Rainfall enhancement of band-shaped convective cloud system in the downwind side of an isolated island

Abstract

During the Baiu period in 1996 the enhancement of rainfall in a slow southward-moving band-shaped convective cloud system was observed in the downwind side of Yaku-shima Island by the PPI-radar of Tanega-shima Meteorological Observatory and in some cases the dual-Doppler radar system of Nagoya University. Common atmospheric conditions in these cases were strong low-level west-southwesterly winds and stability which led to a Froude number between 0.3 and 0.7. Mainly on the basis of a case study on the radar-echo band observed on June 22 and numerical simulation on airflows modified by the orographic effect of Yaku-shima Island, the orographic enhancement of rainfall in the downwind side of the island is considered to have been caused by the combination of the following processes. (1) Convective clouds in the slow southward-moving system developed in the zone of low-level horizontal convergence intensified due to winds modified by the orographic effect of the island and they produced intensive rainfall in the downwind side of the island. (2) New convective clouds formed and developed in the zone of low-level horizontal convergence over the sea north of Yaku-shima Island, which was caused by the confluence of ambient winds around the island and winds modified by its orographic effect. The line of convective clouds which formed to the southeastern side of the slow southward-moving system mentioned in (1) produced rainfall in its downwind side. (3) In the slow southward-moving system, precipitation particles in the new cloud line are provided to convective clouds by ambient winds which turned around the southern end of the island. This process was observed as the merging of the two band-shaped radar-echoes and would have caused the efficient formation of precipitation in the downwind side of the island.