Snow–Atmosphere Exchange in Complex Terrain: Turbulent and Advective Controls on Snow Sublimation and Surface Energy Fluxes

Abstract

This dissertation examines turbulent and advective processes controlling the snow-atmosphere exchange of water vapor, heat, momentum, and turbulent kinetic energy (TKE) in the mountainous East River Valley of Colorado. We used measurements spanning spatial and temporal scales, from eddy covariance point measurements to Doppler lidar measurements of wind fields that span the width of a mountain valley.For Chapter 2, we measured snow sublimation and found that 10% of the seasonal snowpack is lost to the process. We also found that sublimation of suspended, blowing snow induced positive water vapor flux divergence, and, as a result, sublimation estimates are sensitive to instrument deployment height. By quantifying sublimation in a Colorado River headwater catchment, we addressed sublimation’s potential role in declining streamflow efficiency. While models suggest sublimation removes 20–40% of snowpack, our observations indicated it removes less, although future measurement campaigns should investigate sublimation rates above forests and on exposed ridges. To future field campaigns, we suggest that eddy covariance systems should be deployed near the top of the blowing snow layer, which was around 10 m at our field site. For Chapter 3, we measured strong wind shear at the tops of mountain ridges and the occasional formation of rotors and vortices in the lee of a prominent ridge. During these highly turbulent events, TKE propagated down into the valley, and near-surface mixing and sublimation rates increased. Surface fluxes during these events were under-predicted by Monin-Obukhov similarity theory, the predominant method for predicting surface fluxes in weather and land surface models. Our observations suggested that similarity theory fails to predict surface fluxes in complex terrain because the theory assumes that the near-surface TKE budget involves only shear and bouyancy. In complex terrain, TKE transport can be an important term in the near-surface TKE budget. Additionally, we found that eddies transporting TKE also carried upper-atmosphere dry air to the surface. This finding indicates that future surface flux parameterizations should account for non-local exchange in complex terrain. For Chapter 4, we estimated the full surface energy balance over isolated patches of snow. We used an infrared video camera to estimate horizontal sensible heat advection, an eddy covariance system to measure vertical heat fluxes, a scanning lidar to quantify snow melt, and a radiometer to measure radiative fluxes. We found that once the fractional snow-covered area fell below ∼50%, horizontal heat advection supplied ∼50% of the energy consumed by melting snow. This result suggests that advective processes should be incorporated into snowmelt models and motivates further investigation into whether advection influences snowpack disappearance and streamflow timing. Our field measurements of sensible heat advection also diverged from the predictions of an idealized model, potentially reflecting the geometry of the observed snow patch. Future studies should therefore aim to measure sensible heat advection over snow patches situated within the complex and often concave landforms where they typically persist.