Multiple-scale simulations of the impact of PBL and shallow convection schemes on the initiation of convection over the tropical ocean

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Description

The goal of this study is to identify the weakness of current Planetary Boundary Layer (PBL) and convection schemes in capturing the initiation of convection and intra‐seasonal variability of precipitation and transfer our better understanding in those processes into improved parameterizations. We performed a series of regional simulations with the WRF model using a wide range of spatial resolutions (i.e. 2 km, 10 km, and 50 km grid spacing), PBL schemes and shallow convection schemes over the Indian Ocean. Two episodes of the Madden-Julian Oscillation observed during the AMIE/DYNAMO field campaign of the boreal winter of 2011 are selected as a case study. The work consists of four parts: 1) we evaluated the spatial pattern of simulated precipitation with different resolutions against TRMM data, as well as the simulated vertical profiles of cloud, humidity, and temperature against observations collected during AMIE, 2) we conducted simulations with three different PBL schemes (i.e. YSU, MYJ and UW) and compared the boundary layer structure and the transition from clear to cloudy conditions within the simulations. 3) we compared the different convective schemes (i.e. KF vs. KF-CuP) to look at the responses of simulated PBL clouds to the different shallow convection schemes, 4) we conducted another set of simulations similar to (2) and (3) but with nudging applied to the simulated water vapor. The nudging tendency terms are examined to help identify errors in the observed gradual moistening and heating processes that are believed to be critical for simulating low frequency variability of PBL and clouds in the tropics. Results show that all of the parameterizations tested tend to overpredict precipitation when compared to TRMM and surface radars and have a wet bias, and consequently larger moist static energy within the PBL. By analyzing the water vapor nudging tendency and PBL tendency, we find the vertical transports of water vapor are too active in all PBL schemes and moisture nudging tends to suppress the initiation of convection and reduces the overpredicted precipitation. We are conducting another set of free-run simulations by tuning a few key parameters to suppress the moisture vertical transport to ensure a more reasonable simulation for precipitation and convection.