Topoclimate and diurnal cycle of summer rain over the Ethiopian highlands in a convection-permitting simulation

Topoclimate, orographically enhanced rain on windward slopes and leeward rain shadows, control rainfall variability in tropical mountains. Such variability may cause a substantial societal impact in populated highlands such as in Ethiopia. This study aims at understanding the rainfall diurnal cycle and the orographic driving factors over the Ethiopian highlands using the ALARO-0 regional climate model at convection-permitting resolution, over a period of 21 years. While the evaluation against satellite-based observations shows a too early onset of the model diurnal cycle and a too low nocturnal rainfall, several conclusions can be drawn from the model simulations presented here. Elevation is the most important determining factor for modelled rainfall with increased average rainfall and rainfall per rainfall event towards higher elevations. Ethiopia's simulated summer rain exhibits a pronounced diurnal cycle with the highest rainfall occurring during the early afternoon hours and the minimum values occurring in the late night. Average rainfall and rainfall per rainfall event are substantially larger on the windward sides than on the leeward sides of the mountains, except during the peak hours. The diurnal cycles of temperature and humidity start earlier in the morning and recede later than the cycles of wind speed and rainfall. Moreover, rainfall peaks occur earlier in the day at higher elevations, and at night in valleys and in Afar Triangle. Rain intensities around noon on the leeward side are slightly higher than those on the windward side of the highest mountains (>3000 m). This could be due to delayed onset of cloudiness and larger morning insolation on the leeward side of the mountain, but this needs further study. The prevalence of windward over leeward rainfall, the stark contrast in wind-speed diurnal cycle between windward and leeward slopes, and the early peak hour of surface air and dew point temperature all point towards temperature-induced rather than wind-induced convection. These differences are likely to determine hydrology and vegetation distribution, and farmers economy at large. Hence, the need for a way out from the 'one-size-fits-all' agro-ecosystem management approach towards a site-specific policy that takes into account climatic differences.