Study of a Monsoon Depression (I): Synoptic Structure

°This is an observational study of a monsoon depression during its westward passage across India. We present a study based on available conventional radiosonde rawinsonde pilot balloon and commercial aircraft wind reports during August, 1968. Besides showing conventional analysis of the motion and thermal fields, we also show a number of vertical structure diagrams. The important findings of the observational study are that the horizontal scale of the depression is about 2000 km, the vertical scale about 10 km, its westward speed of motion about 5° longitude /day. The monsoon depression is an intense closed vortex that has horizontal wind speeds of about 10 to 15 mps and its closed circulation extends to about 9 km in the vertical. A narrow vertical tube of cyclonic vorticity with a horizontal scale≈1000 km and extending up to 9 km is a characteristic feature of this circulation. This depresson formed over the northern part of the Bay of Bengal and dissipated over the Arabian Sea. The vortex has a very well defined cold core in the lower troposphere and a warm core above 500mb. To the west of the cold core there exists a very intense warm core in the lower troposphere. The intense warm core is a con-sequence of advection of desert air by the storm circulation. Vertical motions show rising motion west of i.e., ahead of the trough line and descending motion to the rear. The westward motion of the monsoon depression is related to intense low level convergence in the region of the strongest ascending motion. In this region rainfall rates exceed 10cm/24 hrs during the passage of the disturbance. The analysis and observational structure derived in this study is examined in a second part of this paper, where the dynamics, energetics, and numerical prediction aspects are stressed.