Dynamical characteristics of monsoon circulation and its modelling

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Monsoon is the atmospheric circulation driven by temperature difference between lands and oceans. The dynamics is controlled by 1)density stratification of the atmosphere, 2)differential heating at the lower boundary, 3)diffusion proces of heat and momentum, 4)Coriolis force and 5)seasonal variation of the temperature contrast between lands and oceans. In this study hydrodynamic characteristics of the atmosphere subject to these factors have been investigated by a linear theory and a laboratory experiment. The objective of the linear theory is to determine the horizontal scale of the circulation produced by differential heating. It was found that the horizontal scale is given by N K^<1/2>w^<-3/2>F(f/w), where N is the buoyancy frequency, K is the diffusion coefficient, w is the angular frequency of the thermal forcing and f is the Coriolis parameter. In the laboratory experiment we investigated effects of the rotation to the flow pattern: When the rotation rate is small, the circulation is axisymmetrical without any frontgenesis. When the rotation rate exceeds a critical value, it turns out to be wavy due to barolinic instability. Formation of the cold front takes place in the unstable situation. The behaviour showed remarkable similarity to that of the actual cold front observed in the atmospehre.