Analysis of the wind and temperature field in an alpine lake basin

Observational data collected during the Lake Tekapo Experiment (LTEX) in January and February 1999 (austral summer) are used to analyse the development of local winds during fair weather conditions. Lake Tekapo and the tributary Godley Valley are located in the MacKenzie Basin, one of the major mountain basins on the eastern side of the Southern Alps, New Zealand. Measurements from surface stations, pilot balloon and tethersonde soundings as well as an instrumented light aircraft provide evidence of multi-scale interacting wind systems, ranging from local slope winds to regional scale coast-to-basin flows. Thermal forcing of the winds occurred due to differential heating as a consequence of orography and heterogeneous surface features, and is quantified by heat budget analysis.

The daytime vertical temperature structure was characterised by distinct layering. Features of particular interest are the formation of thermal internal boundary layers due to the lake-land discontinuity and the development of elevated mixed layers. The latter were generated by advective heating from the basin and valley sidewalls and by a superimposed valley wind blowing from the basin over Lake Tekapo up the Godley Valley.

Daytime heating in the MacKenzie Basin and its tributary valleys caused the development of a strong horizontal temperature gradient between the basin atmosphere and that over the surrounding landscape, and hence the development of a regional scale heat low over the basin. After noon, air from outside of the basin started flowing over mountain saddles into the basin causing cooling in the lowest layers, whereas at ridge top height, the horizontal air temperature gradient between inside and outside the basin still increased. In the early evening, rapid cooling and transition to a rather uniform slightly stable stratification up to about 2000 m agl was observed. The onset time of this cooling varied about 1-2 h between observation sites. It is hypothesised that this rapid cooling was caused by the massive intrusion of air from outside of the basin over wide parts of the surrounding mountain ridges. The onset time of the intrusion was probably triggered by the decay of up-slope winds, which previously prevented the intrusion of air over the surrounding ridges. Although this cooling process drastically reduced the temperature difference between the basin atmosphere and that over the surrounding landscape, the coast-to-basin circulation continued until about mid-night, when a mountain wind from the Godley Valley became dominant. The results indicate the extreme complexity that can result from the operation of thermal forcing processes at a wide range of spatial scales.