Heat Low Over the Saudi Arabian Desert During May 1979 (Summer MONEX)

Abstract

In May of 1979 a unique data set was obtained over the desert area in Saudi Arabia near the surface heat low as part of the GARP Monsoon Experiment (MONEX). Analysis of the data reveals that during the day a well-mixed layer extends up to 650 mb, while a stable layer forms above the desert surface during the night. Calculations of the divergence and kinematic vertical velocity show that the entire troposphere is dynamically active with strong descending motions during the day down to a 1 km deep surface layer which has ascending motion in it. There is daytime surface convergence, divergence around 850 mb, and convergence above 700 mb. During the pre-sunrise hours there is descending motion down to the surface with divergence at nearly all levels from the surface up to 550 mb and convergence above this level. The roles of vertical and horizontal advective, as well as radiative, processes in the thermal budget have been investigated. During the pre-sunrise hours the horizontal advective and longwave radiative processes contribute a cooling which is nearly balanced by the heating due to vertical advective processes. During the midday hours there is net radiative heating and heating due to vertical advective processes. The horizontal advective processes give a cooling up to 400 mb and a heating above. Approximately half of the sensible beat estimated to leave the earth's surface is transferred to the atmosphere between the surface and 975 mb and half between 975 and 650 mb. Satellite observations show that desert areas experience a net loss of radiation to outer space while the thermal stratification remains nearly invariant from one day to the next. This study indicates that a crucial element in the maintenance of stratification over the desert region in Saudi Arabia is the importation of heat into the upper troposphere coupled with subsidence above this region. This energy supply most likely comes from planetary-scale divergent circulations.