Nocturnal cold air drainage in complex terrain, Lake Tekapo, New Zealand

Abstract

This thesis presents results from a summertime field campaign which investigates the structure and development of nocturnal drainage winds within the complex alpine setting of Lake Tekapo, New Zealand. Surface meteorological observations obtained from a network of automatic weather stations and anemographs, were supplemented by pilot balloon, free-flight radiosonde ascents and sodar measurments, conducted during four intensive special observation periods throughout January and February, 1999. Supplementary meteorological data including mean sea level synoptic analyses were also obtained from the New Zealand Meteorological Service. Observations from Mt.Gerald revealed that the onset of katabatic drainage winds is closely related to negative sensible heat flux, while their velocities indicate a high degree of correlation with the amplitude of negative sensible and ground heat flux. Under optimum light wind conditions, katabatic flow started at lower levels on the slope and quickly ascended to higher elevations. A reversal of this trend was observed during katabatic cessation with lower slopes recording the onset of daytime wind regimes before those on upper slopes. Multiple layering of drainage winds was observed to occur simultaneously around Lake Tekapo, resulting in a stratified nocturnal boundary layer according to the origin and temperature of individual drainage winds. Their effect on local nocturnal wind regime of the Tekapo Lake Basin is dramatic with widely varying wind directions, speeds and temperatures recorded over small horizontal distances. The mountain wind was found to develop gradually displaying elevated jets exhibiting maximum velocities shortly before sunrise. A remnant core of the mountain wind was monitored until late morning draining the catchment by which time its erosion by the developing daytime wind regime had been completed. Two windspeed maxima were also observed within the lake basin mountain wind downstream of the confluence of the Godley and Macaulay Valleys. It is believed that drainage from the Macaulay River Valley creates sufficient differences in density within the main mountain wind to account for this phenomenon. Results from this study contribute to our knowledge of nocturnal wind fields in complex alpine settings. As a result, they will have application to future studies of air pollution dispersion and air quality management in complex terrain, as well as the development of agriculture.