Winter Monsoon Convection in the Vicinity of North Borneo. Part I: Structure and Time Variation of the Clouds and Precipitation

Abstract

Radar and satellite observations in the vicinity of northern Borneo obtained during the International Winter Monsoon Experiment (WMONEX) showed that the convection in that region underwent an extremely regular diurnal cycle. Over the sea to the north of Borneo, the general level of convective activity was increased during monsoon surges and during the passages of westward-propagating near-equatorial disturbances. Convective activity was decreased during monsoon lulls. The diurnal cycle was well-defined, regardless of whether, the general level of convective activity was enhanced or suppressed by synoptic-scale events. The cycle of convection over the sea was especially well documented. It was typically initiated at about midnight when an offshore low-level wind began. Where this wind met the monsoonal northeasterly flow, usually just off the coast, convective cells formed. After midnight, the convection continued to develop and by 0800 LST it had evolved into an organized mesoscale system with a precipitation area often continuous over a horizontal distance of 200 km. The structure of this system resembled that of squall lines and other organized mesoscale systems observed in the tropics. The precipitation was composed partially of convective cells, but a considerable portion was also stratiform with a well-defined melting layer extending across much of the system. This precipitation fell from a large mid-to-upper level cloud shield. The mesoscale systems typically began dissipating at midday, when the offshore wind reverted to an onshore wind and low-level convergence became concentrated over land.