Surface energy balance closure at ten sites over the Tibetan plateau

Observations of the surface heat fluxes can be used to evaluate and improve land-surface models (LSMs). There are significant uncertainties, however, in measured surface-energy budgets, especially for the heterogeneous Tibetan Plateau (TP) region where the observation conditions are harsh. In this study, summer (July-October 2014) surface flux data were obtained using the eddy covariance method from ten sites over the TP during the Third Tibetan Plateau Atmospheric Scientific Experiments. Data analysis was performed to assess the surface-energy balance ratio (SEBR= H + LE/(Rn-G)) and associated uncertainties across various land-cover types and elevation heights. Measured latent heat fluxes were positive during nighttime and exhibit substantially greater uncertainty than the sensible heat fluxes. The ten-site averaged SEBR was 74.2 ± 5.4%, largely on par with reported SEBR for other regions. SEBR values were similar among homogeneous sites, and the averaged SEBR (93.4%) for those sites was better than that (67.3%) for the heterogeneous sites. The soil heat storage term represents the most significant source of uncertainty (8.2%) than the canopy storage term (0.22%) to closing the surface energy budget. The SEBR showed a strong diurnal cycle and the midday (10:00∼15:00 local time) values were higher than those nearest sunrise and sunset times. The late-night SEBR (00:00∼6:00 local time) at sites located at higher elevations were more reliable than those at lower elevation sites, because of the frequent occurrence of neutral conditions (instead of stable or very stable conditions) at high terrains. The relationships between SEBR and surface-layer turbulent parameters (ξ, u*,θ*) and wind direction were investigated. An uncertainty range for measured surface heat fluxes was derived to provide a meaningful guidance for applying these observations in evaluating LSMs.