Characteristics and evolution of diurnal foehn events in the Dead Sea valley

Abstract

This paper investigates frequently occurring foehn in the Dead Sea valley. For the first time, sophisticated, highresolution measurements were performed to investigate the horizontal and vertical flow field. In up to 72% of the days in summer, foehn was observed at the eastern slope of the Judean Mountains around sunset. Furthermore, the results also revealed that in approximately 10% of the cases the foehn detached from the slope and only affected elevated layers of the valley atmosphere. Lidar measurements showed that there are two main types of foehn. Type I has a duration of approximately 2-3 h and a mean maximum velocity of 5.5ms -1 and does not propagate far into the valley, whereas type II affects the whole valley, as it propagates across the valley to the eastern side. Type II reaches mean maximum wind velocities of 11ms -1 and has a duration of about 4-5 h. A case study of a type II foehn shows that foehn is initiated by the horizontal temperature gradient across the mountain range. In the investigated case this was caused by an amplified heating and delayed cooling of the valley boundary layer in the afternoon, compared to the upstream boundary layer over the mountain ridge. The foehn was further intensified by the advection of cool maritime air masses upstream over the coastal plains, leading to a transition of subcritical to supercritical flow conditions downstream and the formation of a hydraulic jump and rotor beneath. These foehn events are of particular importance for the local climatic conditions, as they modify the temperature and humidity fields in the valley and, furthermore, they are important because they enhance evaporation from the Dead Sea and influence the aerosol distribution in the valley.