Modeling energy and mass balance of Shallap Glacier, Peru

Abstract

Abstract. We calculated the distributed surface mass and energy balance of Shallap Glacier, Cordillera Blanca, Peru (9° S, 77° W, 4700–5700 m a.s.l., ~ 7 km2), on hourly time steps for two years (September 2006–August 2008) using a process-based model and meteorological measurements as input. Model parameter combinations were optimized against 21 temporal readings of 20 stakes in the ablation zone of the glacier. Uncertainty caused by model input parameters and parameterization schemes was estimated using a leave-one out cross-validation scheme, which yields values of root mean square deviation (RMSD) of surface height change < 1 m (< 10% of the measured amplitude) for all stakes. With the best parameter combination (smallest RMSD) applied, the modeled annual surface mass balance of the glacier was −0.32 ± 0.4 m w.e. (water equivalent) for September 2006–August 2007 and 0.51 ± 0.56 m w.e. for September 2007–August 2008. While the mass balance above 5000 m was similar in both years (Δ 0.33 ± 0.68 m w.e.) due to similar annual sums of solid precipitation, a difference of 1.97 ± 0.68 m w.e. was calculated for the lower parts of the glacier. This difference is associated with more frequent occurrence of higher snow line altitudes during the first year, which was mainly caused by a higher fraction of liquid precipitation due to higher mean air temperatures. As the net shortwave budget was found to be the main source for ablation throughout the year at Shallap Glacier, lower surface albedo especially caused by lower solid precipitation amounts explains most of the difference in modeled ablation and mass balance between the two years.

Climate trends and glacier retreat in the Cordillera Blanca, Peru, revisited

“…However, the sensible heat flux is generally small compared to the energy supplied by shortwave and longwave radiation (Wagnon et al, 1999;Sicart et al, 2010;Gurgiser et al, 2013). An unknown remains the role of humidity and cloud cover.…”

“…Since incoming shortwave radiation is the dominant energy source all over the year (e.g. Sicart et al, 2008;Gurgiser et al, 2013), glacier melt in the tropics depends to a large degree on the surface albedo (Rabatel et al, 2013) and the sensitivity of glaciers to albedo changes is high. The glacier surface albedo is largely influenced by the state of aggregation of precipitation (and ice surface having a much lower albedo compared to a snow surface), and the transition of snow to rain is closely related to air temperature.…”

“…Consequently, net radiation absorption was higher where the glacier was snow-free and more energy was available for melt. If we assume an albedo of 0.2 for ice and 0.75 for snow (taken from Gurgiser et al, 2013) under constant conditions, about 3.2 times more energy from net shortwave radiance is available for the ablation process of an ice surface. The height of the freezing line during precipitation events is therefore crucial for the ablation process.…”

“…For the hydrological year 2011 to 2012, they derived an actual ELA of 4975 masl for Artesonraju, and 4915 masl for Yanamarey (Dávila, 2013). (ii) Gurgiser et al (2013) …”

“…Due to the much lower albedo of a bare ice surface of the elevation band, the outgoing shortwave radiation is smaller and, consequently, more energy is available for ablation. The estimations are based on data from Gurgiser et al (2013) and the glacier hypsography from the year 2003. Seasonal mean values of energy fluxes and surface are utilized, assuming that all fluxes (except for outgoing shortwave radiation) are constant and the fraction of sublimation to total ablation is 12.5% and 75% for the wet and dry season, respectively (Winkler et al, 2009).…”