60 The Simulation and Mechanisms of the Diurnal Variation and Nocturnal Peak of Warm Season Precipitation over Sichuan Basin, China

Monday, 8 January 2018
Exhibit Hall 3 (ACC) (Austin, Texas)
Yuehan Zhang, Nanjing Univ., Nanjing, China; and M. Xue, K. Zhu, and B. Zhou

Handout (15.6 MB)

The Sichuan Basin (SB), located off the eastern slope of the Tibetan Plateau, is one of the rainstorm centers in China. Many precipitation systems also move off SB and propagate and affect the central and eastern parts of China. The precipitation of SCB also exhibits pronounced diurnal cycles, producing a minimum amount of rain in the afternoon, and a prominent peak around mid-night. This study investigates the mechanism of mid-night peak using dense rain-gauge observations and forecasts from three summer months of 2013 at a 4-km convection-permitting resolution. Model forecasts reproduce the observed spatial and temporal distributions and diurnal peak over SCB near midnight (0100-0200 LST) very well.

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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