1977 Volume 55 Issue 3 Pages 286-300
From the analysis of three medium-scale depressions which formed over the East China Sea during the AMTEX'75, the following characteristic features were drawn: (1) The medium-scale depression was a shallow disturbance with a warm core below 700mb. (2) The depression was thought to generate over the East China Sea. The sea-level cyclogenesis, however, commenced when a pre-existing cyclonic vortex at about 850mb moved over the sea from the continent. (3) The intensification of the system was largest in the lowest layer, and the deepening rapidly decreased with height. (4) The depression formed in the largescale frontal zone. The above-mentioned features suggested warming in the lowest layer was a primary factor for formation of sea-level medium-scale depression. Heat balances in the three layers between 1000mb and 500mb were evaluated by using the equation of thickness tendency. The result of evaluation showed the following three simultaneous dynamical processes were responsible for the formation: (1) warm air advection; (2) upward motion and small adiabatic cooling; (3) apparent heat source in the lowest layer. Sometimes such a shallow system seemed to develop into a strong cyclone. This was attributed to development of a synoptic-scale disturbance rather than to self-development of a mediumscale low.