Assessment of the land surface scheme in climate models with focus on surface albedo and snow cover

This work assesses the land surface focusing on surface albedo and snow, along with their representation in numerical models. Based on comprehensive surface datasets and literature, the work presents a number of improvements for the albedoand snow parameterization. These algorithms are validated with ECHAM4 climate simulations at a T42 horizontal resolution. The transformation of the prognostic snow water equivalent into the snow cover fraction was improved over both mountainous and flat areas. Validating the global snow cover fraction gave rise to a substantial overestimation in the snow cover fraction in mountainous areas, while the opposite has been found over flat areas by comparing remotely sensed and ground-based data. Long-term three dimensional ECHAM4 simulations showed that the relationship between snow water equivalent and snow cover fraction largely affects the surface climate in the Northern Hemisphere, including the Monsoon circulation. A model experiment, including the effects of subgrid scale vaiiations in topography on surface temperature, snow water equivalent and surface albedo demonstrated that a significant deviation from the control climate primarily occurs over mountainous areas in autumn. The incorporation of the BATS snow submodel in ECHAM showed that it is beneficial to introduce a prognostic snow-aging variable for simulating the snow albedo. However, it is crucial to correctly determine the parameter set in regions with minimal snowfall, such as Antarctica. The adoption of the submodel for snow albedo as used in CLASS, combined with a simple snow interception model demonstrated the ability to capture the main physical processes of snow covered canopies, including the albedo. The mean surface albedo over the boreal forests decreases by approximately 0.1 during winter and spring, which is in better agreement with ground-based observations. The enhanced solar absorption at the Earth's surface induces a significant rise in the surface temperature over extended parts of Eurasia and North America in late spring, which yields a faster snowmelt in spring and an accelerated retreat of the snow line. The separate treatment of the solar radiation and surface albedo in the visible and nearinfrared spec! rum yielded changes in the total (radiation weighted) surface albedo by up to 0.05, primarily in regions with high seasonal variations in the precipitation and the precipitable water vapour. The inclusion of a reasonable relationship between the surface albedo and soil moisture demonstrated that neglecting direct soil moisture-albedo feedback fails to accurately simulate the annual amplitude of the surface albedo, primarily in regions with both rainy and dry seasons. The model's ability to simulate the snow-rain ratio could be considerably improved by reducing the threshold temperature where the melt of falling snow flakes is initialized. The main results, which are based on off-line simulations and sensitivity tests using the land surface schemes incorporated in ECHAM and EM, yielded the following results. The two land surface schemes produced similar results for surface temperature, net ra¬ diation and, to a lesser extent, for total water discharge and latent heat flux. Signifi¬ cant differences are found in simulations of the soil moisture content, surface runoff and drainage, as well as transpiration and bare soil evaporation, primarily due to differences in parameterization algorithms and the structure of the soil water model. The diurnal amplitudes of the ground heat fluxes are substantially overestimated in both models due to the reasonably thick first subsurface layer. The EM fails in the simulation of the tran¬ spiration rate in late spiing due to the neglect of the biophysical control by the stomatal resistance of the canopy. These sensitivity studies clearly demonstrated that it is of great importance to test the