The role of convection in orographic precipitation

Orographic precipitation is often lumped into a single category in textbooks, although it varies widely in its physical nature. Here, we consider the occurrence of convection in mid-latitude orographic precipitation. Using observations from the DEEPWAVE field project in June and July 2014 (and other sources), we examine characteristics of the incoming flow during periods of heavy precipitation on the western (windward) slopes of the Southern Alps. During DEEPWAVE, the largest orographic precipitation events typically occurred when the water vapor flux (WVF) was strong and the atmosphere conditionally unstable. A WRF-model simulation for the duration of the field project reproduces this finding. 10-years of ERA-Interim data suggests these are the dominant flow characteristics during wintertime orographic precipitation events in New Zealand. For a global context, we find strong WVFs and conditionally unstable flow are associated with heavy orographic events at other mid-latitude mountain locations at various times of the year. This suggests convection is a common occurrence in mid-latitude orographic precipitation.

To explore the role of convection in orographic precipitation, we examine one of the best cases of heavy, orographic precipitation during DEEPWAVE. The flow was from the northwest during the passage of a complex frontal cyclone, and coincided with repeated passes over Mt Cook by the NFS/NCAR HAIPER Gulf Stream V. Flight-level observations reveal unsteady flow above the Alps, with wave breaking directly above the Alpine ridge. A dropsonde “curtain” released from the aircraft shows low-level instability in the upstream flow that diminished above the Alps (probably released via convection). Satellite imagery is consistent with this picture. We use high-resolution WRF simulations to examine the influence of latent heat release (crucial to moist convection), and find it plays a major role in affecting the windward flow dynamics and precipitation distribution.