Sensitivity Analysis of Ensemble Simulations on a Torrential Rainfall Case over South China Using Multiple PBL and SL Parameterizations

KeyWords:PBL  SL  parameterization  torrential rain

Abstract:

      A good representation of the interaction between the planetary boundary layer (PBL) and the surface layer (SL) in numerical models is of great importance for predicting the initiation and development of convection. This study examined an ensemble that consists of the available suites of PBL and SL parameterizations based on a torrential rainfall event over South China. The sensitivity of the simulations was investigated against objective measurements using multiple PBL and SL parameterization schemes. The main causes of the bias from different parameterization schemes were further analysed by comparing the good and bad ensemble members. The results showed that good members tended to underestimate the rainfall amount but presented a decent evolution of mesoscale convective systems that were responsible for the torrential rainfall. Using the total energy mass flux (TEMF) scheme, the bad members overestimated the amount and spatial coverage of rainfall. The failure of the bad member was due to a spurious convection initiation (CI) resulting from the overestimated high-θe elevated air. The spurious CI developed and expanded rapidly, causing intensive and extensive rainfall over South China. Consistent with previous studies, the TEMF scheme tends to produce a warmer and moister PBL environment. The detailed sensitivity analysis of this case may provide reference for the operational forecast of rainfall over South China using multiple PBL and SL parameterizations.