Simulation and Analysis of the Initiation and Mesoscale Structures of a Squall Line occurring within a Meiyu Frontal System in East China

A squall line that formed on the southern flank of a Meiyu front precipitation system in East China is simulated using the Advanced Regional Prediction System (ARPS) at 2 km grid spacing from an initial condition obtained from the ARPS 3DVAR assimilating observations from a dense surface observation network including automated weather stations.  Analyses of the simulation results show that convergence between southwesterly monsoonal flows and northerly flows behind a cold dome produced by on-going mesoscale convective systems within the Meiyu front helps to establish a primary convergence zone that favors convective initiaion along the band, but the exact locations of first convection initiations are determined by the intersepting points between the primary band, and secondary convergence band assocaited with horizontal convective rolls (HCRs) that form in the southwesterly monsoon flow region to the south of the primary band, in a way similar to the interacion between HCRs with a dryline convergence band discussed in earlier studies for the U.S. Great Plains dryline case. Trajectory analyses indicate that the lifting assocaited with primary convergence and the HCR circulations is able to bring near surface parcels to their level of free convection, and this is confirmed by an experiment in which the condesation process and therefore latent heating is turned off.

A sensitivity experiment is performed in which shorwave radiation is turned off in the model. The HCRs ceases to develop in this case, and no convection is initiated along the primary convergence line, providing additional evidence on the essential role pf HCR circulations. These results are interesting because Meiyu season is usually thought to be wet and cloudy, and cold pool and solar insulation tend to be weaker, and therefore both outflow boundaries and HCRs maybe of less importance.