Numerical Simulations of Conditionally Unstable Flows over a Mountain Ridge

Abstract

Numerical simulations of conditionally unstable flows impinging on a mesoscale mountain ridge have been performed with an explicitly resolving cloud model to investigate the statistically stationary features of the solution precipitation characteristics. The simulations are performed on a three-dimensional domain and at high resolution (grid spacing: 250 m) to properly resolve cellular-scale features. Although the environmental conditions are specified by a simplified idealized conditionally unstable sounding, there are still quite a few external parameters, so only a limited portion of the parameter space was explored. Numerical solutions were first carried out for different uniform-wind profiles impinging on a bell-shaped ridge 2000 m high. In the experiments with weaker environmental wind speeds (2.5 m s 21 ), the cold-air outflow, caused by the evaporative cooling of rain from precipitating convective cells, is the main mechanism for cell redevelopment and movement; this outflow produces new convective cells near the head of the up-and downstream density currents, which rapidly propagate far from the ridge so that no rainfall is produced close to the ridge at later times. For larger wind speeds (10 and 20 m s 21 ), there is less time for upwind, evaporation-induced cold-pool formation before air parcels reach the ridge top and descend downwind. For the intermediate wind speed (10 m s 21 ), evaporation is effective in generating a cold pool only on the downstream side of the ridge, in a region where the air is unsaturated and slow moving. Further experiments with different ridge heights and half-widths were carried out in order to analyze their effect on the distribution and intensity of precipitation. Dimensional analysis reveals that the maximum (nondimensional) rainfall rate mainly depends on the ratio of mountain height to the level of free convection, the ridge aspect ratio, and a parameter that measures the ratio of advective to convective time scales.

Under what conditions does embedded convection enhance orographic precipitation?

“…Another orographic process that potentially influences I is upstream-propagating cold pools created by precipitating convection (e.g. Miglietta and Rotunno, 2009). However, no evidence of these features was found in our simulations.…”

An observational and numerical study of a flash-flood event over south-eastern Italy

“…The unsaturated moist Froude number H (H =h m N/U -with N moist unsaturated Brunt-Väisälä frequency, h m the maximum mountain height, U the average wind speed in the layer −) is the parameter relevant for the classification of the flow regime in the surroundings of an isolated mountain for moist stably stratified flows (Smith, 1979;Miglietta and Buzzi, 2001). It is important to stress that the use of H is appropriate for the classification of the flow regime only for moist stably stratified flows, since for conditionally unstable flows other nondimensional parameters may be equally important, as shown in Miglietta and Rotunno (2007); here H is evaluated just to roughly estimate the tendency of the flow to pass over and not around the mountain. If one evaluates H in the lower 1 km from the Brindisi sounding at 12:00 UTC, 22 October, the large wind speed together with the small mountain height determines a value of H <1 (H ∼0.2), which corresponds to a "flow over" regime.…”

Wind patterns associated with the development of daytime thunderstorms over Istria

“…15b). Recently Miglietta and Rotunno (2009) have shown, that a non-dimensional number CAPE/speed 2 represents the ratio of the control parameters (still considering idealised conditions) that can display the effect of the orography on a conditionally unstable flow. Here four points in the smallest domain (Pazin, S1, S3 and S4 in Fig.…”