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Yoshiaki Shibagaki, Toyoshi Shimomai, Toshiaki Kozu, Shuichi Mori, Yasushi Fujiyoshi, Hiroyuki Hashiguchi, Masayuki K. Yamamoto, Shoichiro Fukao, and Manabu D. Yamanaka

Abstract

Multiscale aspects of convective systems over the Indonesian Maritime Continent in the convectively active phase of an intraseasonal oscillation (ISO) during November 2002 are studied using Geostationary Meteorological Satellite infrared data and ground-based observational data from X-band rain radar, equatorial atmosphere radar, L-band boundary layer radar, and upper-air soundings at Koto Tabang (KT; 0.20°S, 100.32°E; 865 m above mean sea level), West Sumatera, Indonesia. In the analysis period, four super cloud clusters (SCCs; horizontal scale of 2000–4000 km), associated with an ISO, are seen to propagate eastward from the eastern Indian Ocean to the Indonesian Maritime Continent. The SCCs are recognized as envelopes of convection, composed of meso-α-scale cloud clusters (MαCCs; horizontal scale of 500–1000 km) propagating westward. When SCCs reach the Indonesian Maritime Continent, the envelopes disappear but MαCCs are clearly observed. Over Sumatera, the evolution and structure of a distinct MαCC is closely related to the organization of localized cloud systems with a diurnal cycle. The cloud systems are characterized by westward-propagating meso-β-scale cloud clusters (MβCCs; horizontal scale of ∼100 km) developed in eastern Sumatera, and an orographic cloud system formed over a mountain range in western Sumatera. Ground-based observations further revealed the internal structure of the orographic cloud system around KT. A meso-β-scale convective precipitation system with eastward propagation (E-MβCP; horizontal scale of ∼40 km) is found with the formation of the orographic cloud system. This is associated with a low-level wind change from easterly to westerly, considered to be local circulation over the mountain range. The E-MβCP also indicates a multicell structure composed of several meso-γ-scale convective precipitation systems (horizontal scale of <10 km) with multiple evolution stages (formation, development, and dissipation).

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Shuichi Mori, Hamada Jun-Ichi, Yudi Iman Tauhid, Manabu D. Yamanaka, Noriko Okamoto, Fumie Murata, Namiko Sakurai, Hiroyuki Hashiguchi, and Tien Sribimawati

Abstract

The diurnal cycle of rainfall and its regional variation over Sumatera Island, Indonesian Maritime Continent, are examined using Tropical Rainfall Measuring Mission (TRMM) satellite precipitation radar (PR) and intensive rawinsonde sounding data. The TRMM PR sensor can detect raindrops directly, regardless of ground and cloud conditions, and can distinguish between convective and stratiform types of rainfall. Rainfall variation over this area was found to have the following characteristics: 1) convective rainfall with a broad peak between 1500 and 2000 LT predominates over the land region of Sumatera Island, whereas rainfall in the early morning, composed almost equally of stratiform and convective types, is predominant over the surrounding sea region. 2) A rainfall peak in the daytime and one in the nighttime migrate with time starting from the southwestern coastline of the island into the inland and offshore regions, respectively. The distance of each rainfall peak migration from the coastline is up to 400 km, and the average speed of migration is approximately 10 m s−1. 3) Using intensive rawinsonde sounding data, it was also found that remarkable diurnal variations of wind, humidity, and stability appear in the lower troposphere corresponding to the migrating rainfall peaks over both the inland and the coastal sea regions.

The mechanism of the diurnal land–sea rainfall peak migration is discussed comprehensively using TRMM PR, intensive rawinsonde soundings, Geostationary Meteorological Satellite (GMS) data, objective reanalysis, and ground-based observation data. Finally, a crucial difference in rainfall peak migrating mechanisms is suggested between those toward the inland region in the daytime and the offshore region in the nighttime.

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