The Occurrence and Influence of Convection in Orographic Precipitation over New Zealand

Orographic precipitation (OP) varies widely in its physical nature, but it is often lumped into a single category in textbooks. Here, we consider the occurrence and influence of convection in mid-latitude OP over the Southern Alps of New Zealand using observations from the DEEPWAVE field campaign in June and July 2014 (and other sources). We find that the environment upstream of the Southern Alps during periods of heavy precipitation is typically conditionally unstable, suggesting convection is a common occurrence. Dropsonde “curtains” released by the NSF/NCAR HAIPER Gulf Stream V across the Southern Alps show that the low-level instability in the upstream flow is diminished above the mountains, probably released via convection. Satellite imagery is consistent with this picture. Through a suite of high-resolution WRF simulations, convection is found to reduce water vapor available for orographic precipitation by i) enhancing precipitation upstream of the Southern Alps and ii) enhancing the vertical transport of moisture from the marine boundary layer (the primary source of water vapor for OP) to the mid- to upper-troposphere. Our results have broad application to other coastal mountain ranges.