Investigation Of The Mesoscale Predictability And The Convective Initiation Of An Extreme Rainfall Along A Mei-yu Front: An Ensemble Approach

Forecast uncertainties and physical mechanisms of a quasi-linear-extreme-rain-producing mesoscale convective system(MCS) along a mei-yu front in Eastern China during the midnight-to-morning hours of 8 July 2007 are studied using ensembles of 24-h convection-permitting simulation with the nested grid spacing of 1.11 km. The simulations reveal a very strong sensitivity to uncertainties in the initial state despite the synoptic environment favorable for heavy rainfall production. Linear changes of a less skillful member’s initial state toward a skillful member’s lead to monotonic improvement of the precipitation simulation with the most significant contribution from changes in the moisture field. The sensitivities to physics parameterizations representing subgrid-scale processes fail to account for the larger simulation errors(missing the MCS) with the physics variation examined, but could result in large spread in the location and amount of accumulative rainfall. The sensitivities to the planetary boundary layer(PBL) and land surface processes of the precipitation simulation are attributed to the differences of the simulated moisture and temperature profiles in the PBL caused by the different physics schemes, which leads to different convective available potential energy(CAPE) of the environmental inflow air to support the development of the rainy storm. A robust feature of the best-performing members that reasonably simulate the MCS associated heavy rainfall is the presence of a cold dome ahead of the mei-yu front generated by previous convection. The cold dome helps the nocturnal convective initiation by lifting the high-e? air in the southwesterly flow to its level of free convection. The skillful members reproduce the convective backbuilding and echo-band training processes that are observed during this and many other heavy rainfall events over China. In contrast, the less skillful members that miss the development of the MCS either do not simulate the previous convection or produce a cold dome that is too shallow to initiate the MCS.