大对流有效位能和条件不稳定下地形降水的三维理想数值研究

An idealized three-dimensional numerical study of the topographic precipitation under air flow of large CAPE and conditionally unstable stratification

利用WRF(V3.01)中尺度数值模式,对具有大对流有效位能和条件不稳定层结下中小尺度地形对降水的影响进行了一系列三维理想数值试验,探讨了不同地形高度、尺度、形状(水平形态比)以及环境气流对降水模态和分布的影响。结果发现:在大对流有效位能(2875J/kg)和条件不稳定(Nw≈0.01s-1)下,地形重力波破碎触发对流是地形对流降水的主要机制之一,地形重力波的形成、特征和传播受到环境条件制约,也受到地形自身参数的影响。当对流形成后,对流系统自身的下曳气流和降水蒸发冷却形成的向外流出的低层辐散气流可激发新的对流,同时对流还可激发重力波与地形重力波相互作用,使地形降水模态复杂化。在不同地形高度、尺度、形状和环境气流下,地形降水存在着多种模态,其中,迎风坡、背风坡、山顶附近以及地形上、下游存在准静止的降水模态,地形上、下游分别存在向上、下游传播的降水模态,地形两翼还存在向下游传播的降水模态。对流降水、稳定层结降水以及对流与稳定层结混合的降水均可出现在这些降水模态中,这取决于对流是否触发。地形降水对地形无量纲参数v/(Nwh)和Nwa/v非常敏感,单一地形无量纲参数并不能决定降水的分布。地形下游和两翼降水模态和分布的差异主要与不同地形参数和环境气流下地形激发的重力波特性不同有关。

Using Weather Research and Forecast model（WRF V3.01）,a series of three-dimensional numerical experiments are carried out to investigate the effects of the different topographic height,scale,shape（horizontal aspect ratio） and environmental flow on the topographic rainfall mode and the distribution under the condition of large convective available potential energy（CAPE） and conditionally unstable stratification.It is found that,under large CAPE（2875 J/kg） and conditionally unstable stratification（Nw≈0.01 s-1）,the breaking of the topographic gravity wave（GW） is the main triggering mechanism for convection.The formation and propagation of the topographic GW and its features are influenced by the environmental condition and topographic parameters.The downdraft airflow of convection itself and the low-level divergence airflow due to the rainfall evaporation cooling may induce new convection.Besides,the convection perturbation also may trigger gravity waves that are able to interact with the topographic GW,resulting in the complexity of the topographic rainfall mode and the distribution.Under the different topographic height,scale,shape and environmental flows,there may be several topographic rainfall modes.The quasi-stationary rainfall mode may occur on the upslope,the down-slope,the mountain top,even the upstream and downstream of the topography.There also exists the upstream and downstream propagating rainfall mode in the upstream and downstream of the topography,respectively.The downstream propagating rainfall may also exist in the two flanks of the topography.The convective rainfall,stably stratified rainfall and the mixing of them may occur in all of these rainfall modes,which is dependent of whether the convection is triggered or not.The topographic rainfall is sensitive to the dimensionless topographic parameters of v/（Nwh） and Nwa/v and it cannot be predicted by any one of them,that is,only one of such dimensionless topographic parameters cannot totally determine the topographic rainfall distribution.The differences of the rainfall mode and the distribution between the downstream and the two flanks of the topography are mainly related to the topography-generated GW of different features due to different topographic parameters and environmental flows.