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 pro?ler (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-1, 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.
2009 by Meteorological Society of Japan