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

Auteur : Weiß, Cornelius Immanuel

Université de soutenance : Universität Innsbruck

Grade  : Master of Science in Atmospheric Sciences 2021

Résumé
Local scientists reported regularly occurring strong winds at Lake Abaya, located in the Great Rift Valley, Ethiopia. The driving forces for these winds, however, are unexplained. Hence, the main goal of this Master’s thesis was to give a first qualitative description of the phenomenon that induces strong winds in the Great Rift Valley and especially at Lake Abaya.

As a first step, two possible events were selected based on a preanalysis of automatic weather station data of a local network as well as on ERA5 reanalysis data. Subsequently to the preanalysis, two case studies were conducted based on mesoscale numerical simulations using the Weather Research and Forecasting (WRF) model.

The simulations revealed that in both cases a gap flow in the Great Rift Valley led to high wind speeds of roughly 25\,m s$^-1$. Two types of gap flow were identified : a northeast gap flow and a southwest gap flow. The directions are in line with the orientation of the valley axis and depend on the air mass distribution north and south of the valley, and the resulting along-valley pressure gradient. The air mass distribution was a result of the position of the Intertropical Convergence Zone. The colder air mass was upstream in both case studies. During the day, the convective boundary layer in the warmer air mass on the downstream side heated up stronger and quicker than in the colder air mass. The most important variable describing the occurrence of the gap flow was found to be the pressure gradient at pass height, which corresponds roughly to the 800\,hPa pressure level, rather than the surface pressure gradient. In both cases the gap flow exhibited a strong daily cycle, which illustrates the importance 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 duration of the gap winds within in the valley were location-dependent. For both cases, the strongest winds occurred after sunset and in the ongoing night downstream of the gap and on the corresponding lee slope.

Applying reduced-gravity shallow-water theory supported the expected transition from a sub- to a supercritical flow but is inaccurate due to the lack of a precise determination of the inversion height and the degree of the valley narrowing.

In the ERA5 reanalysis, both events are represented in a qualitative sense but with weaker wind speeds than in the mesoscale numerical simulations.

Présentation

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