Vertical Motion in Near - Equatorial Winter Monsoon

Observations from the International Winter Monsoon Experiment (Winter MONEX) have been analyzed to determine the nature of the vertical motion field within mesoscale deep convective systems that occurred during December 1978 over the southern portion of the South China Sea. Vertical motion has been computed using rawinsonde data from a nearly equilateral triangle (*400km on a side) of Soviet research vessels situated off the north coast of Borneo. It has been found that over the South China Sea to the north of Borneo a diurnal cycle of convective activity occurs throughout December. During its mature stage the convection is characterized by a deep (from near 500mb to *l00 mb) lightly-precipitating stratiform cloud layer of mesoscale (*200-400km) dimension. Using triangle-computed vertical velocities and an assumption that the clear environment surrounding clouds is subsiding at a rate just sufficient to offset net radiational cooling to space, it has been determined that within mesoscale clouds upward motion occurs with, on the average, a peak vertical velocity in the very high troposphere near 250mb (10km) of *12cm s-1. Beneath the mesoscale cloud systems downward motion is observed with a peak near 650mb or 3.5km of *3cm s-1. Comparison is made with vertical motion reported in mesoscale convection in other tropical and midlatitude regions. Because the mesoscale cloud systems cover large areas, they probably contribute in an important way to synoptic-scale atmospheric mass, heat and moisture budgets in the near equatorial winter monsoon region.