Abstract

Abstract. For physics-based snow cover models, simulating the formation of dense ice layers inside the snowpack has been a long-time challenge. Their formation is considered to be tightly coupled to the presence of preferential flow, which is assumed to happen through flow fingering. Recent laboratory experiments and modelling techniques of liquid water flow in snow have advanced the understanding of conditions under which preferential flow paths or flow fingers form. We propose a modelling approach in the one-dimensional, multilayer snow cover model SNOWPACK for preferential flow that is based on a dual domain approach. The pore space is divided into a part that represents matrix flow and a part that represents preferential flow. Richards' equation is then solved for both domains and only water in matrix flow is subjected to phase changes. We found that preferential flow paths arriving at a layer transition in the snowpack may lead to ponding conditions, which we used to trigger a water flow from the preferential flow domain to the matrix domain. Subsequent refreezing then can form dense layers in the snowpack that regularly exceed 700 kg m−3. A comparison of simulated density profiles with biweekly snow profiles made at the Weissfluhjoch measurement site at 2536 m altitude in the Eastern Swiss Alps for 16 snow seasons showed that several ice layers that were observed in the field could be reproduced. However, many profiles remain challenging to simulate. The prediction of the early snowpack runoff also improved under the consideration of preferential flow. Our study suggests that a dual domain approach is able to describe the net effect of preferential flow on ice layer formation and liquid water flow in snow in one-dimensional, detailed, physics-based snowpack models, without the need for a full multidimensional model.

Highlights

  • Ice layers form a marked microstructural transition inside the snowpack (Fierz et al, 2009)

  • We proposed a dual domain approach for modelling liquid water flow in snow, which separates the pore space in a part that is representing matrix flow and a part that is representing preferential flow

  • The difference between the first modelled and measured 20 mm w.e. cumulative snowpack runoff decreased from approximately 8 days too late to 3 days too early

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Summary

Introduction

Ice layers form a marked microstructural transition inside the snowpack (Fierz et al, 2009) Their formation is generally considered to be tightly coupled to the presence of preferential flow in snow (Marsh and Woo, 1984; Pfeffer and Humphrey, 1998; Fierz et al, 2009). Despite their often small vertical extent, (thin) ice layers may have a profound impact on large-scale processes in a snowpack, such as liquid water, heat and vapour flow (Colbeck, 1991; Hammonds et al, 2015).

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