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

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 ( N w≈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 ofv/(Nwh) and Nw a/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.