Mesoscale Convective Systems Represented in High Resolution E3SMv2 and Impact of New Cloud and Convection Parameterizations
Mesoscale convective systems (MCSs) consist of an assembly of cumulonimbus clouds on scales of 100 km or more and produce mesoscale circulations. As the largest form of deep convective storms, MCSs contribute to 30% – 70% of annual and warm-season rainfall in the U.S. and in the global tropics. Since MCSs contribute importantly to mean and extreme precipitation in the U.S. and many other regions around the world, understanding how well they are simulated by the recently released U.S. Department of Energy (DOE) Energy Exascale Earth System Model version 2 (E3SMv2) and impact of several new developments in cloud and convection parameterizations for E3SMv3 on MCS may guide future development towards more skillful modeling of convective storms and associated hydrologic impacts.
This study indicates the challenge in simulating MCSs at a scale that MCSs cannot be fully resolved. Even at ~ 25km grid spacing, the simulated MCS precipitation is substantially underestimated in E3SMv2 due to insufficient MCS genesis and rain rate in individual MCSs. The newly developed cloud and convection parameterizations developed for E3SMv3 show little improvement in the simulation of MCSs. The future direction of improving MCS simulation in E3SM should involve both increasing model resolution to better resolve key dynamical processes and improving model physics to better represent MCSs.
In this study, we evaluate MCS simulations in E3SMv2. E3SMv2 atmosphere model (EAMv2) is run at ~25km horizontal resolution. We track MCSs consistently in the model and observations using the PyFLEXTRKR algorithm, which defines MCSs based on both cloud top brightness temperature (Tb) and surface precipitation. Results from using only Tb to define MCSs are also discussed to understand the impact of different MCS tracking algorithms on MCS evaluation and provide additional insights into model errors in simulating MCSs. Our results show that EAMv2 simulated MCS precipitation is largely underestimated in tropical and extratropical regions. This is mainly attributed to the underestimated MCS genesis and underestimated precipitation intensity in EAMv2. Comparing the two MCS tracking methods, simulated MCS precipitation is increased if MCSs are defined with only cloud top Tb. The Tb-based MCS tracking method, however, includes cloud systems with very weak precipitation. This illustrates the model issues in simulating heavy precipitation even though the convective cloud shield is overall well simulated from the moist convective processes. Furthermore, sensitivity experiments are performed to examine the impact of new cloud and convection parameterizations developed for EAMv3 on simulated MCSs. The new physics parameterizations help increase the relative contribution of convective precipitation to total precipitation in the tropics, but the simulated MCS properties are not significantly improved. This suggests that simulating MCSs still remains a challenge for the next version of E3SM.