Idealised numerical experiments of Alpine flow regimes and southside precipitation events

Heavy precipitation events to the south of the Alps are usually associated with a southerly pre-frontal low-level jet advecting moisture toward the so uthern slopes of the Alps. Here we use idealised numerical simulations to a ssess the nature of the associated flow regimes and the mechanisms leading to vertical lifting and precipitation. The idealisations comprise: a simpli fied are-shaped barrier-like orographic obstacle of Alpine scale; neglectio n of the tropopause; a stationary two-dimensional upstream flow configurati on that includes a frontal structure and a low-level jet; hydrostatic dynam ics with free-slip lower boundary conditions; and a simplified set of param eterizations to address dry, moist absolutely stable, and moist conditional ly unstable upstream flow configurations. Within the dry dynamics, typical settings lead to Alpine-scale flow splitti ng with pronounced left/right asymmetries with respect to the incident sout herly flow. Strong vertical lifting occurs over the western portion of the upstream slopes, within the stream of air that tries to circum go the elong ated obstacle on the western flank. Thus, despite belonging to the "flow-ar ound" regime, these flow configurations can exhibit vertical lifting over t he whole height of the obstacle. The responsible asymmetry is primarily ind uced by the Coriolis effect in the presence of an elongated mountain, but i t can further be intensified by the impinging low-level jet and the are-sha pe of the Alpine topography. With a conditionally unstable moist upstream p rofile, the flow is able to surmount the obstacle without pronounced horizo ntal deflections. Maximum precipitation rates of approximate to 100 mm (24h )(-1) are obtained. When moist convection is suppressed by using a moist ab solutely stable upstream profile, the how is again substantially deflected and shows the typical characteristics of the dry flow regime discussed abov e, with somewhat reduced precipitation rates as compared to the convective case. Overall there is evidence that the asymmetry introduced by the Coriol is effect, a pronounced low-level jet, and a moist upstream profile, all fa cilitate vertical lifting and thereby provide a suitable environment for he avy condensation and precipitation.