The dynamics of the Congo air mass and its influence on the rainfall over Western Uganda

Abstract

The rainfall patterns over Uganda particularly western Uganda shows a high degree of spatial and temporal variability compared to other meteorological parameters such as temperature, pressure and wind among others. The study's main objective was to examine the relationship between the Congo Air mass and the rainfall characteristics over Western Uganda. The datasets used in this study included monthly rainfall data obtained from four meteorological stations (Kabale, Mbarara, Kasese and Masindi) spanning a period of 29 years from 1989 to 2018, monthly wind data from ERA-Interim reanalysis, with spatial resolution of 0.250 by 0.250 and specific humidity data at different pressure levels (500,700 and 850hPa) all retrieved from (http://apps.ecmwf.int). Annual rainfall results showed increasing trends in only three stations (Masindi, Mbarara and Kabale) (0.013, 0.016 and 0.002) while Kasese (-0.048) had a decreasing trend, however these trends at all the four stations are not statistically significant (P>0.05). Coefficient of variation results indicated lower Coefficient of Variations (≤ 20%) for all the stations in Western Uganda implying that the annual rainfall weakly deviated away from the mean value during the study period. Moisture results from the four stations generally indicated significant moisture increase at 500hPa level while insignificant moisture variations at 700 and 850hPa at all the four stations. Generally, results for linkages between circulations over Congo basin and rainfall over Western Uganda depicted weak relationship between wind speed at 500,700 and 850hPa levels at all the four stations however, significant relationship was more observed at 500hPa level compared to other levels. This therefore concludes that Congo westerlies are associated with wet days in all seasons, and they are the largest contributor to rainfall during the second rains. However, the frequency of Congo westerlies does not appear to be related to inter-annual rainfall variability and thus not the cause of increased rainfall in western Uganda.