Mountain climate: one of the multipronged challenges in Ethiopia’s Agriculture

The Ethiopian highlands are home to more than 90% of the Ethiopian population and constitute for 90% of total area suitable for agriculture (Hurni et al., 2010). The livelihood of 66% of Ethiopia’s population depends on subsistence agriculture, mostly rainfed. Little rainfall variability can therefore cause massive economic loss for farmers reliant on rain-fed agriculture, making differences in wealth among farmers on different sides of the mountain. This study aims at understanding the sub daily distribution of summer rain over the Ethiopian highlands using the ALARO-0 regional climate model at convection-permitting resolution of 4 km. The dependence on factors such as leeward or windward conditions, and elevation are explored to categorize and relate the diurnal cycles of surface variables including precipitation, wind speed, humidity, and temperature. Rainfall occurrence in these mountains is mainly influenced by circulation patterns, orography, surface heating and convection, making its distribution very heterogeneous. Elevation is the most important determinant factor leading to increased average rainfall and rainfall per rainfall event towards higher elevations. Ethiopia’s summer rain exhibits a pronounced diurnal cycle with the highest rainfall occurring during the early afternoon hours (12:00–16:00) and the minimum occurring in the late night (04:00–11:00). Windward average rainfall and rainfall per rainfall event are on average 0.05mm h^-1 and 0.08 mm hr^-1 (respectively) larger than leeward events, except during peak hours when leeward events have 0.05 mm higher average rainfall and rainfall per rainfall event. In contrast to average rainfall events, extreme events in the afternoon are often followed by another peak rainfall event at night. Leeward wind speed features a weak diurnal variation as compared to the strong contrast between day and night for windward wind speed. 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. The prevalence of windward over leeward event probability, the stark contrast in wind speed diurnal cycle between windward and leeward events, and the early peak hour of dewpoint and air temperature all point towards temperature-induced rather than wind-induced convection. Rainfall-temperature dependence, in other words Clausius-Clapeyron relationship, in the lowlands such as the Afar triangle is however at its lowest due to moisture deficit. These differences are very likely to determine hydrology and vegetation distribution, and farmers economy at large.