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.
