Dominant rainfall processes in New Zealand's southern Alps

Abstract

Orographic rainfall in the Southern Alps of New Zealand is extreme. Over ten meters rainfall per year is recorded in many locations within a narrow zone running the length of the windward side of the Alps. Within the high rainfall zone, the physical mechanism for rainfall generation is not well understood. While wide-scale up-slope rain appears to play a significant role, questions remain as to how this rainfall is generated in the available time. This is because airflows generally take approximately half an hour to traverse the steep windward slopes of the Alps, which is insufficient time for accepted droplet coalescence processes to result in heavy stratiform rain. This question is investigated in detail in this thesis. High resolution rain gauges and small meteorological radars were deployed within and upwind of the high rainfall zone. Data from these sources was supplemented with polar orbiting satellite data and, to a lesser extend, standard meteorological data. Analysis begins with multiscaling characterisation of rain gauge data. This revealed systematic orographic trends in rainfall nature during northwesterly airflow, heavy rain periods. These trends were attributed to the influence of the mountain wave on the stability of the air mass, and a conceptual explanation of this mechanism is presented. IR satellite data, after analysis to reveal cloud top temperature as a function of distance from the alpine divide, provided some experimental support for this mechanism. IR satellite data were also used to determine the amount of offshore pre-existing cloud impinging on the Alps. The pre-existing cloud information was then used to modulate a simple forecast model, based on advection of moist air parallel to the topography. Results were compared with rain gauge data and indicate that heavy rainfall in the Alps generally depends on pre-existing cloud in the onshore flow. This conclusion is not independent of the influence of the mountain wave on stability, which provides a conceptual explanation for the range of synoptic conditions in which pre-existing cloud appears to be significant. Finally, the role of the seeder feeder mechanism in further enhancing precipitation in the high rainfall zone is investigated. Vertical pointing radar images of a brief but clear example of the seeder feeder process in the Southern Alps are presented.