A numerical study of moisture build‐up and rainfall over West Africa

Abstract

ABSTRACT:Empirical studies have shown that a necessary condition for abundant rainfall in any year at Kano in Nigeria is a pre-rainy season moisture build-up for 6-8 weeks at the surface or below 800 hPa. The International Centre for Theoretical Physics (ICTP) regional climate model (RegCM3) is used to study the mechanism and variability of the inflow of this moisture over West Africa over a 21-year period . The study focused on three dry (1983, 1989 and 1990) and three wet years (1980, 1988 and 1996) and uses rainfall and other surface data such as specific humidity, over Nigeria for the same years for model validation. Comparison of RegCM3 simulation of rainfall and moisture patterns over West Africa with both CRU and Nigerian stations observations shows that the model performed very well with respect to the observations over the region, particularly over the Sahel and Savana zones.The composites of moisture inflow and wind anomalies for the wet years are found to be associated with areas of boundary layer moisture build-up, overlain by relatively drier (but still moist) air driven by strong west to southwest wind vector anomalies at 700 hPa (African Easterly Jet, AEJ, level). These strong wind anomalies, which occur south of the AEJ between May and September and transport moisture towards it, seem unconnected with the boundary layer winds and are not found in the dry composites. In sharp contrast, the dry composites are characterized by negative moisture anomalies in the boundary layer overlain by drier than normal air associated with northerly to easterly wind anomalies at the AEJ level. Secondly, while the ITD reached the same northernmost limit of about 20°N in August during both dry and wet periods, the AEJ advanced faster across West Africa, reaching a mean position of 16.5°N in August during wet years but 15°N in dry years. Also, on a monthly and inter-annual basis, moisture adequacy and variability analysis show that, for dry years, moisture build-up is delayed but starts early and strongly during wet years. Furthermore, rainfall does not begin properly until about 2.5 months after the start of the moisture build-up. Finally, RegCM3 predicted rainfall correlates very well (r ≥ 0.6) with observed rainfall for stations within Nigeria.

CAM-EULAG: A non-hydrostatic atmospheric climate model with grid stretching

“…The southward retreat of the rainbelt starts in September in the models and GPCP. The seasonal variation of the rainfall closely follows the latitudinal position of the Inter-Tropical Discontinuity (ITD) and the African Easterly Jet (AEJ), in agreement with previous studies (Le Barbé et al 2002, Omotosho and Abiodun 2007, Sylla et al 2010a). The models underestimate the strength of the AEJ, Figure 4 shows that CEU-UNI and CEU-SG simulate the vertical structure of the monsoon system better than CFV does.…”

Changes in Precipitation Seasonality in West Africa Predicted by RegCM3 and the Impact of Dynamic Vegetation Feedback

“…Among the three different convection parameterizations in RegCM3, the MIT-Emanuel scheme [34] option is chosen based on performance over the study domain. RegCM3 has been shown to perform well in its simulation of precipitation over West Africa [35][36][37].…”

Evaluation of the ECMWF Sub-seasonal to Seasonal Precipitation Forecasts during the Peak of West Africa Monsoon in Nigeria

“…However, there exist no commensuration between the operational use of S2S models and studies of their abilities to reproduce basic atmospheric variables. This present study will explore this knowledge gap over Nigeria-the most populous country in Africa whose food security depends entirely on rainfed agriculture (Omotosho and Abiodun, 2007;Okwori et al, 2015).…”

“…Few of these, discussed in this study, are the El Nino Southern Oscillation (ENSO; Latif and Grotzner, 2000;Camberlin et al, 2001;Newman et al, 2003), the Inter-Tropical Discontinuity (ITD; Grist and Nicholson, 2001;Redelsperger et al, 2002;Omotosho and Abiodun, 2007), thermal lows (Parker et al, 2005;Lavaysse et al, 2006Lavaysse et al, , 2009Lavaysse et al, , 2010, African Easterly Jet (AEJ; Diedhiou et al, 1998;Grist and Nicholson, 2001;Afiesimama, 2007), and the sea surface temperature (SST) anomalies over the Gulf of Guinea (GOG; Omotosho and Abiodun, 2007;Odekunle and Eludoyin, 2008).…”