O. Fuhrer, C. Schär Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland

Properties of orographically induced precipitation are strongly dependent on the stability and moisture content of the impinging airflow. In the present study, consideration is given to quasi-steady stratified flow over a mountain ridge with an upstream profile of uniform wind speed, constant moist buoyancy frequency and constant relative humidity. Numerical experiments are undertaken with the Canadian non-hydrostatic mesoscale model MC2, using explicit warm-rain Kessler-type microphysics and a TKE-based turbulence scheme. Results will be presented using a two-dimensional model topography, but within a fully three-dimensional model set-up.

Numerical experiments confirm that the nature of the flow regime is approximately determined by the convective available potential energy (CAPE) of the incoming airstream. Both stratiform (regime I) and convective precipitation (regime II) are considered. In regime I, the incoming airflow is stable and precipitation is due to forced lifting of airmasses above the lifting condensation level (LCL). In regime II, the incoming airflow is conditionally unstable and deep convective cells are triggered as airmasses are lifted over the ridge. The individual convective cells are mainly produced by orographic forcing, by gravity waves associated with other convective cells, and by upstream propagating density currents caused by evaporative cooling. As the cells propagate downstream and decay in the lee of the mountain ridge, they give rise to a temporally and spatially highly variable field of intense precipitation.

THE transition from regime I to regime II is studied using the upstream surface temperature (and implied specific humidity and CAPE) as control parameter. Time averages of relevant dynamical and microphysical fields such as the precipitation distribution are used to characterise the transition. The main purpose of these experiments is the investigation of the presence and role of embedded convection for settings close to the regime boundary.

Presenter: Oliver FUHRER Coauthors: Christoph SCHAER

Type: ORAL