Investigating cloud effects on the surface energy and mass balance of Brewster Glacier, New Zealand

Glaciers in the Southern Alps of New Zealand are seen as sensitive indicators of changing regional climate because periods of anomalous mass balance are associated with rearrangement of the regional ocean-atmosphere system. However the mechanisms that transfer these anomalies to changes in glacier mass balance are still not well established. Though the effects of changes in air temperature and precipitation have received some attention, the influence of clouds on glacier mass balance variability in the Southern Alps is not well understood. High quality micrometeorological datasets from a two year period over the Brewster Glacier, New Zealand, are used to characterize the influence of clouds on energy and mass exchanges over both snow and ice surfaces. Four-component radiation measurements enable an all-sky broadband radiation model to be tested and show the large and seasonally variable effect of clouds on shortwave, longwave and net all-wave radiation fluxes. A surface energy and mass balance model is used to detail the fundamental effect of cloudy conditions on the surface energy balance and to show how this impacts the sensitivity of melt to changes in air temperature. In contrast to other mid-latitude areas, melt energy in overcast conditions is similar to clear-sky conditions, due to enhanced turbulent latent heat and incoming longwave radiation fluxes that allow a large increase in the time glacier melt occurs. This dramatically increases the sensitivity of snow and ice melt to increased air temperature. This improved understanding of the role clouds play in modifying the surface energy and mass balance will lead to more confident attribution of past glacier states and glacier response to future climate in the Southern Hemisphere mid-latitudes.