THE EFFECTS OF A MOUNTAIN ON THE PROPAGATION OF PRE-EXISTING CONVECTION FOR DIFFERENT FROUDE NUMBER FLOW REGIMES

Abstract: In this research, the tendency for squall lines to stagnate upstream of mountain ranges is investigated through a series of 2-dimensional, idealized simulations where the basic state wind was varied from 1 m s−1 to 20 m s−1. These simulations included a set of simulations with no pre-existing convection but with a mountain (MO), a set with pre-existing convection, but no mountain (SO), and a set with both the pre-existing convection and the mountain (SM). These simulations show stagnation is dependent on the Froude number of the basic state flow with stagnation appearing to occur for smaller Froude number flow regimes. For subcritical Froude number flow, the greatest precipitation accumulations were found well upstream of the mountain. This maximum in precipitation was larger than that for either the MO and SO simulations and, additionally, was farther upstream than the maxima in either of these simulations. For critical Froude number flow, the SM simulations exhibited two precipitation maxima. The upstream maximum was colocated with the precipitation maximum in the SO simulation, while the downstream maxima was colated with that in the MO simulation. Finally, for supercritical flow, the precipitation maximum in the SM simulation was positioned over the peak of the mountain. This maximum was smaller than in the MO simulation