Exploration of some convective regimes over the Alpine orography

Abstract

AbstractA numerical exploration of several airflow regimes is performed by means of the non‐hydrostatic model Meso‐NH. The Alpine orography has been selected together with a uniform conditionally unstable profile extracted from the Mesoscale Alpine Programme database. The stationary solution is obtained at the fine scale after three successive simulations using three nested levels (40, 10 and 2.5 km). The variation of the non‐dimensional height of the Alps leads to the following regimes: no convection, upstream propagating convection and stationary convection over the Alps. The last two regimes correspond to regimes I and II of the Chu and Lin (2000) study. The sensitivity of the convective response to the fine‐scale orography is found to be more important for regime II than for regime I. Moisture budgets confirm the importance of the barrier wind to determine the rain distribution: in the centre of the Alpine orography for regime II and away from it for regime I. The drying ratio is more influenced by the comparative value of the advective time to a characteristic microphysical time‐scale than by the orographic forcing itself. Finally, for this sounding, the diabatic effects change the critical value of the non‐dimensional height for the transition between the ‘flow‐over’ and ‘flow‐around’ configurations. Copyright © 2004 Royal Meteorological Society