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A parameterization of turbulent dissipation and pressure damping time scales in stably stratified inversions: Moving beyond Mellor's single master length scale
郭准
中国科学院大气物理研究所
利用气候模式准确的模拟出观测中的低云和辐射分布一直是模拟界的难题之一.特别是对采用“moist turbulence closure”机制的模式而言,同时模拟出“明亮”(高反射率)的海岸层积云(SC)和“暗淡”的浅对流积云(cu)尤为困难.这是因为模拟层积云湍流机制必须削弱云顶处的湍流通量,以减少干空气的卷入;但模拟浅对流又需要产生足够强的次网格温度、湿度扰动.这两个看似“矛盾”的目标分别被两个截然不同的物理过程所控制,即“湍流耗散”(Turbulent dissipation)和“气压脉动”(non-hydrostatic pressure fluctuations),不能被单一的湍流长度尺度机制(turbulent length scale)所表述.因此,我们引入了多湍流尺度机制,并构建了新的“湍流耗散”和“气压脉动”参数化,大幅提高了全球低云的模拟技巧.It is difficult for coarse-resolution global models of the atmosphere to accurately simulate the observed distribution of low clouds. In particular, it is difficult for moist turbulence closure models to simulate sufficiently bright near-coastal stratocumulus (Sc) without simulating overly bright marine shallow cumuli (Cu).To parameterize bright Sc, a turbulence parameterization must damp the turbulent fluxes of heat and moisture above cloud top in order to prevent excessive entrainment of dry air into cloud top. To parameterize dim shallow Cu, the subgrid variances of temperature and moisture must remain large, in order to permit partial cloudiness. However, damping the fluxes but not the variances just above cloud top is difficult if a parameterization uses a single ``master" time scale to damp both. In nature, the above-cloud fluxes are damped by pressure fluctuations, whereas scalar variances are damped by a different process, namely, turbulent dissipation. In a stably stratified inversion above cloud, pressure damping is large but turbulent dissipation is small.To avoid this problem, a multi-time-scale parameterization for damping has been developed. The damping parameterization has been implemented in a global model and evaluated. The parameterization is capable of dimming shallow Cu while producing adequately bright Sc.