Usefulness of Stochastic Entrainment Rate in the Simulation of South Asian Summer Monsoon

The formulation of the atmospheric mixing process or entrainment in the deep convective parameterizations used in the atmospheric component of models is highly sensitive to the model simulations. We introduced stochastic entrainment in the deep convection scheme of NCAR CAM5 and analysed the advances in model simulation, particularly on the South Asian Summer Monsoon (SASM), as compared to the default version of the model's deterministic entrainment formulation. Multiple metrics linked with the SASM showed that simulations using stochastic entrainment (StochCAM5) outperformed simulations using the default model (DefCAM5). Some of the long-standing SASM biases seen in DefCAM5, such as precipitation pattern and magnitude over the Arabian Sea and western Equatorial Indian Ocean, early monsoon withdrawal, and the overestimation of light precipitation and underestimation of large-to-extreme precipitation, were significantly alleviated by StochCAM5. Stoch-CAM5 improves related SASM dynamical and thermodynamical characteristics including the Somali Jet, low-level westerly winds, and meridional tropospheric temperature gradient (MTTG). StochCAM5 also improves the modelling of monsoon intra-seasonal oscillation (MISO), Madden Julian Oscillation (MJO), and equatorial Kelvin waves.