Effects of Orography on the Generation and Propagation of Mesoscale Convective Systems in a Two-Dimensional Conditionally Unstable Flow

Abstract

Effects of orography, cold-air outflow, and gravity waves on the generation and propagation of convective systems in a conditionally unstable airstream over a mesoscale mountain are studied using a two-dimensional cloud model. Based on the propagation of convective systems, three regimes are identified: (I) an upstream propagating convective system, (II) a quasi-stationary convective system, and (III) quasi-stationary and downstream propagating systems. In regime I [low moist Froude number (Fw)], the convective cells are generated by upstream deceleration associated with orographic forcing, by gravity waves associated with convective cells over the upslope area at earlier stages, and by the upstream propagating density current at later stages when the density current is fully developed. In this flow regime, quasi-continuous and heavy rainfall is produced over the upslope and plain areas as individual convective cells develop farther upstream at the head of the density current and then propagate downstream once they form. In regime II (moderate Fw), the convective system becomes quasi-stationary over the upslope and in the vicinity of the mountain peak. The individual convective cells are mainly produced by orographic forcing and gravity waves associated with the mountain-induced convective system. A balance between the orographic forcing and the cold-air outflow forcing has been reached in this flow regime. In addition, the convective cells are able to merge into a large single cell due to the phasing of orographic forcing and gravity wave forcing. In regime III (large Fw), two modes of convective systems are identified: the quasi-stationary and downstream propagating modes. For the quasi-stationary convective system, the formation mechanisms are the same as those in regime II. For the downstream propagating convective system, the convective cells are mainly generated by convergence associated with an internal hydraulic jump over the lee slope. Without evaporative cooling, the mountain-induced convective system cannot trigger new cells far upstream of the mountain. The dominant convective system is advected downstream slowly by the basic flow. With the presence of upstream deceleration, the convective system tends to occur far upstream on the plain area rather than over the upslope of the mountain.