scholarly journals Rainfall Types in the West African Sudanian Zone during the Summer Monsoon 2002

2006 ◽  
Vol 134 (8) ◽  
pp. 2143-2164 ◽  
Author(s):  
A. H. Fink ◽  
D. G. Vincent ◽  
V. Ermert
Keyword(s):  
Summer Monsoon ◽  
Monsoon Season ◽  
Integrated Approach ◽  
West African ◽  
Annual Rainfall ◽  
Major Type ◽  
Synoptic Situation ◽  
The West ◽  
Convective Systems ◽  
Sudanian Zone

Abstract Enhanced surface and upper-air observations from the field campaign of the Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) project are used to partition rainfall amounts over the West African Sudanian zone during the 2002 summer monsoon season into several characteristic types and subtypes of precipitating systems. The most prominent rainfall subtype was fast-moving, long-lived, and extensive cloud clusters that often developed far upstream over the central Nigerian highlands in the afternoon hours and arrived at the Upper Ouémé Valley (UOV) after midnight. These organized convective systems (advective OCSs, subtype Ia) accounted for 50% of the total rain amount in the UOV catchment in Benin. Subtypes Ia and IIa (i.e., locally developing OCSs) were found to pass by or organize when a highly sheared environment with deep and dry midtropospheric layers was present over the UOV. These systems were most frequent outside the peak of the monsoon season. The second major type of organized convection, termed mesoscale convective systems (subtypes Ib, IIb, and IIIb) in the present study, contributed 26% to the annual UOV precipitation. They occurred in a less-sheared and moister tropospheric environment mainly around the height of the rainy season. A third distinct class of rainfall events occurred during an unusual synoptic situation in which a cyclonic vortex to the north of the UOV led to deep westerly flow. During these periods, the African easterly jet was lacking. The so-called vortex-type rainfalls (subtypes IIIa, IIIb, and IIIc) contributed about 9% to the annual rainfall totals.

Precipitation modes over the Western Ghats orography during the summer monsoon season

2021 ◽  
Author(s):  
Jayesh Phadtare ◽  
Jennifer Fletcher ◽  
Andrew Ross ◽  
Andy Turner ◽  
Thorwald Stein ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
West Coast ◽  
Western Ghats ◽  
Monsoon Season ◽  
Radiosonde Data ◽  
Summer Monsoon Season ◽  
The West ◽  
Radiosonde Station ◽  
Westerly Flow ◽  
The Western Ghats

<p>Precipitation distribution around an orographic barrier is controlled by the Froude Number (Fr) of the impinging flow. Fr is essentially a ratio of kinetic energy and stratification of winds around the orography. For Fr > 1 (Fr <1), the flow is unblocked (blocked) and precipitation occurs over the mountain peaks and the lee region (upwind region). While idealized modelling studies have robustly established this relationship, its widespread real-world application is hampered by the dearth of relevant observations. Nevertheless, the data collected in the field campaigns give us an opportunity to explore this relationship and provide a testbed for numerical models. A realistic distribution of precipitation over a mountainous region in these models is necessary for flash-flood and landslide forecasting. The Western Ghats region is a classic example where the orographically induced precipitation leads to floods and landslides during the summer monsoon season. In the recent INCOMPASS field campaign, it was shown that the precipitation over the west coast of India occurred in alternate offshore and onshore phases. The Western Ghats received precipitation predominantly during the onshore phase which was characterized by a stronger westerly flow. Here, using the radiosonde data from a station over the Indian west coast and IMERG precipitation product, we show that climatologically, these phases can be mapped over an Fr-based classification of the monsoonal westerly flow. Classifying the flow as 'High Fr' (Fr >1), 'Moderate Fr' ( 0.5 < Fr ≤ 1) and 'Low Fr' ( Fr ≤ 0.5 ) gives three topographical modes of precipitation -- 'Orographic', 'Coastal' and 'Offshore', respectively.  Moreover, these modes are not sensitive to the choice of radiosonde station over the west coast.</p>

scholarly journals Three MCS Cases Occurring in Different Synoptic Environments in the Sub-Sahelian Wet Zone during the 2002 West African Monsoon

2006 ◽  
Vol 63 (9) ◽  
pp. 2369-2382 ◽  
Author(s):  
Jon M. Schrage ◽  
Andreas H. Fink ◽  
Volker Ermert ◽  
Epiphane D. Ahlonsou
Keyword(s):  
West Africa ◽  
West African Monsoon ◽  
Lower Troposphere ◽  
Integrated Approach ◽  
West African ◽  
Easterly Waves ◽  
Convective Systems ◽  
Efficient Management ◽  
Mesoscale Convective ◽  
Wet Zone

Abstract Three mesoscale convective systems (MCSs) occurring in the sub-Sahelian wet zone of West Africa are examined using observations from the 2002 Integrated Approach to the Efficient Management of Scarce Water Resources in West Africa (IMPETUS) field campaign, the European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, and Meteosat infrared imagery. These datasets enable the analysis of the synoptic-scale environment in which the MCSs were embedded, along with a high-resolution monitoring of surface parameters during the systems’ passages. The available data imply that cases I and II were of a squall-type nature. Case I propagated into a moderately sheared and rather moist lower and middle troposphere over the Upper Ouémé Valley (UOV). In contrast, case II was associated with a well-sheared and dry lower troposphere and a large, moist instability. In either case, behind the convective cluster a westward-propagating cyclonic vorticity maximum that was likely captured by the ECMWF analysis as a result of the special upper-air station at Parakou (Benin). In case I, the fast-moving vorticity signal slowed down over the Guinean Highlands where convection dissipated. Farther downstream, it might have played a role in the consolidation of an African easterly waves (AEW) trough over the West African coast and the eastern Atlantic. Case III proved to be a more stationary pattern of convection associated with a vortex in the monsoon flow. It also exhibited a moist and low shear environment.

Stochastic tracking of mesoscale convective systems: evaluation in the West African Sahel

2015 ◽  
Vol 30 (2) ◽  
pp. 681-691 ◽  
Author(s):  
Alexandros Makris ◽  
Clémentine Prieur ◽  
Théo Vischel ◽  
Guillaume Quantin ◽  
Thierry Lebel ◽  
...  
Keyword(s):  
West African ◽  
Mesoscale Convective Systems ◽  
The West ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
West African Sahel ◽  
African Sahel

Linkage of the Boreal Spring Antarctic Oscillation to the West African Summer Monsoon

1500 ◽  
Vol 999991 (9991) ◽  
pp. 9915-9928 ◽  
Author(s):  
Jianqi dummySUN ◽  
Huijun dummyWANG ◽  
Wei dummyYUAN
Keyword(s):  
Summer Monsoon ◽  
West African ◽  
Antarctic Oscillation ◽  
The West ◽  
Boreal Spring

scholarly journals Soil Moisture Influence on Seasonality and Large-Scale Circulation in Simulations of the West African Monsoon

2017 ◽  
Vol 30 (7) ◽  
pp. 2295-2317 ◽  
Author(s):  
Alexis Berg ◽  
Benjamin Lintner ◽  
Kirsten Findell ◽  
Alessandra Giannini
Keyword(s):  
Soil Moisture ◽  
Seasonal Cycle ◽  
Large Scale ◽  
Monsoon Season ◽  
West African Monsoon ◽  
West African ◽  
Monsoon Precipitation ◽  
The West ◽  
Large Scale Circulation ◽  
Soil Moisture Variability

Prior studies have highlighted West Africa as a regional hotspot of land–atmosphere coupling. This study focuses on the large-scale influence of soil moisture variability on the mean circulation and precipitation in the West African monsoon. A suite of six models from the Global Land–Atmosphere Coupling Experiment (GLACE)-CMIP5 is analyzed. In this experiment, model integrations were performed with soil moisture prescribed to a specified climatological seasonal cycle throughout the simulation, which severs the two-way coupling between soil moisture and the atmosphere. Comparison with the control (interactive soil moisture) simulations indicates that mean June–September monsoon precipitation is enhanced when soil moisture is prescribed. However, contrasting behavior is evident over the seasonal cycle of the monsoon, with core monsoon precipitation enhanced with prescribed soil moisture but early-season precipitation reduced, at least in some models. These impacts stem from the enhancement of evapotranspiration at the dry poleward edge of the monsoon throughout the monsoon season, when soil moisture interactivity is suppressed. The early-season decrease in rainfall with prescribed soil moisture is associated with a delayed poleward advancement of the monsoon, which reflects the relative cooling of the continent from enhanced evapotranspiration, and thus a reduced land–ocean thermal contrast, prior to monsoon onset. On the other hand, during the core/late monsoon season, surface evaporative cooling modifies meridional temperature gradients and, through these gradients, alters the large-scale circulation: the midlevel African easterly jet is displaced poleward while the low-level westerlies are enhanced; this enhances precipitation. These results highlight the remote impacts of soil moisture variability on atmospheric circulation and precipitation in West Africa.

scholarly journals Large-scale overview of the summer monsoon over West Africa during the AMMA field experiment in 2006

2008 ◽  
Vol 26 (9) ◽  
pp. 2569-2595 ◽  
Author(s):  
S. Janicot ◽  
C. D. Thorncroft ◽  
A. Ali ◽  
N. Asencio ◽  
G. Berry ◽  
...  
Keyword(s):  
Summer Monsoon ◽  
Land Surface ◽  
Large Scale ◽  
Monsoon Season ◽  
Regional Scale ◽  
West African ◽  
Future Research ◽  
Atmospheric Conditions ◽  
African Monsoon ◽  
Continental Hydrology

Abstract. The AMMA (African Monsoon Multidisciplinary Analysis) program is dedicated to providing a better understanding of the West African monsoon and its influence on the physical, chemical and biological environment regionally and globally, as well as relating variability of this monsoon system to issues of health, water resources, food security and demography for West African nations. Within this framework, an intensive field campaign took place during the summer of 2006 to better document specific processes and weather systems at various key stages of this monsoon season. This campaign was embedded within a longer observation period that documented the annual cycle of surface and atmospheric conditions between 2005 and 2007. The present paper provides a large and regional scale overview of the 2006 summer monsoon season, that includes consideration of of the convective activity, mean atmospheric circulation and synoptic/intraseasonal weather systems, oceanic and land surface conditions, continental hydrology, dust concentration and ozone distribution. The 2006 African summer monsoon was a near-normal rainy season except for a large-scale rainfall excess north of 15° N. This monsoon season was also characterized by a 10-day delayed onset compared to climatology, with convection becoming developed only after 10 July. This onset delay impacted the continental hydrology, soil moisture and vegetation dynamics as well as dust emission. More details of some less-well-known atmospheric features in the African monsoon at intraseasonal and synoptic scales are provided in order to promote future research in these areas.

scholarly journals Integration of Weather System Variability to Multidecadal Regional Climate Change: The West African Sudan–Sahel Zone, 1951–98

2006 ◽  
Vol 19 (20) ◽  
pp. 5343-5365 ◽  
Author(s):  
Michael A. Bell ◽  
Peter J. Lamb
Keyword(s):  
Rainy Season ◽  
Regional Climate ◽  
West African ◽  
Seasonal Rainfall ◽  
Annual Rainfall ◽  
Large Set ◽  
The West ◽  
Weather System ◽  
Linear Type ◽  
System Variability

Abstract Since the late 1960s, the West African Sudan–Sahel zone (10°–18°N) has experienced persistent and often severe drought, which is among the most undisputed and largest regional climate changes in the last half-century. Previous documentation of the drought generally has used monthly, seasonal, and annual rainfall totals and departures, in a standard “climate” approach that overlooks the underlying weather system variability. Most Sudan–Sahel rainfall occurs during June–September and is delivered by westward-propagating, linear-type, mesoscale convective systems [disturbance lines (DLs)] that typically have much longer north–south (102–103 km) than east–west (10–102 km) dimensions. Here, a large set of daily rainfall data is analyzed to relate DL and regional climate variability on intraseasonal-to-multidecadal time scales for 1951–98. Rain gauge–based indices of DL frequency, size, and intensity are evaluated on a daily basis for four 440-km square “catchments” that extend across most of the West African Sudan–Sahel (18°W–4°E) and are then distilled into 1951–98 time series of 10-day and seasonal frequency/magnitude summary statistics. This approach is validated using Tropical Applications of Meteorology Using Satellite Data (TAMSAT) satellite IR cold cloud duration statistics for the same 1995–98 DLs. Results obtained for all four catchments are remarkably similar on each time scale. Long-term (1951–98) average DL size/organization increases monotonically from early June to late August and then decreases strongly during September. In contrast, average DL intensity maximizes 10–30 days earlier than DL size/organization and is distributed more symmetrically within the rainy season for all catchments except the westernmost, where DL intensity tracks DL size/organization very closely. Intraseasonal and interannual DL variability is documented using sets of very deficient (8) and much more abundant (7) rainy seasons during 1951–98. The predominant mode of rainfall extremes involves near-season-long suppression or enhancement of the seasonal cycles of DL size/organization and intensity, especially during the late July–late August rainy season peak. Other extreme seasons result solely from peak season anomalies. On the multidecadal scale, the dramatic decline in seasonal rainfall totals from the early 1950s to the mid-1980s is shown to result from pronounced downtrends in DL size/organization and intensity. Surprisingly, this DL shrinking–fragmentation–weakening is not accompanied by increases in catchment rainless days (i.e., total DL absence). Like the seasonal rainfall totals, DL size/organization and intensity increase slightly after the mid-1980s.

scholarly journals Assessing the Capabilities of Three Regional Climate Models over CORDEX Africa in Simulating West African Summer Monsoon Precipitation

2015 ◽  
Vol 2015 ◽  
pp. 1-13 ◽  
Author(s):  
A. A. Akinsanola ◽  
K. O. Ogunjobi ◽  
I. E. Gbode ◽  
V. O. Ajayi
Keyword(s):  
West Africa ◽  
Summer Monsoon ◽  
Climate Models ◽  
Regional Climate ◽  
Regional Climate Models ◽  
West African ◽  
Annual Rainfall ◽  
Climate Projections ◽  
Wind Fields ◽  
African Easterly Jet

This study evaluates the ability of three Regional Climate Models (RCMs) used in Coordinated Regional Climate Downscaling Experiment (CORDEX) to simulate the characteristics of rainfall pattern during the West Africa Summer Monsoon from 1998 to 2008. The seasonal climatology, annual rainfall cycles, and wind fields of the RCMs output were assessed over three homogenous subregions and validated using precipitation data from eighty-one (81) ground observation stations and TRMM satellite data. Furthermore, the ability of the RCMs to simulate response to El Nino and La Nina events was assessed. Results show that two of the RCMs (RCA and REMO) simulated the main features of the rainfall climatology and associated dynamics over the three subregions (Guinea Coast, Savannah, and Sahel) of West Africa. The RCMs also capture the African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ) with little variations in position and intensity. Analysis shows significant biases in individual models depending on subregion and season under consideration which may be attributed to strong cyclonic circulation observed at 850 mb pressure level. In general, the study shows RCA and REMO fairly simulate West Africa rainfall adequately and can therefore be used for the assessment of West African Summer Monsoon and future climate projections.

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