Mean Climate and Seasonal Cycle

Fink, Andreas H.; Engel, T.; Ermert, Volker; Linden, Roderick van der; Schneidewind, Malvin; Redl, Robert; Afiesimama, Ernest; Thiaw, Wassila M.; Yorke, Charles; Evans, M. J.; Janicot, Serge

doi:10.1002/9781118391297.ch1

Cited by 39 publications

(53 citation statements)

References 55 publications

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“…Inland areas are somewhat drier. The coastal stretch along southeastern Ghana, Togo, and Benin is even drier (700-1000 mm.yr −1 ) due to coastal upwelling and low-level wind divergence during the monsoon season [43]. The 99th percentile displays quite similar patterns, with higher values over the Atlantic Ocean and along the coast (55-80 mm), except in southeastern Ghana and southern Togo where P99 values are close to those found inland.…”

Section: Definition Of Intense Rainfall Eventsmentioning

confidence: 75%

“…The contribution of intense events to the monthly total precipitation also peaks in June (29.7% on average over the region). Due to the northern shift of the ITCZ in boreal summer, August records almost no intense event on the coast but quite a number in the north, at the limit of Sudanian climates which display a summer maximum [43]. The spatial patterns of the intense rainfall events were stratified in three main classes, each one showing quite a distinct geography, although some overlaps do exist.…”

Section: Classification Of Intense Rainfall Eventsmentioning

confidence: 99%

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Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

2020

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Daily rainfall in southern West Africa (4-8 • N, 7 • W-3 • E) is analyzed with the aim of documenting the intense rainfall events which occur in coastal Ivory Coast, Ghana, Togo, and Benin. The daily 99th percentile (P99) shows that the coastline experiences higher intensity rainfall than inland areas. Using Tropical Rainfall Measuring Mission (TRMM) rainfall data for 1998-2014, a novel way of classifying the intense events is proposed. We consider their space-time structure over a window of 8 • latitude-longitude and five days centered on the event. A total 39,680 events (62 at each location) are classified into three major types, mainly found over the oceanic regions south of 5 • N, the Bight of Benin, and the inland regions respectively. These types display quite distinct rainfall patterns, propagation features, and seasonal occurrence. Three inland subtypes are also defined. The atmospheric circulation anomalies associated with each type are examined from ERA-interim reanalysis data. Intense rainfall events over the continent are mainly a result of westward propagating disturbances. Over the Gulf of Guinea, many intense events occur as a combination of atmospheric disturbances propagating westward (mid-tropospheric easterly waves or cyclonic vortices) and eastward (lower tropospheric zonal wind and moisture anomalies hypothesized to reflect Kelvin waves). Along the coast, there is a mixture of different types of rainfall events, often associated with interacting eastward-and westward-moving disturbances, which complicates the monitoring of heavy precipitation.

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Section: Definition Of Intense Rainfall Eventsmentioning

confidence: 75%

Section: Classification Of Intense Rainfall Eventsmentioning

confidence: 99%

Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

2020

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“…Northern tropical and subtropical Africa features a wide range of different climates. Annual precipitation ranges from more than 2500 mm yr −1 in coastal perhumid areas of western and central Africa to less than 100 mm yr −1 in the hyperarid areas in the Saharan desert (Fink et al, 2017). In order to make the effect of tropical waves on precipitation anomaly patterns comparable for these very diverse climates, precipitation values need to be normalized.…”

Section: Normalization Of Precipitationmentioning

confidence: 99%

A Systematic Comparison of Tropical Waves over Northern Africa. Part I: Influence on Rainfall

et al. 2019

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Low-latitude rainfall variability on the daily to intraseasonal timescale is often related to tropical waves, including convectively coupled equatorial waves, the Madden-Julian Oscillation (MJO), and tropical disturbances. Despite the importance of rainfall variability for vulnerable societies in tropical Africa, the relative influence of tropical waves for this region is largely unknown. This article presents the first systematic comparison of the impact of six wave types on precipitation over northern tropical Africa during the transition and full monsoon seasons, using two satellite products and a dense rain gauge network.Composites of rainfall anomalies in the different datasets show comparable modulation intensities in the West Sahel and at the Guinea Coast, varying from less than 2 to above 7 mm d −1 depending on the wave type. African Easterly Waves (AEWs) and Kelvin waves dominate the 3-hourly to daily timescale and explain 10-30 % locally. On longer timescales (7-20d), only the MJO and equatorial Rossby (ER) waves remain as modulating factors and explain about up to one third of rainfall variability. Eastward inertio-gravity waves and mixed Rossby-gravity (MRG) waves are comparatively unimportant. An analysis of wave superposition shows that low-frequency waves (MJO, ER) in their wet phase amplify the activity of high-frequency waves (TD, MRG) and suppress them in the dry phase. The results stress that more attention should be paid to tropical waves when forecasting rainfall over northern tropical Africa.

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“…This is quite unusual for the Sahel, where climatologically the maximum convergence occurs at the AEJ level with a weak mean ascent underneath and sometimes even a subsidence due to rain evaporation (fig. 1.1 of Fink et al ., ).…”

Section: Contribution Of Thermodynamics Features To the Extreme Eventmentioning

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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Mean Climate and Seasonal Cycle

Cited by 39 publications

References 55 publications

Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

Classification of Intense Rainfall Days in Southern West Africa and Associated Atmospheric Circulation

A Systematic Comparison of Tropical Waves over Northern Africa. Part I: Influence on Rainfall

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

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