Sensitivity of the Indian summer monsoon rainfall and its interannual variation to model time step

The sensitivity of the Indian summer monsoon rainfall (ISMR) and its interannual variation (IAV) to model time step is investigated using NCAR-Community Atmosphere Model version 3 (CAM3). A set of multiyear numerical experiments is performed using the atmospheric model inter-comparison project (AMIP) protocol with observed sea surface temperature (SST). The default value of time step for 64 × 128 horizontal resolution with semi-Lagrangian dynamical core is 60 min. The model overestimates the mean and underestimates the standard deviation. The mean and standard deviation of ISMR systematically decrease with decrease of time step size. With respect to observations, the mean becomes more reasonable but standard deviation becomes less reasonable. There is a decrease in precipitation over the Saudi Arabia, Maritime Continent, and northwestern Arabian Sea with decrease in time step, while over the Eastern Indian Ocean, Eastern Arabian Sea, and Eastern Bay of Bengal there is an increase in precipitation. The pattern correlation of precipitation with observation systematically increases with decrease of time step. In regard to the IAV of ISMR, simulation with 20 min time step outperforms the other time steps i.e. 60, 40, 30, and 05 min. When it is decreased to 20 min, the model bias in precipitation climatology is reduced and the low-level westerly jet over the Indian peninsular becomes more realistic. There is an overall improvement in the climatology of rainfall and winds in the vicinity of Indian summer monsoon region with 20 min time step.

Research Highlights
► The Indian summer monsoon rainfall (ISMR) and its interannual variation (IAV) is sensitive to model time step.
► The mean and standard deviation of ISMR monotonically decrease with decrease of time step size.
► The pattern correlation of precipitation with observations systematically increases with decrease of time step.
► In regard to the IAV of ISMR, simulation with 20 min time step outperforms the other time steps i.e. 60, 40, 30, and 05 min.
► Based on the overall performance, 20 min time step is a reasonable choice for NCAR CAM3 at 64 × 128 resolutions.