The role of synoptic cloud in orographic rainfall in the Southern Alps of New Zealand

Abstract

Orographically enhanced rainfall is the dominant climatic feature of the alpine regions of South Island, New Zealand. Most of this rain occurs in strong north-westerly winds although a forecast difficulty exists in that near-saturated north-westerly flows do not always produce heavy rain. Air masses impinging on the Southern Alps are generally carried over the alpine divide in times that are normally regarded as insufficient for clouds to produce precipitation-sized particles. Hence a possible explanation for these observations is that precipitation occurs only when pre-existing clouds advected onshore contain cloud drops that are sufficiently mature to allow for raindrop growth in the available time. A model combining advection of moist air over the topography and IR satellite data was used to show that synoptic cloud is necessary for the formation of heavy rainfall in the Alps.