Diurnal variation of convective activity and precipitable water over the semi-basin - Preliminary study on the mechanism responsible for the evening convective activity maximum

The relationship between the diurnal variation of convective activity and precipitable water was examined in summer seasons using water vapor radiometer, C-band radar and radiosonde data. Convective activity over the semi-basin exhibited a diurnal variation with dual peaks at 15-17 JST and 19-20 JST, and the SSI (Showalter Stability Index) decreased from morning to evening due to the increase of water vapor in the lower layers associated with a thermally induced local circulation. When precipitable water exhibited a pronounced diurnal variation, the evening convective activity maximum (19-20 JST) was evident and cumulonimbus clouds had a tendency to develop over a limited region south and east of Mt. Haruna. On average the convective activity maximum occurred about an hour before the precipitable water maximum, and when precipitable water reached a maximum at an early (late) time, cumulonimbus clouds also formed at an early (late) time. Furthermore, the mechanism responsible for the evening convective maximum over the semi-basin is discussed, based on the results of the diurnal variation of convective activity and precipitable water.