Mechanisms related to the diurnal cycle of tropical deep convection over a complex terrain were investigated in the Bandung basin, West Java, Indonesia. Observational data were analyzed from X-band radar, Global Navigation Satellite System (GNSS) receivers, and radiosondes, in conjunction with high-resolution numerical model data.

 Significant diurnal variation of GNSS-derived precipitable water vapor (PWV), which peaked in the early evening, was observed from 13 to 19 March 2013. During this period, the X-band radar detected convective initiation at approximately 1200 local time over the southern slope of the basin. A 2-km-mesh model successfully simulated the observed diurnal variations of PWV and rainfall from 15 to 17 March 2013. In the model, moist air was present at the bottom of the basin early in the morning, which was transported to the southern slope of the basin by valley wind circulation after sunrise. In contrast, humidity was lower in the northern part of the basin due to a downward circulating valley wind. The valley wind decreased static stability around the southern slope of the basin by transporting moisture. It also caused a low-level wind convergence, resulting in convective initiation on the southern slope of the basin. The GNSS receiver network also recorded this simulated water vapor variability associated with the valley wind.

 These results suggest that water vapor in the bottom of the basin during mornings and its advection by the valley wind strongly influences convective initiation in Bandung.