Synoptic Systems

Cornforth, Rosalind; Mumba, Zilore; Parker, Douglas J.; Berry, Gareth; Chapelon, Nicolas; Diakaria, Kone; Diop-Kane, Mariane; Ermert, Volker; Fink, Andreas H.; Knippertz, Peter; Lafore, Jean Philippe; Laing, Arlene; Lepape, Serge; Maidment, Ross; Methven, John; Orji, B. N.; Osika, David; Poan, Emmanuel; Roca, Rémy; Rowell, Simon; Smith, Roger K.; Spengler, Thomas; Taylor, Christopher M.; Vincendon, Jean-Christoph; Yorke, Charles; Thorncroft, Chris D.

doi:10.1002/9781118391297.ch2

Cited by 17 publications

(34 citation statements)

References 164 publications

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“…At synoptic scales the Ouagadougou extreme event was associated with a well pronounced ‘breaking’ AEW, whose six favourable factors have been identified by forecasters in several case‐studies and proposed in the synoptic chapter 2 of the ‘West African Forecaster's Handbook’ (Cornforth et al ., ). The present study confirms four of them: A pre‐existing significant monsoon flow penetration over the eastern Sahel, resulting in a large‐scale wet spell (positive anomaly of PW, Figure (b)) over the eastern Sahel, that propagates westward. A train of AEWs (Figures and (a)).…”

Section: Discussionsupporting

confidence: 62%

“…On this date, this vortex located over Niamey has wrapped the wet anomaly which almost splits it into two parts (grey area in Figure (f)). A key feature of this event is that contrary to the usual schematic of an AEW (Cornforth et al ., – see their Fig. .10) the wet anomaly is ahead (west) of the trough T2 within the northerly flow, instead of being located behind the trough in the monsoon flow.…”

Section: Propagating Modes Of Variability and Wave Activitymentioning

confidence: 99%

“…The AEW breaking – defined by Cornforth et al . () as a very steep north–south oriented curvature of the wave streamlines (Figure (c)) – reaches Ouagadougou on 31 August, 1200 UTC (Figure (b)). The breaking is so strong that a northwesterly flow at 700 hPa (i.e.…”

Section: From Synoptic To Convective Scalesmentioning

confidence: 99%

“…The main goal of this first article is to present a multi‐scale analysis of the extreme rain event of Ouagadougou that occurred on 1 September 2009, with a focus on regional and synoptic scales and to identify key factors for this type of event. This case has been used in the recently published West African Forecaster's Handbook (Parker and Diop‐Kane, ) to illustrate AEW breaking events (Cornforth et al ., ) as seen by forecasters. In a companion article we will analyse the physical mechanisms involved in the small‐scale development of a moist vortex associated with this extreme rain event, owing to the computation of budgets of heat, moisture and vorticity from a convection‐permitting simulation.…”

Section: Introductionmentioning

confidence: 99%

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A multi‐scale analysis of the extreme rain event of Ouagadougou in 2009

Lafore

Beucher

Peyrillé

et al. 2017

Quart J Royal Meteoro Soc

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This study presents a multi‐scale analysis of an extreme rain event that occurred in Burkina Faso on 1 September 2009 with an absolute record of 263 mm rainfall observed at Ouagadougou. This high‐impact weather system results from the combination of several favourable ingredients at different scales. The sea‐surface temperature anomaly patterns in July–August 2009 of both the Atlantic cold tongue, the Tropical Atlantic Dipole and the Mediterranean Sea are favourable factors for the northward penetration of the West African monsoon. The intense convective activity of the last 10‐day period in August is associated with the crossing of a convectively coupled Kelvin wave increasing the African easterly wave (AEW) activity, and of an equatorial Rossby wave. At the synoptic scale this event corresponds to the passage of a train of three AEWs with increasing magnitude. Behind the first AEW trough axis, an intense and deep southerly monsoon burst develops. It contributes to the amplification of the second AEW and its breaking is associated with the formation of an intense meso‐vortex on the southern flank of the African easterly jet. Compared to the fast‐moving squall line, the dominant type of precipitating weather system over the Sahel, the Ouagadougou precipitating system appears to be a moist vortex propagating slowly, allowing rainfall accumulation, without wind gusts or convective cold pools observed at the surface. The main precipitation area is located about 2° longitude downshear (westward due to the African easterly jet) of the centre of this strong meso‐vortex.

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Section: Discussionsupporting

confidence: 62%

Section: Propagating Modes Of Variability and Wave Activitymentioning

confidence: 99%

Section: From Synoptic To Convective Scalesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

A multi‐scale analysis of the extreme rain event of Ouagadougou in 2009

Lafore

Beucher

Peyrillé

et al. 2017

Quart J Royal Meteoro Soc

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“…As this scenario is based on only one extreme case‐study, it is a priority to check this scenario for other cases. Nevertheless, forecasters have analysed similar cases, as reported in the Synoptic Chapter 2 of the West Africa Forecasters Handbook (Cornforth et al ., ). Recently, Vischel et al .…”

Section: Resultsmentioning

confidence: 99%

Simulation and analysis of the moist vortex associated with the extreme rain event of Ouagadougou in 2009

Beucher

Lafore

Chapelon

2019

Quart J Royal Meteoro Soc

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This study analyses the mechanisms involved in the formation of a moist meso‐vortex associated with an extreme rain event that occurred in Burkina Faso on 1 September 2009, on the basis of high‐resolution convection‐permitting simulations. After an evaluation of the 6‐day simulation skill to capture the main characteristics of this event, budgets of heat, moisture and relative vorticity are calculated. Results allow us to propose the following scenario for the occurrence of this extreme rain event, completing the large‐scale analysis of the same event performed in the companion article. Firstly, the arrival of a large‐scale wet spell is a key factor, creating a quite unusual wet environment over the Sahel, resulting in weak rain evaporation. As over oceanic regions, it favours intense heating at low levels and thus strong mean ascent and convergence. Secondly, the combination with the arrival of the trough of an African easterly wave provides a second key factor. The associated relative vorticity maximum on the southern flank of the African easterly jet (AEJ) core is boosted by the strong low‐level convergence through the stretching term. The vortex deepening is favoured by the tilting and by eddies at upper levels. Thirdly, in turn the quasi‐balanced vortex circulation brings warm and moist air from the north, favouring the convection triggering to the west of the vortex in the down‐shear direction, in agreement with Raymond and Jiang's theory for long‐lived mesoscale convective systems. This scenario exhibits a positive feedback that could explain its extreme character.

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Upper‐level midlatitude troughs in boreal winter have an amplified low‐latitude linkage over Africa

Ward

Fink

Keane

et al. 2022

Atmospheric Science Letters

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In boreal winter, strong upper‐level midlatitude troughs across the Atlantic–Africa–southwestern Asia sector generate substantial tropical–extratropical interaction and have become recognized as important factors in some extreme weather events. As such, they represent important dynamic features to understand and capture in weather forecasts, as well as in climate models for projections on longer timescales. Here, we empirically study the 20% of winter days with strongest trough signatures during 1982–2020 at each longitude across the sector, and show that the trough impact over northern Africa, most notably in central parts, is particularly strong in magnitude, low‐latitude extent and persistence, leading to the characterization of a northern Africa mode of several‐days weather fluctuation. Weather conditions that follow strong troughs from the eastern Atlantic to the Central Mediterranean include: (i) a warming tendency across much of northern Africa, generally of several Celsius magnitude ahead of the trough, and >1°C even extending to the south of 10° N in central parts and continuing eastward until the Ethiopian Highlands; (ii) precipitation development further north than normal across northern tropical Africa, especially strong over longitudes corresponding to a northward extension of the main Congo rain belt. The intertropical discontinuity and low‐level heat low are also shifted significantly north, with the complex of anomalies persisting for several days, beyond the timescale of the trough. For context, at all other trough longitudes across the sector, a warming signal does emerge (statistically significant), but with much shorter persistence (2–3 days), smaller magnitude and extending southward clearly only to 15–20° N. Mid‐level tropical plumes of moisture are also typically present for strong troughs from the eastern Atlantic to southwestern Asia, and these alone can lead to weather extremes. However, low‐level warming and mid‐level moistening are uniquely juxtaposed at low latitudes over central Africa, where a near‐equatorial signature develops.

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Synoptic Systems

Cited by 17 publications

References 164 publications

A multi‐scale analysis of the extreme rain event of Ouagadougou in 2009

A multi‐scale analysis of the extreme rain event of Ouagadougou in 2009

Simulation and analysis of the moist vortex associated with the extreme rain event of Ouagadougou in 2009

Upper‐level midlatitude troughs in boreal winter have an amplified low‐latitude linkage over Africa

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