Added value of a high-resolution regional climate model in simulation of intraseasonal variability of the South Asian summer monsoon

2017-06-06

By J. Karmacharya, M. New, R. Jones, R. Levine • 2017

The South Asian summer monsoon (SASM) exhibits large variability in the intraseasonal scale, and active and break cycles of monsoon constitute dominant mode of this variability. This has been subject of many model-based studies as improved simulation of intraseasonal features also leads to better representation of the seasonal mean characteristics. We evaluate a recent Hadley centre regional climate model's performance, at low and high resolutions and forced by a reanalysis, in simulating various characteristics associated with intraseasonal variability of SASM. In particular, we compare the spatial patterns of precipitation and upper level circulation composites for active and break spells and, timing, frequency and duration of those spells. We found improvements in simulation of active and break composite precipitation in the high-resolution simulation. These improvements probably come from improved position of the monsoon trough particularly over west India for active composite and improved low-level flow particularly south of the Himalaya for break composite. Moreover, enhanced capacity of the model at high resolution to resolve atmospheric motions and interaction of moisture laden low-level flow with the steep Himalayan orography is likely to contribute in reduction of excess precipitation over Indo-Gangetic plain. Similarly, improvements over east Nepal for break spells are likely to come from model's ability to effectively capture precipitation enhancements that arise from orographic-forced and mid-tropospheric ascending motions. However, mixed results are obtained for temporal statistics associated with occurrence of active and break events. We also compare the model performance in simulating precipitation extremes over Nepal. The timing of extreme precipitation occurrence in relation to peak monsoon months and break spells, and contribution of the extreme cumulative precipitation to the seasonal total are improved in the high-resolution simulation.

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