Sensitivity of cloud invigoration/suppression effects during Indian summer monsoon to model resolution in a global climate model
Authors: Sharma Puneet , Ganguly Dilip 
Published: March 2024

Tropical convection is usually associated with processes having high spatio-temporal variability at the scale of individual clouds to the cloud clusters. Deep convective clouds (DCCs), which result from organized convection contribute to Earth&#8217;s hydrological cycle in a major way. Previous studies based on observations and mesoscale/cloud resolving model results suggest that injecting aerosols into DCCs can have both suppressing or invigorating effects. However, very few studies have used a global climate model (GCM) to study this effect. Since the interaction between clouds and aerosols is strongly connected with large-scale circulation, the study of aerosol-induced cloud invigoration (AIVe) warrants the use of GCMs to account for any non-linear relationships in AIVe. In this work, we attempt to investigate and understand the capability of the atmospheric component of the Community Earth System Model, which is the Community Atmosphere Model version 5 (CESM-CAM5) in simulating AIVe during the Indian summer monsoon season (ISM). The AIVe signatures are examined for mid/high level deep convective clouds using the aerosol and cloud parameters from CESM-CAM5 and satellite observations for a period of 4 years (2005&#8211;2008) over the ISM region, which is the central India region for summer monsoon months June&#8211;July&#8211;August&#8211;September (JJAS). Three sets of AMIP style simulations with a horizontal resolution of 2-degree, 1-degree, and 0.5-degree are carried out in order to ascertain the impact of model resolution on simulating AIVe. We chose the August month for our analysis, as during the August month, a strong break phase leading to buildup of aerosols due to suppressed rainfall is seen in both observations and model simulations, creating a testbed to understand AIVe in the presence of DCCs. A significant improvement in simulated Aerosol Optical Depth (AOD) and precipitation over central India and the Indo-Gangetic plain is seen in case of 0.5-degree simulation compared to 1- and 2-degree simulation when compared to Moderate Resolution Imaging Spectroradiometer (MODIS) and Indian Meteorological Department (IMD) observations, respectively, indicating improved simulation of aerosol lifecycle and cloud processes at higher resolution. Further, we note a strong linear relationship between aerosol and cloud parameters in MODIS-CERES observations, which is a strong signature of AIVe over ISMR. Although the simulated frequency of DCCs is similar in all three simulations, we notice a difference in aerosol-cloud parameter relationships over ISMR between the three model simulations. As the model resolution increases from 2 degree to 1 degree and further to 0.5 degree, a smooth transition effect from invigoration to suppression emerges as we go from a low aerosol to a high aerosol regime, which suggests a sensitivity of AIVe to model resolution. Further investigation revealed the presence of a high amount of dust aerosols, leading to invigoration/suppression transition effects signaling the sensitivity of DCCs to initial aerosol concentration and size distribution. More results with greater detail will be presented.