Snow Cover Fraction in a General Circulation Model

Swiss Federal Institute of Technology, Zurich, Switzerland

Abstract: Snow cover fraction (SCF) has a significant influence on the surface albedo and thus on the radiation balance and surface climate. Long-term three dimensional simulations with General Circulation Models (GCMs) showed that the SCF greatly affects the climate in the Northern Hemisphere. By means of both ground observations and remotely sensed data, several deficiencies in the SCF parameterization used in the current ECHAM4 GCM were identified: over mountainous areas a substantial overestimation in the SCF was found whereas flat areas showed a distinctly underestimated SCF. This paper proposes a new parameterization of the SCF for use in GCMs. Evaluations illustrate that it is beneficial to include the effects of (i) flat, non-forested areas, (ii) mountainous regions and (iii) forests.

A new SCF parameterization for flat, non-forested areas was derived by using global datasets of ground-based snow depth and remote sensing observations of snow cover data. A 3-dimensional ECHAM4 simulation showed that this modification raises the SCF by up to approximately 20%, mainly in areas with a relatively thin snow cover.

The comparison between remotely sensed and simulated mean monthly surface albedo revealed a significant overestimation of the surface albedo in snow covered mountainous areas. The extension of the current SCF parameterization in ECHAM4, according to the French climate model Arpège, yielded a close agreement with satellite-derived surface albedo.

Using remotely-sensed SCF data in ECHAM4 over forested areas produced unrealistic results due to the masking of snow cover on the ground underlying the canopy. Therefore, we adopted the submodel for snow albedo as used in the Canadian Land Surface Scheme (CLASS) to simulate the SCF of snow-covered canopies. This model combined with a newly-developed simple snow interception model demonstrated the ability to capture the main physical processes of snow covered canopies, including the albedo. This modification has a beneficial impact on the delayed snow melt in spring, a well-known problem in many current GCMs: The simulated surface albedo over the boreal forests decreases by approximately 0.1 during winter and spring, which is in better agreement with ground-based observations. This induces a significant rise in the surface temperature over extended parts of Eurasia and North America in

Remote Sensing and Climate Modeling: Synergies and Limitations, 203-232. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.

late spring, which subsequently yields a faster snowmelt and an accelerated retreat of the snow line.

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