条件不稳定大气中二维小尺度双脊地形上空对流及降水特征

Characteristics of the orographiclly induced convection and precipitation over an idealized two-dimensional small-scale two-ridge orography in the conditionally unstable atmosphere

在条件不稳定大气条件下,二维小尺度双脊钟形地形上空对流触发、传播和降水分布特征主要决定于地形上游基流强度、双脊地形配置形式、地形高度及其山谷宽度。双脊地形在沿基流方向上有两种配置:高脊地形位于上游和低脊地形位于上游。对于高脊地形位于上游的双脊地形,上游高地形将起主导作用,山地上空对流及降水特征与单脊地形类似。对于低脊地形位于上游的双脊地形,上游低地形可明显地改变下游高地形的前方来流,同时,下游高地形也能够对上游低地形背风侧流动产生影响,从而导致出现地形上空复杂的对流传播、降水分布特征。对于低脊地形位于上游的双脊地形,其山谷宽度主要决定了双脊地形与单脊地形之间在对流、降水分布等的差异;当山谷宽度较小时,双脊地形可以近似为一个包络地形,此时地形上空的对流、降水特征与单脊地形类似;当山谷宽度较大时,双脊上空流动相互影响较小,此时双脊地形可以分成两个单脊地形;当山谷宽度在一定范围内,其上空的对流及其降水分布与单脊地形有明显差异。对于低脊地形位于上游、中等山谷宽度的双脊地形上空降水主要呈现4种类型:(1)山谷与低脊迎风坡降水;(2)高脊迎风坡降水;(3)低脊山峰与高脊迎风坡降水;(4)低脊背风侧、双脊山峰准静止降水。

The formation of orography-induced convections and precipitation over a two-dimensional small-scale two-ridge mountain in the conditionally unstable atmosphere depended mainly upon the upstream flow, the height of the two ridges and the width between them. There are two configurations for two-ridge orography considered, with a higher ridge in either the up- stream or downstream of basic flow. For the case of the higher ridge located in the upstream, the formation and propagation of convection and/or precipitation over the mountain are mainly determined by the effect of the higher ridge, whose features are similar to that the single-ridge orography. For the case of the higher ridge located in the downstream, the upstream ridge could change the basic flow for the downstream ridge, and the interaction with the perturbation induced by the two-ridge orography, which results in the different regimes of convection and/or precipitation over the mountain. The width of valley primarily influ- ences the discrepancies for the convection and/or precipitation over mountain between single- and two-ridge orography. For the narrow valleys, the two-ridge orography may be approximated by an envelope orography, so that the regimes of convection and/or precipitation are analogous to that of the single-ridge orography. For the wide valleys, the interactions between the flows over two different ridges are trivial, and then each ridge may be treated as the single-ridge orography. When the valley width lies in a certain range, however, the regimes for convection and/or precipitation are quite different from that for the single-ridge case. Four types of precipitation distribution are detected for the higher ridge located in the downstream with a moder ate valley width： （1） precipitation located in the valley and in the upslope of the lower ridge; （2） precipitation located in the upslope of the higher ridgel （3） precipitation located in the peak of lower ridge and in the upslope of the higher ridge; and （4） quasi-stationary precipitation located in the peaks of two ridges and in the lee-side of lower ridge.