All-sky radiation over a glacier surface in the Southern Alps of New Zealand: characterizing cloud effects on incoming shortwave, longwave and net radiation

Clouds are important features of many high-altitude and glaciated areas, yet detecting their presence and specifying their effects on incoming shortwave (SW), longwave (LW) and net all-wave radiation (Rnet) remains challenging in these environments. These limitations hamper efforts to understand atmospheric controls on glacier surface mass balance (SMB) in the Southern Alps of New Zealand, as both cloud and airmass forcing accompanies key synoptic controls on SMB. Multi-year datasets of four-component broadband radiation from two sites at Brewster Glacier, Southern Alps of New Zealand, are used here to develop cloud metrics to account for the effects of clouds on SW, LW and Rnet. On average 23% of top-of-atmosphere shortwave radiation (SWTOA) is attenuated by the clear-sky atmosphere, while clouds attenuate a further 31%, resulting in <50% of SWTOA being received at the surface. The transmission of shortwave radiation by clouds (trc) during overcast conditions is found to vary with season and airmass characteristics. A simple parameterization is developed to account for lower trc observed during periods of higher water vapour pressure. Cloud metrics derived at the site show overcast conditions are frequent (45% of period) and strongly dependent on wind direction, highlighting the dominant role of orography in cloud formation and enhancement in the Southern Alps. The effect of clouds on Rnet exhibits a distinct seasonal variation; during summer when albedo and trc are lower, clouds decrease Rnet by 20-40Wm(-2), while during autumn, winter and spring, clouds enhance Rnet by approximately 20Wm(-2). Idealized modelling shows that these patterns are strongly dependent on albedo and extend across the elevation range of glaciers in the Southern Alps. Thus, overcast conditions appear to aid the extension of ablation into spring and autumn by increasing the energy available for snow and ice melt.