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IAHS: Wallingford, UK, pp 255–260. Warren RA, Kirshbaum DJ, Plant RS et al. 2014. A ‘Boscastle-type’ quasi- stationary convective system over the UK Southwest peninsula. Q. J. R. Meteorol. Soc. 140: 240–257. Correspondence to: Jonathan G. Fairman Jr jonathan.fairman@manchester.ac.uk © 2015 The Authors. Weather published by John Wiley & Sons Ltd on behalf of Royal Meteorological Society. This 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. doi:10.1002/wea.2486 Atmospheric divergence over equatorial East Africa and its influence on distribution of rainfall Kiprop Vincent Koech Department of Meteorology, University of Nairobi, Kenya Introduction In East Africa, rainfall amounts and distribu- tion are the most important factors gov- erning crop yields (Muti and Kibe, 2009). Since most food production systems over the region are mainly rain-fed, weather forecasts are crucial. Therefore, this analysis focuses on divergence of the atmospheric flow with the aim of contributing to the understanding of regional weather. The idea that surface convergence and rainfall are related is not new. Marshall et al. (2001) found that initial rainfall morphology is related not only to the amount of low-level convergence but to the depth of the conver- gence. Convergence is the piling up of air above a region, whereas divergence is the spreading of air above some region (Ahrens, 2011). Convergence and divergence of air may result from changes in wind speed or wind direction (Ahrens, 2011). In addition, convergence may be due to frictionally driven, cross-isobaric flow (Zehnder, 2001). Area of study The area under the focus of this analysis is equatorial East Africa (Figure 1). Twenty- one stations have been selected to repre- sent the region, based on homogeneous rainfall zones. East Africa experiences two climatological rainy seasons. During southern hemisphere summer, the weather of equatorial East Africa is influenced by the northeast monsoon. During northern hemisphere summer, the region is under the influence of the south- east monsoon. The southeast monsoon is cool, moist and shallow, and is generally associated with cool, cloudy and dry condi- tions over the region (Christian et al., 2011). Data Two datasets were used in the analysis. These are monthly mean rainfall and diver- gence data for the period 1979–2008. The rainfall data were obtained from the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Centre (CPC). The spatial resolution of the CPC dataset is 0.5° × 0.5°. Divergence data were obtained from European Centre for Medium Range Weather Forecasting (ECMWF) ERA-Interim dataset. The spatial resolution of the ECMWF dataset is 0.125° × 0.125°. Seasonal variation of upper tropospheric divergence As atmospheric sounding and instability indi- ces reveal, most of the tropical zone is essen- tially convective, although variations occur on diurnal, latitudinal, and seasonal scales, as well as with altitude (Galvin, 2008). In this section, we focus on the seasonal variation of upper tropospheric divergence over the region. The seasonal variation is based on the monthly mean for the period 1979–2008. The seasonal march of upper tropo- spheric divergence has a bimodal pattern (Figure 2). During March-April-May (MAM) and October-November-December (OND), there is peak upper level divergence at 300 and 200hPa over the region. During