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Problems Associated with Modelling South Asian Monsoon Rainfall

Paper Reviewed
Sabeerali, C.T., Rao, S.A., Dhakate, A.R., Salunke, K. and Goswami, B.N. 2015. Why ensemble mean projection of south Asian monsoon rainfall by CMIP5 models is not reliable? Climate Dynamics 45: 161-174.

Sabeeralie et al. (2015) begin their instructive paper by noting that the Indian summer monsoon rainfall (ISMR) is a "lifeline for more than one-fifth of the world population living in the area where the seasonal mean (June-September) rainfall strongly correlates with the gross domestic product (GDP) as well as the total food production of the country," as described by Gadgil and Gadgil (2006). And, therefore, they go on to state that the "projection of south Asian monsoon rainfall several decades into the future is very important for policy makers for long-term policy on economy as well as water resource management."

Aware of these facts, the five Indian researchers proceeded to diagnose any inconsistencies that may exist between CMIP5 model projections of ISMR and what has actually been observed in the real world. And in doing so, they discovered that (1,2) "most models produce too much (too little) convective (stratiform) precipitation as compared to observations," that (3) there is "a strong local moisture-precipitation relation in many models as compared to observation," that (4) the "correlation between the atmospheric moisture and precipitation is much higher in all models as compared to observations," that (5) "most of the atmospheric moisture content produced by the models immediately converts to precipitation even though the large-scale thermodynamic index in the model weakens," that (6) "these models tend to produce unrealistic local convective precipitation often not in turn with other large-scale variables," that (7) "all 16 models produce excessive convective precipitation relative to stratiform precipitation compared to that in observations," that (8) "this is a generic problem in all current parameterizations of convection in models," that (9, 10, 11) "cloud microphysics is generally poorly formulated in almost all models leading to serious biases in simulating ice, snow, grupel, etc.," and that (12, 13) "this can lead to improper vertical distribution of heating and improper organization of convection."

And so it is, unfortunately, that Sabeerali et al. conclude their paper by stating that their results "highlight the need to improve the convective parameterization schemes in coupled models for reliable projections of the ISMR." How true!

Gadgil, S. and Gadgil, S. 2006. The Indian monsoon, GDP and agriculture. Economic and Political Weekly 41: 4887-4895.

Posted 8 September 2015