The Importance of Turbulent Fluxes in the Surface Energy Balance of a Debris-Covered Glacier in the Himalayas

Abstract

Surface energy balance models are common tools to estimate melt rates of debris-covered glaciers. In the Himalayas, radiative fluxes are occasionally measured, but very limited observations of turbulent fluxes on debris-covered tongues exist to date. We present measurements collected between 26 September and 12 October 2016 from an eddy correlation system installed on the debris-covered Lirung Glacier in Nepal during the transition between monsoon and post-monsoon. Our observations suggest that surface energy losses through turbulent fluxes reduce the positive net radiative fluxes during daylight hours between 10 and 100%, and even lead to a net negative surface energy balance after noon. During clear days, turbulent flux losses increase to over 250 W m −2 mainly due to high sensible heat fluxes. During overcast days the latent heat flux dominates the turbulent losses and together they reach just above 100 W m −2. Subsequently, we validate the performance of three bulk approaches in reproducing the observations from the eddy correlation system. Large differences exist between the approaches, and accurate estimates of surface temperature, wind speed, and surface roughness are necessary for their performance to be reasonable. Moreover, the tested bulk approaches generally overestimate turbulent latent heat fluxes by a factor 3 on clear days, because the debris-covered surface dries out rapidly, while the bulk equations assume surface saturation. Improvements to bulk surface energy models should therefore include the drying process of the surface. A sensitivity analysis suggests that, in order to be useful in distributed melt models, an accurate extrapolation of wind speed, surface temperature and surface roughness in space is a prerequisite. By applying the best performing bulk model over a complete melt period, we show that turbulent fluxes reduce the available energy for melt at the debris surface by 17% even at very low wind speeds. Overall, we conclude that turbulent fluxes play an essential role in the surface energy balance of debris-covered glaciers and that it is essential to include them in melt models.

Spatial and temporal variations in glacier aerodynamic surface roughness during the melting season, as estimated at the August-one ice cap, Qilian mountains, China

“…Ice surface z 0 was generally larger than snow surface and smaller than patch snow-covered surface. Our results match values reported in studies reporting results ranging from 0.1 to 6.9 mm in Qilian mountain glaciers (Guo et al, 2018;Sun et al, 2018). Our results showed that z 0 reached its maximum at the end of the summer melt, which matched wind profile measurements by Smeets and Broeke (2008).…”

“…Agisoft Photoscan Professional is a commercial package, which implements all stages of photogrammetric processing (James et al, 2017). It has previously been used to generate three-dimensional point clouds and digital elevation models of debris-covered glaciers (Miles et al, 2017;Quincey et al, 2017;Steiner et al, 2018), ice surfaces and braided meltwater rivers (Javernick et al, 2014;Smith et al, 2016). In our study, we found that after new snowfall it was difficult to match feature points in the photo sets.…”

“…The roughness of ice surfaces is an important control on air-ice heat transfer, on the ice surface albedo, and thus on the surface energy balance (Greuell and Smeets, 2001;Hock and Holmgren, 2005;Irvine-Fynn et al, 2014;Steiner et al, 2018). The snow and ice surface roughness at centimeter and millimeter scales is also an important parameter in studies of wind transport, snowdrifts, snowfall, snow grain size and ice surface melt (Denby and Smeets, 2000;Brock et al, 2006;McClung and Schaerer, 2006;Fassnacht et al, 2009a, b).…”

Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya

“…Knowledge about actual debris thickness is limited to few field obser-T. van Woerkom et al: Sediment supply from lateral moraines to a debris-covered glacier in the Himalaya vations (Nicholson and Benn, 2006;Ragettli et al, 2015;McCarthy et al, 2017) and attempts to derive it from thermal bands of satellite imagery (Mihalcea et al, 2008;Pfeffer et al, 2014;Rounce and McKinney, 2014;Schauwecker et al, 2015;Rounce et al, 2018). Several studies suggest that glaciers with prolonged periods of negative mass balance are associated with an increase in debris cover (Deline, 2005;Mihalcea et al, 2006;Stokes et al, 2007;Shukla et al, 2009;Kirkbride and Deline, 2013;Gibson et al, 2017a). Glaciers that have a neutral or positive mass balance, such as in the Karakoram in Pakistan, do not show any positive trends in debris-covered area (Herreid et al, 2015).…”

Comparison of turbulent structures and energy fluxes over exposed and debris-covered glacier ice

“…This is potentially significant as this moisture, along with moisture from precipitation events, can impact the bulk thermal properties of the layer (Nicholson and Benn, 2012). Latent heat flux measured at other debris-covered glaciers shows a stronger and more consistent diurnal variability than is seen at Suldenferner, and typically remains negative during ablation season conditions (Collier and others, 2014;Yao and others, 2014;Steiner and others, 2018). This again could potentially be a result of lower nocturnal surface temperatures over Suldenferner due to the thin debris cover.…”