METEOROLOGICAL CONTROLS ON WIND EROSION DURING FOEHN WIND EVENTS IN THE EASTERN SOUTHERN ALPS, NEW-ZEALAND

Foehn winds in the lee of mountain barriers create highly favourable m eteorological conditions for wind erosion in alpine areas and over adj acent lowlands. This paper presents meteorological observations made d uring winter and summer foehn wind erosion events as part of a 3 year investigation into the physical nature and incidence of eolian process es in the eastern Southern Alps, New Zealand. Foehn winds were observe d to initiate eolian activity, including dust storm genesis when mean near-surface wind speeds exceeded 7-8 m.s(-1) over a 20 min period in the absence of precipitation. Wind speeds of 25-30 m.s(-1) were freque ntly recorded during typical foehn events, and wind speed maxima of 40 -50 m.s(-1) were monitored during severe foehn windstorms. Saltation c louds sampled at 0.5 and 1 m above glaciofluvial deposits during foehn wind erosion events were found to display a mean grain size of betwee n 300 and 435 mu m. The entrainment of fine-grained surface sediments by the airstream appeared to be enhanced in late autumn (May) and earl y spring (September) by freeze-thaw cycles that were observed to resul t in needle-ice growth in exposed glaciofluvial and lacustrine deposit s. The needle ice fragmented such deposits, producing surfaces that we re aerodynamically rough and highly susceptible to deflation by the ai rstream following a thaw. Observations made by this study are thought to be similar to foehn wind erosion events reported in the lee of othe r substantial mountain ranges, such as the Rocky Mountains, Andes, Hin du Kush, and Karakoram.