Qi (Steve) Hu
Date of this Version
This study aims to analyze what processes are mainly responsible for nocturnal convective precipitation during the 1991-2000 period for May-June-July over the Great Plains. Firstly, based on the Weather Research and Forecasting model (WRF) coupled with the NCAR Community Land Model (CLM) and the North American Regional Reanalysis (NARR) reanalysis data, the simulations of the diurnal cycles of LLJ and the convective precipitation are examined. Then, the LLJ-related moisture transport is evaluated since the moisture supply is critical for the development of the heavy rainfall. Results show that the WRF model fails to simulate the nocturnal peak rainfall shown in the reanalysis data. The failure in simulating nocturnal maximum precipitation is related to the bias occurring in the modeled nighttime moisture flux divergence/convergence, which suggests that the nocturnal peak convective precipitation may the result of the large-scale processes.
To further figure out the mechanisms controlling the nighttime convection, based on the composite analysis, three 10-case groups are classified: (a) cases that the WRF cannot simulate the timing of the nocturnal peak convective precipitation; (b) cases that the timing of the nocturnal maximum rainfall can be simulated in the model; (c) cases that the maximum precipitation is shown during the afternoon hours in both the NARR data and the WRF model. The comparisons among the three groups suggest that the suppression of daytime convective precipitation and the favoring of nighttime convective precipitation over the Great Plains is closely related to the diurnal cycle of zonal circulation, which is induced by the mountain-plain baroclinic instability. In addition, during nighttime, the intensified anomalous southerly wind, together with the southward forcing above, may have a positive effect on the maintenance of the mountain-plain circulation.
Advisor: Qi (Steve) Hu