Abstract : E.6
Numerical simulations of conditionally unstable flows over a ridge

Mario Marcello Miglietta, Richard Rotunno

Numerical simulations of conditionally unstable flows impinging on a mesoscale mountain ridge have been performed with an explicitly resolving cloud model in order to investigate the generation and the propagation of convective cells. The environmental conditions are the same studied by Chu and Lin (2000), but the horizontal resolution has been increased (grid interval = 250 m) in order to resolve properly the cellular-scale features. The solutions have been analysed for different uniform-wind profiles impinging on a bell-shaped ridge 2000 m high.

In the experiments with weaker environmental wind speeds (U = 2.5m/s, 10m/s), a density current is generated during the first few hours of the experiment as a consequence of the evaporative cooling associated with the rainfall on the upstream side of the mountain. The outflow produces convective cells upstream of the head of the current, so that no rainfall is produced close to the mountain in the quasi-stationary state. For larger wind speeds (U = 20m/s, 30m/s), the condensation is stronger and the evaporation mechanism less efficient. As a consequence, no density current is generated and the cells remain close to the mountain top. Experiments with different mountain heights h, show that the qualitative response of the system remains the same if the wind speed is kept constant, showing that the Froude number (= U/(Nm h), Nm^2 moist static stability) is not a very relevant parameter in the convective case.