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 in autumn 2016 from the 26$^{th}$ of September to the 12$^{th}$ of October from an eddy correlation (EC) tower 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 EC observations. 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. \textcolor{red}{By applying the best performing bulk model over a complete melt period, we show that turbulent fluxes to 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.
Highlights
Debris-covered glaciers constitute a significant minority of the total glacierized area in High Mountain Asia
Very similar versions of these energy balance models were applied on ice cliffs (Sakai et al, 1998; Reid and Brock, 2014; Steiner et al, 2015; Buri et al, 2016a,b) and supraglacial ponds (Sakai et al, 2000; Miles et al, 2016), features frequently found over debris-covered glaciers that have been identified as one possible reason for larger thinning rates on these glaciers than expected (Immerzeel et al, 2014a)
The results suggest that an accurate estimation of the roughness length is crucial for the derivation of turbulent fluxes using a bulk approach, as suggested earlier (Rounce et al, 2015)
Summary
Debris-covered glaciers constitute a significant minority of the total glacierized area in High Mountain Asia. A more detailed model was developed by Reid and Brock (2010) who used a numerical iteration of debris temperatures to model the propagation of the energy flux through the debris They introduced stability corrections for the calculation of turbulent fluxes. Very similar versions of these energy balance models were applied on ice cliffs (Sakai et al, 1998; Reid and Brock, 2014; Steiner et al, 2015; Buri et al, 2016a,b) and supraglacial ponds (Sakai et al, 2000; Miles et al, 2016), features frequently found over debris-covered glaciers that have been identified as one possible reason for larger thinning rates on these glaciers than expected (Immerzeel et al, 2014a)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
