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

Abstract

ABSTRACT: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 (SW TOA ) is attenuated by the clear-sky atmosphere, while clouds attenuate a further 31%, resulting in <50% of SW TOA 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 20-40 W m −2 , while during autumn, winter and spring, clouds enhance Rnet by approximately 20 W m −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.

The Role of Atmospheric Rivers for Extreme Ablation and Snowfall Events in the Southern Alps of New Zealand

“…Direct mass balance monitoring has been carried out at Brewster Glacier since 2004 and has been recently consolidated by Cullen et al (), making it the longest complete record of its kind in New Zealand. Brewster Glacier has also been the site of extensive surface mass and energy balance modeling (e.g., Anderson et al, ; Conway & Cullen, , ; Conway et al, ; Cullen & Conway, ; Gillett & Cullen, ). Brewster Glacier is one of 50 “index glaciers” in the Southern Alps monitored as part of the end‐of‐summer snowline survey program that was established in 1977 (Chinn et al, ; Willsman et al, ).…”

Cloud effects on surface energy and mass balance in the ablation area of Brewster Glacier, New Zealand

“…The data set and specific methods used are presented in Conway et al (2015), but a brief summary is given below. At each half-hourly interval a theoretical upper limit for LW↓ is set by applying the Stefan-Boltzmann law to the observed T a and an emissivity of 1.…”

“…The inclusion of e a , as well as T a , as a dependent variable in the calculation of theoretical clear-sky LW↓was necessary, as clear-sky LW↓is strongly dependent on both variables at this temperate location (Durr and Philipona, 2004;Conway et al, 2015). The effect of this is to include a larger proportion of days in the clear-sky category, as some clear-sky days with high e a (and LW↓) would have been excluded had only T a been used in the calculation of clear-sky LW↓.…”

“…High-quality surface climate data presented in Cullen and Conway (2015) are used to force an SMB model (Mölg et al, 2008) to estimate both SEB and SMB terms over this period (measurement period). The cloud metrics presented in Conway et al (2015) are used to identify clear-sky and overcast conditions and thus characterize surface climate, SEB, and melt energy during each condition. To test the sensitivity of SMB to changes in surface climate and radiative components, a more heavily parameterized version of the model is used.…”

Reconstructing the mass balance of Brewster Glacier, New Zealand, using MODIS-derived glacier-wide albedo

“…), the glacier has a southwesterly aspect and a mean slope of 31 • while the lower portion of the glacier has a southerly aspect and a mean slope of 10 • . Brewster Glacier is one of the most well-researched New Zealand glaciers, with recent studies focusing on the atmospheric/meteorological controls on the surface energy and mass balance (Gillett and Cullen, 2011;Conway et al, 2015;Cullen, 2013, 2016;Cullen and Conway, 2015), and the climate sensitivity of Brewster Glacier (Anderson et al, 2010). Brewster Glacier belongs to the 50 index glaciers monitored by the EOSS survey programme with a record of the end of summer snowline altitude (SLA) since 1978 (Chinn et al, 2005;Willsman et al, 2015).…”