Abstract
North-westerly airflow and associated atmospheric rivers (ARs) have been found to profoundly influence New Zealand's west coasts, by causing flooding, landslides and extreme ablation and accumulation on glaciers in the Southern Alps. However, the response of local glacier mass balance to synoptic-scale circulation, including events with ARs, has typically not been investigated by considering mesoscale processes explicitly. In this study, high-resolution atmospheric simulations from the Weather Research and Forecasting model are used to investigate the mesoscale drivers of an extreme ablation event on Brewster Glacier (Southern Alps), which occurred on February 6, 2011 during the landfall of an AR on the South Island. The following processes were found to be crucial for transferring the high temperature and water vapor contained in the AR into energy available for melt on Brewster Glacier: First, the moist-neutral character of the air mass enabled the flow to pass over the ridge, leading to the development of orographic clouds and precipitation on the windward side of the orography, and foehn winds on the leeside. These processes fueled melt through longwave radiation and strong turbulent and rain heat fluxes within the high-condensation environment of the orographic cloud. Second, orographic enhancement occurred due to both cellular convection within the cloud and the combined effect of multiple precipitating systems by the seeder-feeder-mechanism. These results indicate the potential importance of AR dynamics for New Zealand's glaciers. They also illustrate the benefit of mesoscale atmospheric modeling for advancing process understanding of the glacier-climate relationship in New Zealand.