A study of South Asian monsoon energetics

Abstract

Monsoon forecasting is one of the most difficult components of the global weather prediction problem. The operational forecasts over the Asian monsoon region are known to have useful skill only for roughly 2-3 days. The rapid deterioration of monsoon forecasts can be attributed to a number of factors such as data deficiencies, physical parameterization in the forecast models, and representation of orography and surface boundary conditions, such as details of sea surface temperature, snow cover, etc. The study of energetics of the model output helps in understanding the above problems. This study is aimed toward the examination of the monsoon energetics. Here it is shown that the use of kinetic energies of the rotational and divergent motions have a special advantage. In order to show these features in the maintenance of a fully developed monsoon, the authors have taken the results of a global model. Such a model is internally consistent and given a reasonable forecast of the motion, thermal, and precipitation fields, the authors believe that such a model-generated dataset can provide useful insights. Direct use of observations and their analysis make it difficult to perform such studies because of data voids and the data errors and inconsistencies. Models tend to produce somewhat more consistent fields during the course of short-range prediction. Although the results obtained contain a model bias, the authors nevertheless performed short-range forecasts with a high-resolution global model and rely only on these if the forecasts appear to be quite reasonable. The Florida State University Global Spectral Model at the resolution T170 (170 waves triangular truncation) was run to carry out several experiments to investigate the issue of the maintenance of the Asian monsoon. In this context the authors examined the issue of the maintenance of the monsoon over a south Asian domain. The computations show that differential heating (i.e., the covariance of heating and temperature) leads to the growth of available potential energy, which is next passed on to the divergent motions via the covariance of vertical velocity and temperature. The final link in this scenario is the transfer of energy from the divergent to the rotational part of the motion field that describes the monsoon. These are largely described by ψ-χ interaction via the covariance of ∇ψ and ∇χ (where ψ is a streamfunction and χ is a velocity potential). It is noted that lateral boundary fluxes are also important for the maintenance of the monsoon. Lateral coupling with the monsoon of southeast Asia and with the Southern Hemispheric circulation are also some of the crucial elements of the overall energetics.