairly realistic. At intraseasonal and daily time scales, differences to RK are more sizable. These differences are not easy to interpret in sectors where the rain gauge network is less dense and the quality of RK more uncertain, as over the eastern slopes of Piton de la Fournaise volcano where WRF seems to simulate more realistic rainfall than RK. Finally, the heavy rainfall associated with TC Ando on 6 January 2001, is documented. WRF shows weak disagreements with RK, indicating its capability to regionalize rainfall during extreme events. [ABSTRACT FROM AUTHOR]
- Published
- 2014
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6. Understanding the West African monsoon variability and its remote effects: an illustration of the grid point nudging methodology.
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Bielli, Soline, Douville, Hervé, and Pohl, Benjamin
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CLIMATOLOGY ,METHODOLOGY ,MONSOONS ,RAINFALL - Abstract
General circulation models still show deficiencies in simulating the basic features of the West African Monsoon at intraseasonal, seasonal and interannual timescales. It is however, difficult to disentangle the remote versus regional factors that contribute to such deficiencies, and to diagnose their possible consequences for the simulation of the global atmospheric variability. The aim of the present study is to address these questions using the so-called grid point nudging technique, where prognostic atmospheric fields are relaxed either inside or outside the West African Monsoon region toward the ERA40 reanalysis. This regional or quasi-global nudging is tested in ensembles of boreal summer simulations. The impact is evaluated first on the model climatology, then on intraseasonal timescales with an emphasis on North Atlantic/Europe weather regimes, and finally on interannual timescales. Results show that systematic biases in the model climatology over West Africa are mostly of regional origin and have a limited impact outside the domain. A clear impact is found however on the eddy component of the extratropical circulation, in particular over the North Atlantic/European sector. At intraseasonal timescale, the main regional biases also resist to the quasi-global nudging though their magnitude is reduced. Conversely, nudging the model over West Africa exerts a strong impact on the frequency of the two North Atlantic weather regimes that favor the occurrence of heat waves over Europe. Significant impacts are also found at interannual timescale. Not surprisingly, the quasi-global nudging allows the model to capture the variability of large-scale dynamical monsoon indices, but exerts a weaker control on rainfall variability suggesting the additional contribution of regional processes. Conversely, nudging the model toward West Africa suppresses the spurious ENSO teleconnection that is simulated over Europe in the control experiment, thereby emphasizing the relevance of a realistic West African monsoon simulation for seasonal prediction in the extratropics. Further experiments will be devoted to case studies aiming at a better understanding of regional processes governing the monsoon variability and of the possible monsoon teleconnections, especially over Europe. [ABSTRACT FROM AUTHOR]
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- 2010
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7. Implication of the Madden–Julian Oscillation in the 40-Day Variability of the West African Monsoon.
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Pohl, Benjamin, Janicot, Serge, Fontaine, Bernard, and Marteau, Romain
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MONSOONS , *CLIMATE change , *CLIMATOLOGY , *RAINFALL , *ATMOSPHERIC circulation - Abstract
Madden–Julian oscillations (MJOs) are extracted over the Indo-Pacific basin using a local mode analysis. The convective perturbations are then projected over a larger domain to evaluate their remote consequences over the West African monsoon (WAM) intraseasonal variability. Rather weak (4–6 W m-2) convective fluctuations occurring in phase with those over the southern Indian basin are found over Africa, confirming the results of Matthews. In reverse, 40-day fluctuations in the WAM, similarly detected and projected over a widened area, demonstrate that a large majority of these events are embedded in the larger-scale patterns of the MJO. The regional amplitude of intraseasonal perturbations of the West African convection is not statistically associated with the amplitude of the MJO over the Indian basin but is instead closely related to background vertical velocity anomalies over Africa, possibly embedded in changes in the regional Walker-type circulation. Subsiding motion over Africa is recorded during the most energetic convective perturbations in the WAM. Composites analyses over the MJO life cycle, as depicted by the real-time daily indices developed by Wheeler and Hendon, show that positive outgoing longwave radiation (OLR) anomalies during the dry phase are of larger amplitude and spatially more coherent than negative anomalies during the wet phase, especially over the Sahel region. Over West Africa, the phase of suppressed convection is thus of greater importance for the region than the phase of enhanced convection. Rain gauge records fully confirm these results. The MJO appears to be significantly involved in the occurrences of dry spells during the monsoon over the Sahel, whereas large-scale convective clusters are only restricted to the equatorial latitudes and thus affect the Guinean belt, which experiences its short dry season at this time of the year. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
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8. Observed Changes in the Lifetime and Amplitude of the Madden–Julian Oscillation Associated with Interannual ENSO Sea Surface Temperature Anomalies.
- Author
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Pohl, Benjamin and Matthews, Adrian J.
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OCEANOGRAPHIC research , *LONG-range weather forecasting , *SEASONAL variations in biogeochemical cycles , *WATER temperature , *CLIMATOLOGY - Abstract
The Madden–Julian oscillation (MJO) is analyzed using the reanalysis zonal wind– and satellite outgoing longwave radiation–based indices of Wheeler and Hendon for the 1974–2005 period. The average lifetime of the MJO events varies with season (36 days for events whose central date occurs in December, and 48 days for events in September). The lifetime of the MJO in the equinoctial seasons (March–May and October–December) is also dependent on the state of El Niño–Southern Oscillation (ENSO). During October–December it is only 32 days under El Niño conditions, increasing to 48 days under La Niña conditions, with similar values in northern spring. This difference is due to faster eastward propagation of the MJO convective anomalies through the Maritime Continent and western Pacific during El Niño, consistent with theoretical arguments concerning equatorial wave speeds. The analysis is extended back to 1950 by using an alternative definition of the MJO based on just the zonal wind component of the Wheeler and Hendon indices. A rupture in the amplitude of the MJO is found in 1975, which is at the same time as the well-known rupture in the ENSO time series that has been associated with the Pacific decadal oscillation. The mean amplitude of the MJO is 16% larger in the postrupture (1976–2005) compared to the prerupture (1950–75) period. Before the 1975 rupture, the amplitude of the MJO is maximum (minimum) under El Niño (La Niña) conditions during northern winter, and minimum (maximum) under El Niño (La Niña) conditions during northern summer. After the rupture, this relationship disappears. When the MJO–ENSO relationship is analyzed using all-year-round data, or a shorter dataset (as in some previous studies), no relationship is found. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
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9. Correction to: Simulated effects of land immersion on regional arid climate: a case study of the pre-Saharan playa of Chott el-Jerid (South of Tunisia).
- Author
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Fathalli, Bilel, Castel, Thierry, and Pohl, Benjamin
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CASE studies ,CLIMATOLOGY - Abstract
In the original article, the last panel of Fig. 10 should be labeled "h". Also some data and Table 2 caption were unfortunately incorrect. [ABSTRACT FROM AUTHOR]
- Published
- 2020
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10. CORRIGENDUM.
- Author
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Macron, Clémence, Pohl, Benjamin, Richard, Yves, and Bessafi, Miloud
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CLIMATOLOGY ,TROPICAL climate - Abstract
A correction to the article "How do tropical temperate troughs form and develop over southern Africa?" that was published in a 2014 issue is presented.
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- 2014
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11. Influence of the Madden-Julian Oscillation on Southern African summer rainfall
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Benjamin Pohl, Nicolas Fauchereau, Yves Richard, Centre de Recherches de Climatologie ( CRC ), Université de Bourgogne ( UB ) -Centre National de la Recherche Scientifique ( CNRS ), Department of Oceanography, University of Cape Town, Pohl, Benjamin, Centre de Recherches de Climatologie (CRC), Université de Bourgogne (UB)-Centre National de la Recherche Scientifique (CNRS), Department of Oceanography [Cape Town], and Faculty of Science
- Subjects
Dynamical climatology ,Atmospheric Science ,010504 meteorology & atmospheric sciences ,Flux ,02 engineering and technology ,Forcing (mathematics) ,subtropical zone ,Atmospheric sciences ,01 natural sciences ,tropical zone ,Déclenchement ,[SDU.STU.CL] Sciences of the Universe [physics]/Earth Sciences/Climatology ,020701 environmental engineering ,Atmospheric convection ,Atmospheric dynamics ,Mécanisme ,Convection atmosphérique ,Madden–Julian oscillation ,Hydroclimatology ,atmospheric precipitation ,[ SDE.MCG ] Environmental Sciences/Global Changes ,[SDU.STU.CL]Sciences of the Universe [physics]/Earth Sciences/Climatology ,Climatology ,Climatologie dynamique ,Outgoing longwave radiation ,[ SDU.STU.CL ] Sciences of the Universe [physics]/Earth Sciences/Climatology ,Southern Africa ,Geology ,Triggering ,Summer ,[SDE.MCG]Environmental Sciences/Global Changes ,rainfall ,0207 environmental engineering ,mechanism ,Subtropics ,Latitude ,Variation interannuelle ,[SDE.MCG.CG]Environmental Sciences/Global Changes/domain_sde.mcg.cg ,Composite analysis ,[ SDE.MCG.CG ] Environmental Sciences/Global Changes/domain_sde.mcg.cg ,Madden Julian oscillation ,Climate variability ,0105 earth and related environmental sciences ,Interannual variation ,Tropics ,[SDE.MCG] Environmental Sciences/Global Changes ,13. Climate action ,Intraseasonal variation ,Africa - Abstract
Rain-causing mechanisms over Southern Africa (south of 15˚S) involve both tropical and temperate dynamics. Most studies focused on the synoptical timescale, while the intraseasonal (20-120 days) variability has more been neglected to date. This study aims at determining whether the dominant mode of intraseasonal variability in the Tropics, namely the Madden-Julian Oscillation (MJO), has a significant impact on Southern African rainfall and associated atmospheric dynamics. The examination of outgoing longwave radiation (OLR) over Southern Africa shows indeed significant intraseasonal fluctuations at the 30-60 day timescale, i.e. in the pe- riods that are typically reminiscent of the MJO. In order to confirm the implication of the latter, composite analyses are computed, based on the real-time MJO indices defined in Wheeler and Hendon (2004). Strong intraseasonal convective signals are particularly recorded over the region dur- ing its rainy season (November through March). Large-scale organized convective perturbations are seen to propagate eastwards, mainly between 10˚S and 20˚S, and then northwards, over the Rift Valley and the African Great Lakes. They finally reach the MJO-associated equatorial clusters over Tanzania, which complete their circuit towards the East over the Indian Ocean. The corresponding response of the rainfall field, obtained through the analysis of daily rain-gauge records in 7665 stations over Southern Africa, presents the alternation, over the intraseasonal cycle, of a dry and a humid phase, which are both significant. The influence of the MJO on the rainfall field is however not homogeneous spatially. While the southern part of the domain (Western Cape Province and surrounding countries) is very partially influenced, and more closely relates to the mid-latitude dynam- ics, the tropical parts of the domain (Northern Province of South Africa, Namibia, Botswana and Zimbabwe) logically show stronger dependency to the MJO forcing. Rainfall records exhibit there sharp periodicities in the 30-60 day timescale. Moisture flux anomalies, derived from the NCEP-DOE II reanalyses, reveal an in- traseasonal modulation of the mid-tropospheric easterly flow over the Congo basin at 700hPa; these fluctuations are coupled to northerly anomalies that extend from the tropical to the subtropical austral latitudes. They are hypothesized to convey mois- ture from the tropical air masses, and hence to favour wet conditions over the region. During the dry phase, southerly anomalies tend on the contrary to prevail, and are hypothesized to convey dryness from the mid-latitude air masses.
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- 2007
- Full Text
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