## 3 Earth Surface Modelling Advances and Links with EO Datasets
### Inland-Waters
Lake depth estimates computed by this approach correspond well with in situ measurements, especially in the tundra zone; however, the approach needs to be further tested and improved for automatic use [235]. While at the moment still no complete global lake depth dataset exist, local bathymetry dataset can be found at least for large lakes (see Section 6.3.4 of the GCOS report [60]). Beyond a few large lakes, the collection of local datasets is a difficult task; therefore, the most common practise for lake modeling in global NWP is to use in situ measurements and indirect estimates of lake mean depths based for instance on the geological origin of lakes [236]. Prior to its implementation in the IFS, the influence of FLake was evaluated in a series of preparatory studies: first, in an offline experimental framework [227; 237], and then extended to fully-coupled lake-atmosphere simulations [230]. More recently, the possibility of treating sub-grid water bodies (lakes and coastal waters) using the land surface tiling methodology has been included. An important step in the inclusion of lakes in NWP is that the simulated lake surface temperature and lake ice conditions are validated by comparing with observations. The use of remote sensing data for validating lake model outputs in NWP is essential, in particular as in situ observations at a global scale are currently lacking. With this approach, each grid box is divided into fractions of different land use, each with their own tile. Merits and limitations of the tiling methodology when accommodating lakes and forest within the same model grid-box have been assessed by [238]. They conclude that the tiling method captures well the influence on surface fluxes of the contrast between the \"lake\" and \"forest\" surface boundary conditions, on seasonal and diurnal time scales (see Figure 20, upper left and upper right panels, respectively). The behavior of simulated lake surface temperatures (Figure 20, bottom panel) and the period of ice cover in large inland water bodies worldwide was verified using satellite-based products (see [230]). Particularly relevant for validating lake simulations (temperature and ice cover) in NWP is, for example, data provided by the ATSR Reprocessing for Climate, ARC-Lake lake surface water temperature and ice cover dataset [239]. Satellite-derived observations from ARC-Lake have been used in recent years to validate simulations from the Canadian small lake model within the Canadian land surface scheme [240], global lake temperature simulations performed within the CNRM-CM5 climate model [232], and in validating the coupling of FLake to the UK Met Office Unified Model [233], among others. The impact of introducing interactive inland water bodies in ECMWF's IFS has been examined by a set of dedicated analysis experiments. However, the lake model (FLake, [231]) does not currently consider a mass balance of water (only for the ice components), whereby a static dataset is used to represent the extent and bathymetry of the world's lakes. Monitoring lake depth globally is not currently achievable although inversion methods (e.g., Ref. [237] demonstrates that an effective lake depth can be derived on the basis of the remotely-sensed lake water-surface temperature). Figure 20: Sensible heat-flux over a lake (blue line) and a near-by forest (green line) in Finland. An annual cycle (upper left) and a mean July diurnal cycle (upper right) are shown for the model (solid line) and the flux-tower observations (dashed line), with a negative sign indicating an upwards heat transfer. Lake surface temperature on global scale simulated by the model FLake and observed with MODIS (bottom).