Mid-Tropospheric Cyclones of the Southwest Monsoon

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Abstract
Mid-tropospheric cyclones are an important part of the tropical general circulation of the summer season. These are synoptic-scale disturbances which appear in the daily and monthly mean circulation maps with greatest intensity at levels near 500 mb. The structure and energetics of this type over southeast Asia are discussed in this paper. Interesting features include a warm anomaly above the cyclone and a cold anomaly below. A five-level non-geostrophic balanced model is used in this study to obtain the distribution of vertical motion. The model includes a parameterization of cumulus-scale convection. In the middle levels, rising motions are found west of the cyclone and sinking motions to the east. This is primarily due to the thermal structure of the atmosphere and associated advection of colder air from the oceanic regions and warmer air from land areas. A marked diurnal change in the vertical velocity is noted in the computations; magnitudes are large at 0700 local time compared to 1900. This diurnal change is primarily due to changes in the wind direction and speed. The important result of this study is that both the cumulus- and synoptic-scale motions exhibit the following dual roles in the maintenance of this midlevel system: 1) Both scales contribute to a net warming of the air above the cyclone, diabatic warming by cumulus-scale motions and adiabatic warming by the descent of synoptic-scale motions. 2) The two scales oppose each other in the transformation eddy available potential energy into eddy kinetic energy. Cumulus-scale motions contribute to a net generation, while synoptic-scale motions transform kinetic energy into potential energy. This latter result is consistent with 1) because the areas of descent are somewhat closer to the warm temperature anomaly than are the areas of ascent.

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