Rainfall distribution over Nairobi area

Abstract

The distribution of rainfall in time and space over Nairobi area is analysed for the 31-year base period between 1960 and 1990. The high-resolution limited-area numerical model, Geesthacht Simulation Model of the Atmosphere ( GESIMA) is used to simulate the water substance field over Nairobi City in a case study. The model is integrated to 24 hours to simulate the diurnal cycle of the moisture budget for a typical day in the “short rains” season. The model encompasses the real topography, land use/cover characteristics and full physics. The interaction between large-scale flow and local factors in determining the distribution of categorised rainfall over the City is assessed. The observed decreasing trend of annual rainfall amounts and frequency over Nairobi area is attributed to inter-annual variability of large-scale forcing systems. Increasing trends of heavy rainfall intensities are attributed to anthropogenic forcing through changes in the landscape on relatively small scales. The effects of small-scale features on the water substance are demonstrated through numerical simulation: Local features have the capability of modifying the heat budget and airflow patterns affect the moisture distribution and convection in the entire Nairobi province.