Abstract
Abstract. The effect of clouds on glacier surface energy balance (SEB) has received increased attention in the last decade, but how clouds interact with other meteorological forcing to influence surface mass balance (SMB) is not as well understood. This paper resolves the SEB and SMB at a site in the ablation zone of Brewster Glacier over a 22-month period, using high-quality radiation data to carefully evaluate SEB terms and define clear-sky and overcast conditions. A fundamental change in glacier SEB in cloudy conditions was driven by increased effective sky emissivity and surface vapour pressure, rather than a minimal change in air temperature and wind speed. During overcast conditions, positive net long-wave radiation and latent heat fluxes allowed melt to be maintained through a much greater length of time compared to clear-sky conditions, and led to similar melt in each sky condition. The sensitivity of SMB to changes in air temperature was greatly enhanced in overcast compared to clear-sky conditions due to more frequent melt and changes in precipitation phase that created a strong albedo feedback. During the spring and autumn seasons, the sensitivity during overcast conditions was strongest. To capture these processes, future attempts to explore glacier–climate interactions should aim to resolve the effects of atmospheric moisture (vapour, cloud, and precipitation) on melt as well as accumulation, through enhanced statistical or physically based methods.
Highlights
The response of glaciers to atmospheric forcing is of interest as glaciers are seen as useful scalable proxy records of past climate (e.g. Mölg et al, 2009a) and because the rapid changes occurring in many glaciated regions have implications for both global sea level rise (Kaser et al, 2006) and water resources (e.g. Jost et al, 2012)
Recent field studies on Brewster Glacier in the Southern Alps, have shown the high frequency of cloudy conditions during all seasons (> 50 % overcast conditions) as well as the significant and variable effect of clouds on SW↓, LW↓, and net radiation (Rnet) (Conway et al, 2015). In this context it is timely to examine in detail the influence of clouds on glacier surface climate, surface energy balance (SEB), and melt, as well as the manner in which clouds alter the sensitivity of surface mass balance (SMB) to air temperature in the Southern Alps. This paper addresses these issues by resolving the SEB and SMB at a site in the ablation zone of Brewster Glacier over a 22-month period in 2010–2012
By multiplying the contribution of each SEB term to the increase in melt by the fraction melt contributes to the total SMB (77 %; Table 8), we find the contribution of each SEB term to the SMB (Table 9f)
Summary
The response of glaciers to atmospheric forcing is of interest as glaciers are seen as useful scalable proxy records of past climate (e.g. Mölg et al, 2009a) and because the rapid changes occurring in many glaciated regions have implications for both global sea level rise (Kaser et al, 2006) and water resources (e.g. Jost et al, 2012). Reliable attribution of past glacier states and prediction of future ones is dependent on a thorough understanding of the physical processes operating at the glacier surface that link glacier change with climate, that is, the surface mass balance (SMB) and surface energy balance (SEB). For debris-free, mid-latitude glaciers, the SMB is primarily the sum of the relative magnitudes of accumulated solid precipitation and melt. In general, incoming short-wave radiation (SW ↓) is the major source of energy for glacier melt, variations in SMB are considered to be forced by changes in air temperature and precipitation (Oerlemans, 2005), through both accumulation and melt processes. The primary influence of air temperature on melt rate is modulated by other influences on the SEB such as surface albedo (Oerlemans et al, 2009), humidity (Gillett and Cullen, 2011), and cloud transmission (Pellicciotti et al, 2005)
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