Development of convective clouds on August 24, 2009 in northern Kanto, Japan, was investigated using stereo photogrammetric analysis and Ka-band and X-band radars, in order to clarify the behavior and structure of convective clouds developing around a mountainous area. Convective activity was divided into three stages based on the spatial distribution of X-band radar echoes: stage 1 with no echo (1006-1200 JST), stage 2 with echoes limited to the mountainous area (1200-1400 JST), and stage 3 with echoes developing over the plain (1400-1600 JST). During stages 1 and 2, the convective clouds (echoes) initiated over the mountains, and then moved toward the foot of mountains and dissipated, repeatedly. During stage 3, convective echoes formed over the mountains moved to the plain without dissipating at the foot of the mountains. In stage 2, new convective echoes tended to form to the rear (upshear) side of pre-existing echoes, while in stage 3, some new cellular echoes formed to the front (downshear) side of pre-existing echoes. Specific humidity in the mountains increased in stage 1, while GPS-derived precipitable water at the foot of the mountains increased during stages 2 and 3. The relationship between the behavior of convective clouds and the transport of water vapor by local wind circulation is discussed.

It is well known that rainfall rate is enhanced over mountains due to orographic uplifting. It would be beneficial to rainfall nowcasting if the intensity of the orographic enhancement could be estimated using simple parameters. In the present study, we found a clear relationship between orographic enhancement of the rainfall rate and the movement speed of radar echoes in a case study of rainfall over mountains in the southwestern area of the Kanto District in Japan during Typhoon 0709, by using rainfall data derived from X-band polarimetric radar. The increasing rate of rainfall rate per unit altitude ðDR=DHÞ showed a positive correlation ðr ¼ 0:95Þ with the movement speed of radar echoes ðV Þ when V > 10 m s À1. Such a correlation suggests that V is an e¤ective parameter for quickly estimating the orographic enhancement of rainfall, however, more case studies are required before it can be used in practical applications.