Atmospheric controls on summer ablation over Brewster Glacier, New Zealand

Micrometeorological data collected from an automatic weather station over a total of 85 days during the summer of 2007/ 2008 in the ablation zone (1770 m a.s.l.) of the Brewster Glacier, New Zealand, were used to determine the main atmospheric processes controlling the magnitude and variability of daily ablation. During the field season, ablation was measured and modelled using an energy balance model (EBM) and a degree-day model (DDM). Calculation of the energy balance over the glacier using the EBM revealed that net radiation provided the largest source of energy for ablation (52%). The turbulent sensible heat flux was the next largest energy source (25%), followed by the turbulent latent heat flux (20%) and the rain heat flux (3%). While daily measured and modelled (EBM) ablation were both equal to approximately 40 mm w.e. d(-1), the EBM was used to demonstrate that rates were highly variable, ranging from less than 10 mm w.e. d(-1) to 101 mm w.e. d(-1), controlled primarily by changes in synoptic conditions. Days with ablation above 60 mm w.e. d(-1) accounted for almost 25% of total ablation, despite such events occurring on only 11 of the 85 days monitored. The contribution of the individual terms of the energy balance to observed air temperature is complex on Brewster Glacier, leading to a larger uncertainty in DDM ablation estimates (under prediction of total ablation by 11%). Advection of warm and moisture-laden air over Brewster Glacier, which is not uncommon given its close proximity to the ocean, is not well accounted for by the DDM. This is because the turbulent latent heat flux and the rain heat flux are poorly correlated to air temperature but are, nonetheless, important sources of energy during large ablation events. Copyright. (C) 2010 Royal Meteorological Society