Abstract
This study presents idealized numerical simulations to analyze the temperature dependence of small‐scale orographic precipitation enhancement in a non‐convective environment for a variety of mountain heights. The investigation is motivated by a climatological analysis of several rain‐gauge stations in the northern Alps, indicating a conspicuous dependence of precipitation enhancement on temperature and mountain height. Specifically, a station lying on the lower lee slope of a moderately high mountain (about 700 m above the adjacent plain) experiences stronger precipitation enhancement at low temperatures than a station in the lee of a mountain twice as high, whereas the opposite is the case at higher temperatures. In qualitative agreement with the observations, the simulations indicate a marked dependence of the precipitation pattern on the position of the freezing level relative to the mountain peak. For a freezing level lying significantly above or below the peak, a single precipitation maximum in the peak region is predicted. However, for a freezing level close to the peak, the simulations indicate two precipitation maxima over the windward slope and over the lower lee slope, separated by a relative minimum in the peak region. A closer analysis suggests that two factors are responsible for this behaviour. First, the higher accretion efficiency of snow and graupel compared to rain favours particularly strong precipitation growth above the peak, which is advected towards the lee slope by the ambient flow. Second, the fall‐speed difference between snow/graupel and rain tends to induce diverging hydrometeor trajectories where the melting layer intersects the windward slope, whereas converging hydrometeor trajectories occur over the lee slope below the melting layer. For a suitable location of the freezing level, the leeward shift of the precipitation maximum may overcompensate the general increase of precipitation enhancement with mountain height, leading to higher precipitation in the lee of a low mountain than in the lee of a high mountain. Copyright © 2008 Royal Meteorological Society