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 meteorological conditions for wind erosion in alpine areas and over adjacent lowlands. This paper presents meteorological observations made during winter and summer foehn wind erosion events as part of a 3 year investigation into the physical nature and incidence of eolian processes in the eastern Southern Alps, New Zealand. Foehn winds were observed 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 frequently recorded during typical foehn events, and wind speed maxima of 40–50 m∙s−1 were monitored during severe foehn windstorms. Saltation clouds sampled at 0.5 and 1 m above glaciofluvial deposits during foehn wind erosion events were found to display a mean grain size of between 300 and 435 μm. The entrainment of fine-grained surface sediments by the airstream appeared to be enhanced in late autumn (May) and early spring (September) by freeze–thaw cycles that were observed to result in needle-ice growth in exposed glaciofluvial and lacustrine deposits. The needle ice fragmented such deposits, producing surfaces that were aerodynamically rough and highly susceptible to deflation by the airstream following a thaw. Observations made by this study are thought to be similar to foehn wind erosion events reported in the lee of other substantial mountain ranges, such as the Rocky Mountains, Andes, Hindu Kush, and Karakoram.