Handout (15.6 MB)
Past studies have mainly attributed the mid-night peak of SB to the mountain-basin solenoidal circulations which suppress daytime and promote nighttime precipitation. This study finds that apart from the solenoidal effects, prominent diurnal inertial oscillations of boundary layer south-southwesterly low-level jet (LLJ) into the basin appear to play at least as important roles in modulating the diurnal cycles of precipitation on SB. When the perturbation winds associated with the inertial oscillations turn towards the basin at midnight, the enhanced LLJ advect much more moist air into the basin and also enhance flow convergence within the basin, forcing strong precipitation. In the afternoon, the perturbation winds act to ‘pull’ air out of the basin. A basin-wide moisture budget analysis reveals that the moisture flux from the southeast side of the basin dominates over the net moisture flux into the basin, suggesting the crucial role of the enhanced nocturnal LLJ in the formation of nocturnal precipitation. In addition, the LLJ is located at the right side of a mesoscale vortex over the SB and helps increase the vortex intensity and favor more precipitation.
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