Volume 86A (2008) Pages 187-203
This study investigates the diurnal variation of convection over western Kalimantan (Borneo) Island and the adjacent seas through analysis of results from surface meteorological observations, satellite observations of convection and precipitation, and numerical experiments with a mesoscale model. Precipitation measurements from the TRMM satellite indicated that rainfall occurs mostly in the afternoon and evening along the coast of the island, and at night and in the early morning over the plains in the central regions of the island and over the sea adjacent to the coast. Results from surface meteorological observations showed that intense shortwave radiation is observed on the western coast at Siantan on most days, even on days with heavy rain. Consequently, sea breezes are evident in the afternoon, whereas land breezes prevail in the early morning. Occasionally, a strong offshore flow is observed by the QuikSCAT satellite over the sea adjacent to the western coast of the island in the early morning, with calm surface winds on the coastal land.
Climate modeling studies for a 1-month period during the region’s rainy season successfully replicated the main features of the observed regional distribution, diurnal variation of surface winds and convection over the western coast of the island and the adjacent seas. The results showed that convections develop across a wide area along the coast of the island at similar times in the late afternoon with the penetration of the sea breeze. The convections cause a rapid drop in temperature in the lower atmosphere, which produces a temperature contrast between the land and the sea, with lower temperatures occurring over the island. In response to the temperature differences, a strong offshore flow occurs over the sea adjacent to the western coast during nighttime, which creates an intensive wind convergence at the low-levels, and initiates convection offshore near the coast at late night and early morning. The results of this study suggest that the strong offshore flow derived principally from convections that develop earlier in the afternoon and evening over the island plays an important role in the formation of the nocturnal rainfall over the sea in the vicinity of western Kalimantan Island.