Dynamical characteristics of monsoon circulation and its modelling

季风是由陆地和海洋之间的温差驱动的大气环流。其动力学过程受大气密度层结、下边界差异加热、热量和动量扩散过程、科氏力和海陆温差季节变化的控制。本文通过线性理论和实验研究了这些因素对大气水动力特性的影响。线性理论的目的是确定由差异加热产生的环流的水平尺度。发现水平尺度由N K^<1/2>w^<-3/2>F（f/w）给出，其中N是浮力频率，K是扩散系数，w是热力强迫的角频率，f是科里奥利参数。在室内实验中，我们研究了旋转对流态的影响：当旋转速率较小时，环流是轴对称的，没有锋生。当自转速率超过一个临界值时，由于气压不稳定性，自转速率呈现出波动性。冷锋的形成是在不稳定的形势下发生的。其行为与实际观测到的大气冷锋有显著的相似性。

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.