Sensitivity Of A Simulated Squall Line During SCMREX To Parameterization Of Microphysics

A squall line on 22 May 2014 during the Southern China Monsoon Rainfall Exper-iment period is simulated with various bulk microphysical parameterizations using the Weather Research and Forecasting model.Although most BMPs are able to capture the basic features of the squall line,movement,morphology and especially the length of the simulated squall lines differ significantly among different BMPs.Morrsion scheme tends to simulate a slower moving squall line due to the weaker cold pool and better trailling stratiform cloud and precipitation due to the smaller snow fall speed used than WSM6 and WDM6 schemes.In general,extending single moment to double moment in warm-phase categories does not impact the performance of ice-phase particles in this case.Assuming hail or graupel in different schemes could result in different melting profiles of rimed-ice particles,with hail melted substantially below the melting layer and contributing to the cold pool production significantly in WDM6 because of the use of larger fall speed for hail.In contrast,graupel mostly presents above the melting layer in other schemes using graupel because of the small fall speed and faster melting of graupel.Although ice-phase particles simulated differ significantly among these schemes,further analysis reveal that it is rain evaporation parameterization dominates the cold pool generation and maintenance,which is mainly controlled by the size of simulated rain drops in this case.Stronger rain evaporation generally contributes to stronger cold pools and thus faster movement and longer simulated squall lines.Failure of the SBU-YLIN scheme in capturing the squall line was identified to be related to the turnoff of rain evaporation once environmental rel-ative humidity is larger than 90%.With modifications of rain evaporation calculation and saturation adjustment method,the scheme is able to simulate this squall line reasonably well.The study suggests that rain evaporation is critical for the simulation of the length of the squall line,with stronger rain evaporation generally leading to longer squall lines.A better simulation of the squall line length and trailing stratiform precipitation require both realistic ice microphysical processes and rain evaporation parameterization.