Indian Summer Monsoon Simulations In Successive Generations Of The NCAR Community Atmosphere Model

Abstract

Four generations of the NCAR Community Atmosphere Model (CAM-3, 4, 5, and 6) are evaluated for the Indian summer monsoon (ISM) simulations. Total precipitation simulated by successive versions of CAM is overestimated over the western equatorial Indian Ocean (EIO), Arabian Sea, and Western Ghats (WGs), and underestimated over the eastern EIO, Bay of Bengal (BoB), and Indo-Burmese mountains, due to overestimated convective and underestimated large-scale precipitation, respectively. Overestimation of total precipitation over the Himalayan region is increased in successive CAM variants, primarily through enhanced large-scale precipitation. Improvement in total precipitation simulation over BoB is found in successive CAM variants, with the largest improvement in CAM6. The frequency of total precipitation is overestimated for low precipitation rates and underestimated for high precipitation rates in CAM variants, except in CAM4 for high precipitation rates. The subtropical westerly and tropical easterly jets are better simulated in CAM5-6 than CAM3-4 (highly overestimated). The easterly shear of zonal wind during the peak monsoon is highly overestimated in all CAM variants. The monsoon low-level jet over AS and peninsular India is overestimated in all CAM variants with the largest overestimation in CAM6, resulting in increased precipitation over WGs and peninsular India. We find the large underestimation in a tropical easterly jet over peninsular India and EIO in CAM3 to have improved in the successive versions. In addition, overestimation of the strength of the subtropical westerly jet and the Tibetan anticyclone seen in CAM3 has improved in successive variants of CAM. We find improvements in monsoon intra-seasonal oscillation (MISO) and associated internal dynamics and the east-west and north-south heat source in successive variants of CAM. Overall, we find many improvements in the simulation of ISM precipitation and its associated dynamics in successive variants of CAM, however, there still remain some important biases (e.g., eastward component of MISO, monsoon low-level jet, excessive precipitation over Himalayan foothills, early monsoon onset, etc.) that need to be alleviated for more realistic ISM simulations in future versions of CAM.