The Properties Of Orographically Induced Convection And Precipitation Over A Idealized Two-dimensional Mountain: Conditional Unstable Flow

The influences of complex topography on the development and propagation of convective systems are investigated by use of the mesoscale numerical model ARPS. Convections induced by conditional unstable flows over two-dimensional single and double bell-shaped ridges, as well as their interactions with the topographic flows are simulated and analyzed dynamically.The convection systems state and precipitation distribution over double bell-shaped ridges are depended upon the intensity of the basic flow, the heights of the two ridges and the width between them as well as their configuration. The width of the valley primarily determines the discrepancies in the convection and precipitation distribution between single-and double-ridge topography. For narrow valleys, the double-ridge topography may be approximated by their envelop topography, so that the convection state and precipitation distribution are analogous to that of the single-ridge topography. For wide valleys, interactions between the flows over the two ridges are trivial, and hence each ridge may be treated as single-ridge topography. When the width of the valley lies in certain range, however, the convection state and precipitation distribution are quit different from that of the single-ridge topography. There are two types of configuration of the double-ridge topography, with higher ridge in either the upstream or downstream of the basic flow. For the topography with higher ridge in the upstream, the convection state and precipitation distribution are determined by the effect of the higher ridge, and are analogous to that of the single-ridge topography, yet they differ a lot from the single-ridge case when the higher ridge is located in the downstream. In this situation, the basic flow is altered by the lower ridge in the upstream and the higher ridge also affects the flow in the leeside of the lower ridge, which generates the complex convection propagation and precipitation distribution. Four types of precipitation distribution are detected for the case of higher ridge in the downstream with moderate valley width:(i) precipitation in the valley and in the upslope of the lower ridge; (ii) precipitation in the upslope of the higher ridge; (iii) precipitation in the peak of the lower ridge and in the upslope of the higher ridge; (iv) quasi-stationary precipitation in the peaks of the two ridges and in the leeside of the lower ridge. The first three kinds of precipitation distribution occur for weak upstream flow, whereas the last is for intense upstream flow.CAPE has an important effect on the precipitation over the topography, since convections are weak and hard to last for small CAPE, and precipitation are mainly due to the topographic flow. When CAPE is large, intense initial convections may quickly appear and develop continuously. The evolution of the convections is dominated by the density current. For given Fr, initial convections will occur more early with stronger intensity, and convections in the peak of the lower ridge will be trigged more early for larger CAPE. Meanwhile, as CAPE increases, precipitation intensifies gradually in the upstream of the higher ridge, the valley and near the lower ridge. When the convections are strongly developed in the upstream, however, the incoming vapor is cutoff near the higher ridge. Thus precipitation near the higher ridge and in its leeside decreases as CAPE increases. For both small CAPE and Fr, convections in the upstream are weak and precipitation in the upstream has little influence on the flow over higher ridge. Intense precipitation will occur near the higher ridge and in its leeside.