Summertime surface energy budget and ablation modeling in the ablation zone of a maritime Tibetan glacier

The surface energy budget and ablation were measured in the ablation zone of Parlung No. 4 Glacier on the southeast Tibetan Plateau (29 degrees 14'N, 96 degrees 55'E) during boreal summer 2009. The present study examines the summertime surface energy fluxes to identify major atmospheric variables governing the surface melt and their phenomenological links to the progression of the South Asian monsoon. Turbulent sensible heat and latent heat fluxes were calculated using the bulk aerodynamic approach, the accuracy of which was verified through a comparison with eddy-covariance flux measurements. The surface ablation calculated by the energy balance model was also verified by measurements of ablation stakes. Our results found the following percentage contributions to the total melt energy: net shortwave radiation, 98%; net longwave radiation, -12%; sensible heat, 16%; latent heat, -1%; and subsurface fluxes, -1%. The combined roles of cloud cover and surface albedo appear to control the surface energy balance during the onset period of the South Asian monsoon. The cloud variations affect surface melting with the advancement of the monsoon. Intensification of the South Asian monsoon probably accelerates melting in the ablation zone, whereas weakening of the monsoon reduces glacier melting, mainly because of changes in downward longwave irradiance and heat release due to vapor condensation. Moreover, the temperature index model proves useful for long-term mass balance and ablation modeling in cases where the degree-day factors are calibrated. But incorporating incoming shortwave radiation into the model should be more applicable and practical in this region.