Mesoscale convective systems and nocturnal rainfall over the West African Sahel: role of the Inter-tropical front

Vizy, Edward K.; Cook, Kerry H.

doi:10.1007/s00382-017-3628-7

Cited by 53 publications

(48 citation statements)

References 76 publications

(126 reference statements)

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“…However, CAPE, CIN and DCAPE tend to be highest for systems with intense deep convective cores (DCC and DWC, later combined to DEEP). Furthermore, as they are predominantly found early in the WAM season (March to May), the high values likely reflect the typical environmental conditions during this period, comparable to a regime described in Parker et al () and also Vizy and Cook (). The poleward flank of the AEJ, where SWA is situated early in the year, is characterized by a moist low‐level monsoon layer overlain by dry air of high potential temperature which may in some cases originate from the Sahara region through shallow dry convection (also Sultan and Janicot, 2003).…”

Section: Summary and Discussionsupporting

confidence: 72%

Rainfall types over southern West Africa: Objective identification, climatology and synoptic environment

Maranan

Fink

Knippertz

2018

Quart J Royal Meteoro Soc

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Southern West Africa (SWA) is characterised by a wide range of rainfall types, the relative importance of which have never been quantified on a regional level. Here, we use 16 years of three-dimensional reflectivity data from the Tropical Rainfall Measuring Mission-Precipitation Radar (TRMM-PR) to objectively distinguish between seven different rainfall types in three subregions of SWA. Highly organized Mesoscale Convective System (MCS) events are the dominating rain-bearing systems in SWA. They tend to occur in highly sheared environments as a result of mid-level northeasterlies ahead of a cyclonic vortex. Their contribution to annual rainfall decreases from 71% in the Soudanian to 56% in the coastal zone. MCSs in SWA also propagate slower than their Sahelian counterparts and occur predominantly at the start of the first coastal rainy season. However, in terms of numbers, about 90% of rainfall systems are weakly organized classes, particularly small-sized, highly reflective and moderately deep (40 dBZ at altitude <10 km) systems. Contrary to MCSs, less organized convection typically occurs during and after the passage of a cyclonic vortex within a regime of deep westerly anomalies, low wind shear and low to moderate CAPE (convective available potential energy), bearing some resemblance to what has been termed "monsoon" or "vortex rainfall". Combining TRMM-PR rainfall system identification with infrared-based cloud tracking reveals that organized convection over SWA typically lasts for more than >9 h, whereas less intense rainfall types tend to be short-lived, diurnal phenomena. This novel approach stresses the relevance of mid-level (wave) disturbances on the type and lifetime of convective systems and thereby their regionally, seasonally and diurnally varying contribution to rainfall amount. The present study suggests further investigations into the character of the disturbances as well as possible implications for operational forecasting and the understanding of rainfall variability in SWA. KEYWORDSmesoscale convective systems; rainfall climatology; TRMM; West African monsoon INTRODUCTIONRainfall over West Africa is mainly related to the West African monsoon (WAM) system and is known to vary considerably from year to year and on decadal time-scales (e.g. Diatta and Fink, 2014). Due to the prevalence of smallholder farming and rain-fed agriculture in West Africa, delayed onsets of the monsoon rains, monsoon breaks, flash floods, larger-scale seasonal inundations and longer-term droughts have large socio-economic impacts. Against these high vulnerabilities, disaster risk and mitigation measures are, amongst others, limited by the forecast skill of global numerical weather prediction models that barely exceeds climatology (Vogel et al., 2018), and by CMIP5 rainfall projections forThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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Section: Summary and Discussionsupporting

confidence: 72%

Rainfall types over southern West Africa: Objective identification, climatology and synoptic environment

Maranan

Fink

Knippertz

2018

Quart J Royal Meteoro Soc

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“…Convection resolving simulations are coming online and may provide further guidance (Marsham et al, 2013;Stratton et al, 2018;Vizy & Cook, 2017a). These simulations are an important addition to our toolkit.…”

Section: How To Interpret Consensus When All Models Have Common Sysmentioning

confidence: 99%

Rainfall trends in the African Sahel: Characteristics, processes, and causes

Biasutti

2019

WIREs Climate Change

169

100

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Sahel rainfall is dynamically linked to the global Hadley cell and to the regional monsoon circulation. It is therefore susceptible to forcings from remote oceans and regional land alike. Warming of the oceans enhances the stability of the tropical atmosphere and weakens deep ascent in the Hadley circulation. Warming of the Sahara and of the nearby oceans changes the structure and position of the regional shallow circulation and allows more of the intense convective systems that determine seasonal rain accumulation. These processes can explain the observed interannual to multidecadal variability. Sea surface temperature anomalies were the dominant forcing of the drought of the 1970s and 1980s. In most recent decades, seasonal rainfall amounts have partially recovered, but rainy season characteristics have changed: rainfall is more intense and intermittent and wetting is concentrated in the late rainy season and away from the west coast. Similar subseasonal and subregional differences in rainfall trends characterize the simulated response to increased greenhouse gases, suggesting an anthropogenic influence. While uncertainty in future projections remains, confidence in them is encouraged by the recognition that seasonal mean rainfall depends on large‐scale drivers of atmospheric circulations that are well resolved by current climate models. Nevertheless, observational and modeling efforts are needed to provide more refined projections of rainfall changes, expanding beyond total accumulation to metrics of intraseasonal characteristics and risk of extreme events, and coordination between climate scientists and stakeholders is needed to generate relevant information that is useful even under deep uncertainty. This article is categorized under: Paleoclimates and Current Trends > Modern Climate Change

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“…However, investing resources into the development of a new generation of convection parameterizations for use in general circulation models is not the only avenue to take. The ever-increasing availability of computational resources has sparked serious interest in attenuating the "cumulus parameterization problem" (coined so by [8]) by focusing efforts on exploring effective ways of running atmospheric models at convection-permitting grid spacings on large domains, even global, and long time periods (e.g., [9][10][11][12][13][14][15][16]). Such models use grid spacings of a few kilometers, allowing them to resolve convective storms explicitly.…”

Section: Introductionmentioning

confidence: 99%

Different Representation of Mesoscale Convective Systems in Convection-Permitting and Convection-Parameterizing NWP Models and Its Implications for Large-Scale Forecast Evolution

2019

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Representing mesoscale convective systems (MCSs) and their multi-scale interaction with the large-scale atmospheric dynamics is still a major challenge in state-of-the-art global numerical weather prediction (NWP) models. This results in potentially defective forecasts of synoptic-scale dynamics in regions of high MCS activity. Here, we quantify this error by comparing simulations performed with a very large-domain, convection-permitting NWP model to two operational global NWP models relying on parameterized convection. We use one month's worth of daily forecasts over Western Africa and focus on land regions only. The convection-permitting model matches remarkably well the statistics of westward-propagating MCSs compared to observations, while the convection-parameterizing NWP models misrepresent them. The difference in the representation of MCSs in the different models leads to measurably different synoptic-scale forecast evolution as visible in the wind fields at both 850 and 650 hPa, resulting in forecast differences compared to the operational global NWP models. This is quantified by computing the correlation between the differences and the number of MCSs: the larger the number of MCSs, the larger the difference. This fits the expectation from theory on MCS-mean flow interaction. Here, we show that this effect is strong enough to affect daily limited-area forecasts on very large domains.

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Mesoscale convective systems and nocturnal rainfall over the West African Sahel: role of the Inter-tropical front

Cited by 53 publications

References 76 publications

Rainfall types over southern West Africa: Objective identification, climatology and synoptic environment

Rainfall types over southern West Africa: Objective identification, climatology and synoptic environment

Rainfall trends in the African Sahel: Characteristics, processes, and causes

Different Representation of Mesoscale Convective Systems in Convection-Permitting and Convection-Parameterizing NWP Models and Its Implications for Large-Scale Forecast Evolution

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