## 3 Earth Surface Modelling Advances and Links with EO Datasets
### Land-Surface Reservoirs
#### 3.1.2 Seasonal Snow Cover  
Snow acts as a fast climate switch [100] with implications ranging from weather forecasts to climate change projections. Among the main evidence of the importance of snow in the Earth System, we can enumerate: (i) the snow albedo feedback, (ii) driver of sub-seasonal to seasonal atmospheric predictability, (iii) important role in the recent Arctic amplification and (iv) its impact on future water resources [101]. The snowpack lying on top of the soil affects the evolution of atmospheric temperatures via its high albedo and its thermal insulation capacity that can create a decoupling between the soil and the atmosphere [102; 103; 104]. This snow insulating effect causes strong temperature inversions near the surface in winter, which represents a challenge for daily minimum temperature forecasting. Snow also affects the freezing of water in the soil, with an impact on the hydrology in spring, on near-surface temperatures and on the stable boundary layer development [105; 106; 107]. Snow cover also acts as a water reservoir, which is released by snowmelt in the spring, influencing runoff, soil moisture, evapotranspiration and thus precipitation and the entire hydrological cycle [108]. Until 2009, the snow pack was parameterised in H-TESSEL following Douville et al. [109]. The snow pack was represented with a single layer of dry snow (i.e., neglecting liquid water) with four snow prognostic variables: mass, albedo, density and temperature. Snow albedo decreased in time at an exponential or linear rate, for melting and normal conditions respectively, and snow density increased with time according to an exponential relaxation. Figure 8: Surface soil moisture at Little Washita, OK, USA for two versions of the NASA Catchment model (**a**). Average surface soil moisture skill (**b**) in terms of time series correlation (R), bias, and unbiased root mean squared error (RMSE) (or standard-deviation of the error). Skill is measured vs. in situ measurements from 18 SMAP core validation sites [45] and then averaged across the sites. The improved model version (NRv7.2) uses updated land cover, topography, and vegetation parameters based on remote sensing observations and was further calibrated using SMAP soil moisture retrievals. With the participation of H-TESSEL in the snow model inter-comparison project 2 (SnowMIP2, [110]), and after the soil hydrology revision presented in the previous subsection, several shortcomings of the Douville snow pack representation were identified such as the lack of liquid water representation with freeze/thaw cycles during the melting season, unrealistic evolution of snow density, and unrealistic albedo of shaded snow (i.e., snow under high vegetation). These shortcomings were partially addressed with a full revision of the snow pack parameterisation in 2009 [111] including: (i) a new parameterisation of snow density, (ii) a liquid water reservoir and (iii) revised formulations for the sub-grid snow cover fraction and snow albedo. In offline mode, forced with near-surface observations, the revised scheme reduced the end of season ablation biases from ten to two days in open areas, and from 21 to 13 days in forest areas. The evolution of snow mass, depth and density during one winter season at the Fraser forest and open stations is shown in Figure 9, a research location in the Rocky Mountains, CO, USA. The results show the improvements of the snow scheme revision (NEW) with respect to the version used in ERA-Interim, labelled as control (CTR). The new snow density parameterisation increased the snow thermal insulation, reduced soil freezing, improved the hydrological cycle, and substantially reduced a warm winter bias compared to Siberian screen-level (2m) observations (not shown). The dramatic change in surface reflectivity when snow is present has been widely explored using EO data, in particular in the visible range, and is mainly limited by cloud presence and contamination.