Aeolian dust transport and deposition by foehn winds in an alpine environment, Lake Tekapo, New Zealand

Abstract

Observations from an automatic weather station and anemograph network are used in conjunction with recorded dust deposition rates to identify principle sources of wind blown dust within an alpine lake basin. Meteorological observations made throughout the study indicate that foehn windstorms provide ideal conditions for the initiation of aeolian processes, including dust storm genesis. A mean wind speed during foehn windstorms of 7.5 m s −1 at 2.65 m above the surface was observed to initiate entrainment of dust particles from geomorphically active areas within the lake basin, particularly from the dry braid channels of glacier fed rivers, exposed lacustrine deltas and degraded tussock grasslands. This threshold entrainment velocity is similar to others presented within the international literature for such surface types. A diurnal modulation of meteorological parameters monitored during foehn wind events suggests that aeolian processes, such as dust entrainment, are most likely to occur from late morning to early evening. However, where topography enhances wind speeds, for example by channelling airflow down river valleys, favourable conditions for dust entrainment may prevail throughout the night in the absence of precipitation. Potential dust roses were determined for two sites within the lake basin. The dust roses supported anecdotal and observational evidence of principle dust sources within the study area, and appear to be an effective tool for identifying the potential hazard to a particular location from blowing dust, once the physical trigger conditions for dust entrainment are known. This requires detailed and concurrent monitoring of meterological variables in association with dust transportation and deposition rates, especially in complex terrain where many micro-climates often exist.