Over estimated topographic diabatic heating in NCAR CAM simulations during the Indo-Asian monsoon season

Abstract:

This study investigates total diabatic heating on the southern slopes of the Himalayan Mountains and Tibetan Plateau and its role in Indo-Asian monsoon (IAM) in climate models and reanalysis products. Here we compare the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 4 (CAM4) simulations across a range of horizontal resolutions from low (2) to high (1/4) with various atmospheric only models from the Coupled Model Intercomparison Project version 5 (CMIP5), and reanalysis data products. We find that an excessive orographic diabatic heating and precipitation exists in CAM4 simulations with commonly used resolutions during the IAM summer, and is inconsistent with Modern Era Retrospective-analysis for Research and Applications (MERRA) and Japanese 55-year Reanalysis (JRA55). This bias is not found in the Geophysical Fluid Dynamics Laboratory Model with Community Model version 3 (GFDL-CM3) model, while Community Earth System Model with Community Atmospheric Model version 5 (CESM-CAM5) and Max Planck Institute Earth System Model low resolution (MPI-ESM-LR) continually show strong diabatic heating over topography. However, enhancing model grid-resolution in the NCAR CAM4 to 1/4 has the potential to mitigate the heating bias. This results in a smoother zonal mean precipitation rate across Southern Asia, rather than having two distinct convective domains that are seen in lower resolution version of CAM.