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This paper describes a diurnal cycle in systematic cloud system migration observed with the GMS IR1 sensor over Sumatera (approximately 1,500 km in length) from May 2001 to April 2002. Convective clouds developed over mountainous areas in the afternoon, and migrated westward and/or eastward for several hundred kilometers (@500 km) from midnight to morning. Westward migration occurred in almost every month except August over southernmost Sumatera Island. Eastward migration occurred when lower-tropospheric winds were westerly and/or when super cloud clusters moved eastward along the Intertropical Convergence Zone (ITCZ), which moves northward and southward with an annual cycle.

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There is a diurnal cycle of systematic cloud migration over Sumatera Island, i.e., cloud systems developing in the mountainous area in the afternoon migrate westward and/or eastward for several hundreds of kilometers (about 500 km) from night to morning. The regional characteristics and internal structure of migratory cloud systems with a diurnal cycle over Sumatera Island during CPEA-I were examined using data from an X-band Doppler radar (XDR), a VHF wind profiler (Equatorial Atmosphere Radar (EAR)), rawinsondes, and Geostationary Operational Environmental Satellite (GOES9).During CPEA-I, the cloud system had a horizontal scale of several hundred kilometers and migrated both westward and eastward over nearly all of Sumatera Island except for the southernmost part. The cloud system migrated only westward over southernmost Sumatera Island during CPEA-I.From a case study on April 17 and 18, 2004, precipitation systems with horizontal scales of several tens of kilometers were observed in a cloud system by XDR, and they migrated in a direction similar to that of the cloud system at a speed of about 3 m s À1 , which roughly corresponded to the wind direction and speed in the lower troposphere. Convective precipitation was observed mainly in the forward region of the precipitation systems, and stratiform precipitation was observed in the rearward region. The convective precipitation successively generated new convective cells in front of old convective cells. These results suggest that the migratory mechanism of the precipitation systems is self-replication of convective cells and the advection of background wind in the lower troposphere.

A remarkable increase in moisture frequently occurred in August afternoon radio soundings in 2001 in the layer up to 1000 m above the mixed layer (ML) near the mountains of western Sumatra, Indonesia. This moisture enhancement was also apparent in the monthly mean diurnal cycle. The mixing ratio is not vertically uniform in this layer, suggesting that turbulent mixing cannot be a major mechanism of the vertical moisture transport. A climatological numerical study using a cloud‐resolving model suggests that thermally‐induced upslope winds converge over the mountain summits during daytime, forming a moist air band along the mountain range. Ambient winds above the mountain range then advect the moist air into the surroundings, moisturizing the air above the ML over the leeward terrain. This mechanism is important for producing diurnal mesoscale precipitation systems over a wide area of the Indian Ocean, as documented by previous studies analyzing TRMM and GMS observational data.