Constraining Cloud and Airmass Controls on the Surface Energy and Mass Balance of Brewster Glacier, Southern Alps of New Zealand

Abstract

A detailed knowledge of glacier surface energy balance (SEB) is essential if global, regional and local atmospheric controls on glacier mass balance (MB) are to be properly understood. Despite keen interest in the relationship between regional climate and glacier fluctuations in the Southern Alps of New Zealand, the separate effects of cloud and airmass characteristics on SEB are still poorly resolved. This thesis uses high quality radiation, micrometeorological and glaciological datasets to resolve the SEB and constrain the cloud and airmass controls on the MB of a temperate maritime glacier in detail. A 22-month record of surface climate and radiative fluxes from Brewster Glacier allows distinctive maritime, orographic and katabatic influences on glacier climate to be resolved. Eddy covariance measurements are used to address the parameterisation of turbulent heat fluxes, showing assumptions commonly used in MB modelling are not valid and the importance of turbulent heat fluxes in the Southern Alps has likely been overstated in previous work. High quality 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 MB model is used to detail the fundamental effect of cloudy conditions on the SEB and MB sensitivity. 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. Collectively these results suggest that atmospheric moisture exerts a strong control on the response of accumulation and ablation to changes in air temperature in the Southern Alps. Future work to characterise atmospheric controls on MB should aim to resolve these processes in order for past and future glacier states to be properly understood.