Evaluation of mean and intraseasonal variability of Indian summer monsoon simulation in ECHAM5: identification of possible source of bias
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The performance of ECHAM5 atmospheric general circulation model (AGCM) is evaluated to simulate the seasonal mean and intraseasonal variability of Indian summer monsoon (ISM). The model is simulated at two different vertical resolutions, with 19 and 31 levels (L19 and L31, respectively), using observed monthly mean sea surface temperature and compared with the observation. The analyses examine the biases present in the internal dynamics of the model in simulating the mean monsoon and the evolution of the boreal summer intraseasonal oscillation (BSISO) and attempts to unveil the reason behind them. The model reasonably simulates the seasonal mean-state of the atmosphere during ISM. However, some notable discrepancies are found in the simulated summer mean moisture and rainfall distribution. Both the vertical resolutions, overestimate the seasonal mean precipitation over the oceanic regions, but underestimate the precipitation over the Indian landmass. The performance of the model improves with the increment of the vertical resolution. The AGCM reasonably simulates some salient features of BSISO, but fails to show the eastward propagation of the convection across the Maritime Continent in L19 simulation. The propagation across the Maritime Continent and tilted rainband structure improve as one moves from L19 to L31. The model unlikely shows prominent westward propagation that originates over the tropical western Pacific region. L31 also produces some of the observed characteristics of the northward propagating BSISOs. However, the northward propagating convection becomes stationary in phase 5–7. The simulation of shallow diabatic heating structure and the heavy rainfall activity over the Bay of Bengal indicate the abundance of the premature convection-generated precipitation events in the model. It is found that the moist physics is responsible for the poor simulation of the northward propagating convection anomalies.
KeywordsIndian summer monsoon Boreal summer intraseasonal oscillation Vertical resolution AGCM Moist processes
IITM, Pune is fully funded by the Ministry of Earth Sciences (MoES), Govt. of India, New Delhi. We would like to thank GSFC/DAAC, NASA for providing MERRA reanalysis, and GPCP dataset and India Meteorological Department (IMD), New Delhi for gridded rainfall dataset. The authors are grateful to Max-Planck Institute for Meteorology, Hamburg, Germany for providing ECHAM5 AGCM. The work is a part of AS’s Ph.D. dissertation, financially supported by Council of Scientific and Industrial Research (CSIR), Govt. of India. AS would like to acknowledge Dr. Xianan Jiang for EEOF analysis, Sharmila S. for helpful discussions. The authors also acknowledge two anonymous reviewers for their constructive comments, and Dr. A. Hazra and Pharthiphan A. for their support during the installation of the model in IBM P6.
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