The Interaction Between Forced and Instable Convection
强迫流与不稳定流的相互作用

Slantwise convection caused by frontogenesis and moist symmetric instability also can lead to band rain.In virtual atmosphere,it is difficult to separate frontogenesis from moist symmetric instability,because they are associated with each other.In the process of deep moist convection with the characteristics of long duration and large scale,the interaction between forced and instable mesoscale convection become more complex.Based on the numerical method,the interaction between forced and instable convection in the process of the break and rebuilding of equilibrium state is discussed using mesoscale numerical-model output of Meiyu front heavy rain process in China.The instable convection is defined as the convection caused by conditional instability or conditional symmetric instability,and the ascending motion caused by the forced factor except the instable one is named as the forced convection.For moist atmosphere,the atmospheric instable factor is defined by convective condensation heat item.In theory,the ageostrophic omega equation should be used only with convective condensation heat item on the right side of the equation to compute the instable convection.But in practice,the parameterized scheme is used to compute the instable convection,the scheme is not only complex,but also with definite error.The numerical result of sensitivity test is used to resolve the instable convection.There are several kinds of reasons resulting in the forced convection,they are ageostrophic forced effect of atmospheric motion,terrain,environmental thermodynamic force and friction.The whole forced convection caused by all factors could not be computed accurately.So the mesoscale circumfluence caused by ageostrophic force is just considered,the ageostrophic omega equation only with ageostrophic force is used to compute the force convection.The MM5 mesoscale simulation system is chosen to make a numerical experiment for a typical Meiyu-front process standing from 23 to 24 June 1999.Three experiment schemes are designed.In the first scheme,the virtual Meiyu-front process is used as the control experimentation.In the second scheme,the initial atmospheric instability is enlarged,and no change has been made for other factors.In the third scheme,only the initial atmospheric instability is reduced.By using the second and third schemes as sensitivity experiments,the first as control experiment,the computation method is built for the numerical resolution of the instable convection,and the reciprocity of forced and instable convection is discussed.A conclusion is drawn about the interaction of the forced convection and instable convection.The quantity of instable convection is ten times of that of forced convection.If the deep instability in vertical or slantwise direction and the finite-amplitude ascending motion in the forced convection are given,the instable convection will appear in three hours after the production of forced convection,or if the instable convection exists