Topoclimate and spatio-temporal distribution of summer rain over the Ethiopian highlands

Ethiopia is a mountainous country known for its distinct topography ranging from alpine environment to desert-like conditions along the rift valley. Topoclimate, orographic rains on windward slopes and leeward rain shadow control rainfall variability in tropical mountains (Van den Hende et al., 2021). Such variability has a substantial societal impact. For instance they can cause differences in susceptibility to geohazards and wealth of rain-fed agriculture farmers among the windward and leeward sides of the mountain. However, due to lack of detailed scientific knowledge, a one-size-fits-all approach has so far been used, without considering the inter-event rainfall variability. This study aims at characterizing and understanding the spatio-temporal characteristics of the summer rain climatology over the Ethiopian highlands using regional climate model ALARO-0 at 4 km resolution. Multiple determining factors are explored including: wind (windward or leeward), rain (wet or dry day) categories, and elevation. These are used to categorize the diurnal cycles of different meteorological variables including wind, temperature, humidity and rainfall frequency and intensity. Ethiopia’s summer rain exhibits a clear diurnal and semi-diurnal pattern with the highest rainfall during the afternoon hours 1100–1800 EAT and the minimum values identified in the morning hours 0200–0900 EAT. During these peak hours, the mountain peaks receive up to 1.5mm hr-1 for as long as seven hours a day, while lowlands receive little to no rainfall at most of the times. Besides, windward events are found to have extended spatial and diurnal range of rainfall with respect to shorter duration on leeward events. The predominant afternoon maximum rainfall, as well as the prevalence of windward over leeward events and high orographic rainfall suggest terrain-induced ascent owing to thermal rather than wind-induced forcing (Kirshbaum et al., 2018). Moreover, the ALARO-0 model is found to reproduce well rainfall extremes. The recurrent flood hazard in lowland settlements such as Dire Dawa and Awash (Erena et al., 2018), an area of little or no rain, comes after heavy rainfall in the Ahmar mountains. Similarly, flash flood in catchment headwaters and hydrological deficit in the flood-plains (Negash et al., 2020), show higher rainfall with increasing elevation; where highland farmers mainly depend on rainfed agriculture while lowlanders could not harvest almost any crop unless supplemented by spate irrigation. This suggests the need for an alternative approach replacing the one-size-fits-all management approach with a site-specific approach taking into account topoclimatic differences.