Dynamics of gap winds in the Great Rift Valley, Ethiopia: emphasis on strong winds at Lake Abaya

Abstract

. Lake Abaya, located in the Great Rift Valley (GRV) in Ethiopia, is affected by regularly occurring strong winds that cause water waves, which in turn affect the lake’s ecology and food web. The driving forces for these winds, however, are yet unexplained. Hence, the main goal of this study is to provide a physical explanation for the formation of these strong winds in the GRV and especially at Lake Abaya. To this aim, two case studies were performed based on mea surements, ERA5 reanalysis data and mesoscale numerical simulations conducted with the Weather Research and Fore casting (WRF) model. The simulations revealed that in both cases a gap flow downstream of the narrowest and highest part of the GRV (i.e. the pass) led to high wind speeds of up to 25ms 1. Two types of gap flow were identified: a north eastern gap flow and a south-western gap flow. The wind di rections are in line with the orientation of the valley axis and depend ontheair massdistribution north and south of the val ley and the resulting along-valley pressure gradient. The air mass distribution was determined by the position of the In tertropical Convergence Zone relative to the GRV. The colder air mass was upstream of the GRV in both case studies. Dur ing the day, the convective boundary layer in the warmer air mass on the downstream side heated up more strongly and quickly than in the colder air mass. The most suitable variable describing the timing of the gap flow was found to be the pressure gradient at pass height, which corresponds roughly to the 800hPa pressure level. In both cases the gap f low exhibited a strong daily cycle, which illustrates the im portance of the thermal forcing due to differential heating over complex terrain in addition to the large-scale forcing due to air mass differences. The start, strength, and the du ration of the gap winds within the valley depended on loca tion. For both cases, the strongest winds occurred after sunset and in the ongoing night downstream of the gap and on the corresponding lee slope. The ERA5 reanalysis captures both events qualitatively well but with weaker wind speeds than in the mesoscale numerical simulations. Hence, ERA5 is suit able for a future climatological analysis of these gap flows.