Doppler Radar Observation on the Orographic Modification of a Precipitating Convective Cloud in its Landing

Abstract

The orographic modification of a precipitating convective cloud, which formed in association with typhoon 9304 and travelled over the sea, was studied by mainly analyzing the dual-Doppler radar system observational data. From July 24 1900 LST, to July 25 0000 LST, many conective clouds landed at the southeastern coast of the Kii Peninsula. Although they reached the coast in various stages of development, most of them showed similar features. Their radar-echoes were intensified 10 to 20km off the coast before landing, and the radar-echoes were intensified again or broadened over land after their radar-echo intensities were reduced near the coast. Before the first intensification of convective radar-echoes over the sea, the echoes were intensified in their upper rear-parts 30 to 40km off the coast, and intensified in their front-parts near the coast. By averaging horizontal winds derived from the dual-Doppler radar system observational data for about two hours, it was shown that the ambient wind speed decreased near the coast, and the horizontal convergence zone larger than 10^-4s^-1 existed along the coast line about 10km off the coast. On the basis of these observational results the travelling convective clouds orographic modification, and the efficient formation of precipitation in them were discussed, taking into account the results of numerical experiment about the Kii Peninsula orographic effect on ambient winds. It can be inferred that the large rainfall amount recorded in association with the typhoon around the Kii Peninsula coast was caused as a result of the integration of travelling convective clouds orographic modification.