The effects of a mountain on the propagation of a preexisting convective system for blocked and unblocked flow regimes

Observations and previous research of squall lines impinging on mountain ranges have revealed that the squall lines sometimes stall upstream of the mountains for several hours leading to copious accumulations of precipitation. It has been hypothesized that squall-line stagnation may be more prone to occur in flows where the Fronde number (F = U/Nh, where U is the basic-state wind, N is the Brunt-Väisälä frequency, and h is the mountain height) is low. This hypothesis is tested herein through a series of idealized, two-dimensional experiments where a convective system was triggered upstream of a mesoscale mountain in conditionally unstable flow. For simulations with relatively low Froude numbers, stagnation of the preexisting convective system was not observed. In the simulations with high values of F, squall lines were noted to stagnate between 100 and 200 km upstream of the mountain. This result indicates that squall-line stagnation may be more favored for moderate to large values of F for conditionally unstable flow. The mechanisms leading to the formation of the stationary convective system upstream of the mountain in the unblocked flows were explored and it was found that evaporative cooling played a pivotal role in the stagnation of the squall line.