Orographic enhancement of mid-latitude cyclone precipitation

Abstract

As mid-latitude cyclones move across mountain ranges, the distribution of precipitation is altered. The windward slopes experience enhancement of precipitation, which is determined by dynamical and microphysical factors. Detailed radar observations collected near the European Alps and the Oregon Cascade Mountains reveal that the windward terrain-modified flows have characteristics that are repeated from storm to storm and from mountain range to mountain range. These flow patterns and their precipitation formation mechanisms are synthesized in conceptual models of orographic enhancement of precipitation.In Type A pattern, low static stability low-level air rises easily as it encounters the first peaks of the terrain. Lifting of moist low-level air produces high liquid water contents over these peaks, which favor growth of pre-existing precipitation by riming and coalescence. If the upstream flow is potentially unstable, convective cells will be triggered in the upslope ascent. These cells produce pockets of especially high liquid water content where the coalescence and riming processes are accentuated. A mesoscale model simulation suggests that subsequent peaks may also be subjected to this kind of precipitation enhancement.Type B pattern exhibits a shear layer on the windward slopes. The combination of high shear and static stability produces conditions that support dynamical instability manifested in the form of Kevin-Helmholtz billows and turbulent overturning cells. Aggregation of ice particles falling from the baroclinic system into the layer of cells is aided by the turbulent motions. The strong updrafts produce pockets of high liquid water content; which favor riming and coalescence. Idealized numerical simulations indicate that a shear layer may develop on the windward side of a mountain as a result of strong static stability and/or surface friction.During mid-latitude cyclone passage over a mountain range, windward precipitation is enhanced by small-scale updrafts regardless of the character of the low-level flow. In Type A (Type B) storms static (dynamic) instability is responsible for the updraft generation. The updrafts produced in both scenarios are strong enough to activate accretion growth processes (coalescence, aggregation, and riming), which are capable of producing large particles that fallout rapidly on the windward side of the terrain.