Investigation of snow melt dynamics and boundary layer processes over a melting snow surface

In this thesis, we investigate the surface energy balance of a snow pack for a continuous and a patchy snow cover. Snow surface temperatures have been validated to assess the accuracy of the surface energy balance of a continuous snow pack for individual points. As the four radiation components contribute largest to the surface energy balance, accurate radiation measurements (typically only available for the shortwave radiation) are required for the model input. A model error (up to 5 K in snow surface temperatures) is introduced to the surface energy balance of a continuous snow pack by parametrizing the incoming longwave radiation, which is typically not measured. Turbulent heat fluxes are typically calculated in physics-based models with Monin-Obukhov bulk formulations and its parametrization contributes to model errors in the surface energy balance up to 2 K. The parametrization of the turbulent fluxes was found to be the largest error source in the case of measured incoming longwave radiation. Very stable atmospheric conditions over snow and a non-equilibrium boundary layer have a strong but very difficult to quantify influence on turbulent surface fluxes. Stability corrections were typically developed over non-snow surfaces and applied over snow in complex terrain, where several mandatory assumptions of the Monin-Obukhov bulk formulation are heavily violated. Therefore, our validation shows a much better model representation of the surface energy balance in idealized flat terrain in comparison with complex terrain. In this thesis, we develop new stability corrections over snow and assess the error of the Monin-Obukhov bulk formulation with 6 W m-2 and an additional error of 1-5 W m-2 due to state-of-the-art parametrizations of the stability correction. The energy balance of a snow pack significantly alters for heterogeneous land-surfaces in the late ablation period. Warm air from the bare ground can be efficiently transported over the snow patch and modifies the near-surface air temperature field. Terrestrial laser scanning measurements reveal that local snow ablation rates at the upwind edge of the snow patch are 25 % larger than further inside of the snow patch. The strong thermal contrast in surface temperatures in combination with calm wind conditions leads to the development of a stable internal boundary layer, which grows along the fetch and reduces turbulent mixing of warm air masses. Small-scale boundary layer dynamics are typically not resolved in hydrological models. Numerical simulations with the atmospheric model Advanced Regional Prediction System (ARPS) reveal a mean air temperature increase above the patchy snow cover of 2-5 K in comparison with a continuous snow cover, which could lead to an increase in daily mean snow ablation rates up to 30 %. Above-average snow ablation rates at the upwind edge of a snow patch could be resolved by the development of a temperature footprint approach. However, the effect of increasing near-surface mean air temperatures to snow ablation from lateral transport processes is small when considering an entire melting period. Uncertainties in measured precipitation, parametrized incoming longwave radiation and calculated turbulent fluxes with Monin-Obukhov bulk formulation lead to larger errors in modelled snow heights than neglecting lateral transport processes.

Lehning, Michael
Mott-Grünewald, Rebecca Maria
Lausanne, EPFL

 Record created 2018-06-18, last modified 2018-07-11

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