Structure and Dynamics of Two Monsoon Depressions. Part I: Observed Structure

Abstract

Part I of a two-part paper describes the wind, thickness, and cloud-field evolution of two monsoon depressions observed during the 1979 Summer Monsoon Experiment (SMONEX). One depression was associated with the June Arabian Sea monsoon onset; the other formed over the Bay of Bengal in early July. The wind-field analyses showed the southwesterly tilt with height of both depressions. Using the subjectively produced wind-field analyses, height and thickness fields were derived using the balance equation. The thickness fields clearly showed the evolution of key features of both depressions, such as the development of a cold lower troposphere southwest of the depression's surface center and the upper-tropospheric warm-air pool above the region of maximum rainfall, west of the surface center. East-west cross sections showed that the thermal anomalies associated with both depressions tilted westward with height. Mesoscale analyses using research aircraft data showed that key structures evident from the synoptic-scale analyses were also reflected in the mesoscale structure of the depressions. Notable was a feature resembling a weak tropical cyclone eye during the mature stage of both depressions.