Spatiotemporal variability of turbulent fluxes in snow-covered mountain terrain

Abstract

Turbulent exchange of heat and moisture plays an important role in snow cover dynamics in mountain regions and governs boundary layer dynamics. Although these processes are subject to great spatial and temporal variability, especially in complex terrain, measurements of heat, moisture, and momentum fluxes are almost exclusively point observations. To quantify the spatial variability, and assess the representativeness of the observations, numerical modeling of the atmosphere and surface is a useful tool. Nevertheless, there is considerable uncertainty regarding the accuracy of surface models in capturing turbulent fluxes, particularly in complex terrain with large spatial variability on small scales. These uncertainties can be attributed in part to (1) the use of Monin-Obukhov similarity theory, which has limitations in complex terrain because the assumptions of stationarity and spatial homogeneity are usually not fulfilled and (2) the errors in representing wind speeds and near-surface atmospheric gradients in the simulations. In this study, we analyze sources of errors in representing energy exchange over snow in mountain areas by models and specifically look at the spatio-temporal variability during different meteorological events in the region of Davos, Switzerland. To verify common modeling approaches with observations, we use model predictions of turbulent fluxes from CRYOWRF, the atmospheric model WRF coupled to the surface model SNOWPACK. The fluxes at different resolutions are compared to turbulent fluxes measured using the eddy covariance method and calculated with the Monin-Obukhov similarity theory. This model comparison and spatial analysis is carried out for three different meteorological events that are representative of the local climate, particularly föhn events. The results from the model indicate that the fluxes vary strongly spatially. Depending on the weather pattern, elevation plays a large role in the variability of the turbulent fluxes, and they correlate by elevation with wind speed. This shows that local turbulent heat fluxes are not representative of the whole mountain area. This has implications for the calculation of snow melt, sublimation, and accumulation across mountainous terrain. The model resolution also plays an important role in the representation of fluxes, as coarser (1 km) resolutions greatly overestimate wind speeds compared to higher resolutions (200 m). This is due to fewer topography-wind interactions resulting in an overestimation of turbulent fluxes.