On the exchange of sensible and latent heat between the atmosphere and melting snow

The snow energy balance is difficult to measure during the snowmelt period, yet critical for predictions of water yield in regions characterized by snow cover. Robust simplifications of the snowmelt energy balance can aid our understanding of water resources in a changing climate. Research to date has demonstrated that the net turbulent flux (F_T) between a melting snowpack and the atmosphere is negligible if the sum of atmospheric vapor pressure (e_a) and temperature (T_a) equals a constant, but it is unclear how frequently this situation holds across different sites. Here, we quantified the contribution of F_T to the snowpack energy balance during 59 snowmelt periods across 11 sites in the FLUXNET2015 database with a detailed analysis of snowmelt in subarctic tundra near Abisko, Sweden. At the Abisko site we investigated the frequency of occurrences during which sensible heat flux (H) and latent heat flux (λE) are of (approximately) equal but opposite sign, and if the sum of these terms, F_T, is therefore negligible during the snowmelt period. H approximately equaled -λE for less than 50% of the melt period and F_T was infrequently a trivial term in the snowmelt energy balance at Abisko. The reason is that the relationship between observed e_a and T_a is roughly orthogonal to the “line of equality” at which H equals -λE as warmer T_a during the melt period usually resulted in greater e_a. This relationship holds both within melt periods at individual sites and across different sites in the FLUXNET2015 database, where F_Tcomprised less than 20% of the energy available to melt snow, Q_m, in 44% of the snowmelt periods studied here. F_T/Q_m was significantly related to the mean e_a during the melt period, but not mean T_a, and F_T tended to be near 0 W m⁻² when e_a averaged ca. 0.5 kPa. F_T may become an increasingly important term in the snowmelt energy balance across many global regions as warmer temperatures are projected to cause snow to melt more slowly and earlier in the year under conditions of lower net radiation (R_n). Eddy covariance research networks such as Ameriflux must improve their ability to observe cold-season processes to enhance our understanding of water resources and surface-atmosphere exchange in a changing climate.