Orographic Enhancement of Precipitation in Midlatitudes: Results from MAP and IMPROVE II

When midlatitude baroclinic systems move over a mountain range, the precipitation from the system intensifies over and sometimes upwind of the slope of the mountain barrier. The maximum precipitation occurs on the lower windward slopes in almost immediate response to the orographically induced lifting of the air such that the maximum precipitation may occur well below the summit of the range (e.g., Frei and Schaer 1998). How this rapid response to the orographic lifting occurs is the central question of orographic enhancement of midlatitude baroclinic precipitation. Smith (1979) called attention to this question by expressing the rapid fallout of the condensate produced by the orographic component of the lifting in terms of an efficiency. This efficiency may be thought of as the ratio of rainfall on the mountainside to the amount of water condensed in the upslope flow. Reviewing the literature, he noted that this ratio was typically high: “...It is...surprising in light of the difficulties in forming precipitation-size particles, to find release efficiencies of 70% to 100%, ...” That is, this high efficiency could only occur if there was a rapid conversion of the condensate generated by the orographic uplift to particles large enough to fall out quickly before being advected over the crest of the mountain range. He asked further, how “is it possible to convert such a high fraction of the condensed water into precipitation?” Since microphysical processes control the growth rate of precipitation particles, the answer to this question is a matter of determining how the microphysical processes accelerate the growth of precipitation particles in the upslope flow. Current research on orographic enhancement of precipitation focuses on answering the question of how microphysical processes are invigorated to account for the efficiency of the orographic precipitation enhancement. This paper reviews some recent progress toward this end.