Multiple-scale simulations of the impact of PBL and convection schemes on the initiation of convection in the tropics

Description

In this study, we performed a series of regional simulations based on the Weather Research and Forecasting (WRF) regional model using a wide range of spatial resolutions (2-km, 10-km, and 50-km grid spacing) to identify the weakness of current planetary boundary layer (PBL) and convection parameterizations in capturing the initiation of convection and intra-seasonal variability of precipitation over the Indian Ocean. Two episodes of the Madden-Julian Oscillation (MJO) observed during the ARM MJO Investigation Experiment (AMIE)/Dynamics of the Madden-Julian Oscillation (DYNAMO) field campaign of the boreal winter of 2011 are selected as a case study. The work consists of four distinct parts:

1. We evaluated the spatial pattern of simulated precipitation with different resolutions against Tropical Rainfall Measuring Mission (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., Yonsei University, Mellor-Yamada-Janjic, and University of Washington) and compared the boundary-layer structure and the transition from clear to cloudy conditions within the simulations.
3. We tested the different convective schemes (i.e., Kain-Fritsch versus Zhang-McFarlane and Kain-Fritsch versus Kain-Fritsch cumulus potential) to look at the responses of simulated PBL clouds to those schemes. All simulations conducted for (2) and (3) are free runs (without nudging) done at both 10-km and 50-km grid spacing. The cloud properties and their probability density function (PDF) obtained from the 2-km simulations are used to evaluate the initiation of convection in the 10-km and 50-km simulations.
4. We conducted another set of simulations similar to (2) and (3) but with water vapor nudging. The nudging tendency terms are examined to help identify errors in the gradual moistening and heating processes that are believed to be critical for simulating low-frequency variability of PBL and clouds.

By comparing the results from different spatial resolutions, PBL schemes, convection schemes, and with or without nudging, we can investigate the scale-dependence of modeled PBL structure and initiation of convection in the tropics under an idealized scenario in which moisture supply from the surface is unlimited using different PBL and convection schemes commonly applied in community models.