Micrometeorological processes driving snow ablation in an Alpine catchment

R Mott,N Dawes,C Manes,T Grünewald,M Bavay,L Egli,M Lehning

doi:10.5194/tc-5-1083-2011

Abstract

Abstract. Mountain snow covers typically become patchy over the course of a melting season. The snow pattern during melt is mainly governed by the end of winter snow depth distribution and the local energy balance. The objective of this study is to investigate micro-meteorological processes driving snow ablation in an Alpine catchment. For this purpose we combine a meteorological boundary-layer model (Advanced Regional Prediction System) with a fully distributed energy balance model (Alpine3D). Turbulent fluxes above melting snow are further investigated by using data from eddy-correlation systems. We compare modeled snow ablation to measured ablation rates as obtained from a series of Terrestrial Laser Scanning campaigns covering a complete ablation season. The measured ablation rates indicate that the advection of sensible heat causes locally increased ablation rates at the upwind edges of the snow patches. The effect, however, appears to be active over rather short distances of about 4–6 m. Measurements suggest that mean wind velocities of about 5 m s−1 are required for advective heat transport to increase snow ablation over a long fetch distance of about 20 m. Neglecting this effect, the model is able to capture the mean ablation rates for early ablation periods but strongly overestimates snow ablation once the fraction of snow coverage is below a critical value of approximately 0.6. While radiation dominates snow ablation early in the season, the turbulent flux contribution becomes important late in the season. Simulation results indicate that the air temperatures appear to overestimate the local air temperature above snow patches once the snow coverage is low. Measured turbulent fluxes support these findings by suggesting a stable internal boundary layer close to the snow surface causing a strong decrease of the sensible heat flux towards the snow cover. Thus, the existence of a stable internal boundary layer above a patchy snow cover exerts a dominant control on the timing and magnitude of snow ablation for patchy snow covers.

Highlights

In mountains the snow cover becomes patchy over the course of the melting season
We address three main points concerning the understanding of small-scale snow ablation in an Alpine catchment: First, we discuss the different energy terms contributing to the spatial variability of snow ablation including net solar radiation, turbulent flux exchange of sensible and latent heat and the effect of local advection of sensible heat
The investigation into the spatial variability of snow ablation was performed in an Alpine catchment, located in the Wannengrat area (Fig. 1b), where several studies on snow depth variability, snowpack stability and snow-hydrology have been completed in recent years

Summary

Introduction

The patchiness of a snow cover can be attributed to the interaction between spatially variable snow-depth distribution at the time of peak accumulation (Luce et al, 1998; Liston et al, 2007; Anderton et al, 2004; Grunewald et al, 2010; Egli et al, 2011) and the local energy balance at the snow surface. Several research fields regarding snow ablation are investigated. These are the development of internal boundary layers above patchy snow covers, the local advection of sensible heat over heterogeneous surfaces, the relative contribution of energy fluxes to snow ablation and the spatial variability of snow water equivalent. In the following we give a brief overview of the state of the art of these research fields

Objectives

Methods

Results

Conclusion