| This dissertation is composed of three separate papers linked by a common purpose: to investigate the orographic effects on the generation and propagation of mesoscale convective systems using a two-dimensional, non-hydrostatic, nonlinear cloud model. In the first paper, we propose four flow regimes, determined by the Froude numbers with respect to the thermal forcing associated with evaporative cooling and orographic forcing, for a two-dimensional, non-rotating, non-hydrostatic, conditionally unstable airstream over an isolated bell-shaped mountain. They are: (1) regime I, flow is subcritical to both outflows and orographic forcing, (2) regime II, flow is supercritical to outflows and subcritical to orographic forcing, (3) regime III, flow is supercritical to both outflows and orographic forcing, and (4) regime IV, flow is subcritical to outflows and supercritical to orographic forcing. However, regime IV may not occur in the real atmosphere because it is difficult to satisfy both the conditions in terms of relevant Froude numbers. In addition, between regimes I and II, there exists a transition zone where the mesoscale convective system is stationary with respect to the mountain.; In regime I, convective cells are mainly triggered by the upstream propagating density current produced by the mountain-induced convective system. In the transition zone, the convective system is quasi-stationary over the mountain. It is triggered by orographic forcing and gravity waves associated with the mountain-induced convective system. In regimes II and III, the quasi-stationary and downstream propagating convective systems co-exist over the mountain. For the quasi-stationary mode, formation mechanisms are the same as those in the transition zone. For the downstream propagating mode, convective cells are generated by the convergence associated with the downslope wind and the western flank of the gust front downstream of the mountain. However, the speed of the downstream propagating mode in regime III is much faster than that of regime II due to the fast moving internal jump.; In the second paper, the effects of diurnal thermal forcing on the generation of convective system over a mesoscale mountain are studied. Based on the regime diagram, we find that the flow in regime I and transition zone can transit to regime II during daytime. The orographic forcing and the gravity waves associated with the mountain-induced convective system are responsible for the cell generation at early stage. After the convective system moves downstream, it is strengthened by the convergence associated with the downslope wind and the easterly flow of the mountain-plains solenoidal circulation. During the nighttime, convective cells cannot form upstream of the mountain due to weak convergence generated by the incoming flow and the drainage flow. For regimes II and III, both the quasi-stationary and downstream propagating convective systems are similar to those described in the first paper. There exists no flow transition in regime III, which indicates that the flow responses are not sensitive to diurnal variation.; In the third paper, the effects of a two-dimensional mesoscale mountain on propagating preexisting mesoscale convective systems (MCSs) are studied. Based on the regime diagram, we find that a preexisting MCS in regime I becomes quasistationary near the initial position and tends to produce heavy precipitation over the plain upstream of the mountain. The convective cells embedded in the preexisting MCS are triggered by the density currents and gravity waves associated with the preexisting MCS. In the transition zone between regimes I and II, the convective cells tend to occur over the upstream foothill, instead of over the upstream plain area. In regime II, the preexisting MCS propagates toward the mountain and tends to merge with the mountain-induced convective system over the upstream plain area and along the upslope of the mountain. In regime III, weaker... |
