Report 12:  Simulation of the Indian and East-Asian Summer Monsoon in the ECMWF Model: Sensitivity to Horizontal Resolution

Sperber, Kenneth R., Sultan Hameed, Gerald L. Potter and James S. Boyle
November 1993, 37 pp. 

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The ability of the ECMWF model (cycle 33) to simulate the Indian and East Asian summer monsoon is evaluated at four different horizontal resolutions: T21, T42, T63, and T106. Generally, with respect to the large scale features of the circulation, the largest differences among the simulations occur at T42 relative to T21. However, on regional scales, important differences among the high frequency temporal variability serve as a further critical test of the model's ability to simulate the monsoon. More generally, the results indicate the importance of evaluating high frequency time scales as a component of the climate sytem.

T106 best captures both the spatial and temporal characteristics of the Indian and East Asian Monsoon, while T42 fails to correctly simulate the sequence and development of synoptic scale milestones that characterize the monsoon flow. In particular, T106 is superior at simulating the development and migration of the monsoon trough over the Bay of Bengal. In the T42 simulation, the development of the monsoon occurs one month earlier than typically observed. At this time the trough is incorrectly located adjacent to the east coast of India which results in an underestimate of precipitation over the Burma/Thailand region. This early establishment of the monsoon trough affects the evolution of the East-Asian monsoon and yields excessive preseason rainfall over the Mei-yu region. EOF analysis of precipitation over China indicates that T106 best simulates the Mei-yu mode of variability associated with an oscillation of the rainband that gives rise to periods of enhanced rainfall over the Yangtze River Valley. The coarse resolution of T21 precludes simulation of the aforementioned regional scale monsoon flows. (pdf file)

UCRL-MI-123395