条件不稳定湿大气中三维理想地形上空对流的动力学特征

条件不稳定湿大气情况下,气流经过三维地形可以形成不同性质的对流系统以及不同特征的地形流结构,其对流系统、地形流的性质主要取决于地形上空的对流触发、对流-地形流-重力波三者之间的相互作用,同样这些过程对于地形降水的性质、分布起重要的作用.根据不同湿Fr数(Froude number),湿条件不稳定大气经过三维小尺度山地上空时其对流和地形流动存在4种不同的流域(flow regirnes):(1)下游传播对流模态;(2)上游传播和下游传播共存对流模态;(3)山峰附近准静止和下游传播共存对流模态;(4)下坡稳定和下游传播对流共存模态.地形上空对流系统主要可以通过两种不同机制形成:(1)地形直接的抬升或减速作用;(2)在地形流形成后,由于地形流本身特性(如上游分离、背风涡旋和下坡重力波破碎)触发.在较大的Fr数情况下,地形上空对流生成后反过来可以破坏上、下游的地形流结构,但对背风坡的重力波破碎影响较小.不同初始对流有效位能(CAPE)不仅可以影响对流系统的传播、发展,而且可以影响整体地形流性质.较低的初始CAPE有利于地形流的形成,此时对流对地形流结构特征的影响相对较小,其流场性质与低Fr数流域性质相似.

The convection and moist stratified flow over an idealized three-dimensionalmountain: conditional unstable flow.

The properties of orographically induced convection and flow over the three-dimensional mountain within the conditional unstable atmosphere are strongly dependent on the trigger mechanism of convection, and the interaction between orographic flow, convection and gravity wave induced by mountain or convection. In present study, a series numerical experiment to the quasi steady stratified flow over an idealized three-dimensional mountain with an upstream profile of uniform wind speed, constant moist buoyancy frequency and constant relative humidity are undertaken.Based on the moist Froude number, four different flow regimes for the moist stratified flow over an three-dimensional mountain under the conditional unstable atmosphere are identified: (1) an downstream propagating convection mode, (2) an upstream and an downstream propagating convections mode, (3) an over-peak quasi-stationary and an downstream propagating convections mode, (4) an downslope quasi-stationary and an downstream propagating convections mode.There are two different mechanisms for the formation of orographically induced convection: (1) orographic lifting and flow decreasing, and (2) orographic flow forcing, i.e. upstream splitting, lee vortex and gravity wave breaking. In the case of larger Froude number, the convection induced by the orographic forcing could break the upstream and downstream orographic flow structure, but only a litter effect on the gravity wave breaking over the leeside.Initial CAPE have important role not only on the transportation and evolution of convective system, but also on the orographic flow in the moist environment. The sensitive simulation with a initial lower CAPE demonstrates that the decreased initial CAPE makes the flow regime shift back to regimes for lower Froude number.