## 3 Earth Surface Modelling Advances and Links with EO Datasets
### Ocean-Atmosphere Coupling  
Both satellite and in situ observations show evidence of a diurnal cycle in Sea Surface Temperature (SST), with a amplitude in excess of 1K over large regions of the ocean extending from the Tropics to high latitudes (see review by (2017)). There is a growing appreciation for the need to resolve coupled physics at time scales shorter than a day. In terms of representing the diurnal cycle in models, a key factor is the difference between the temperature at a depth of 10m (foundation SST), the temperature at shallower depths which is generally warmer (bulk SST) and the temperature in the ocean's thin skin layer (<1 mm thick) which can be cooler at night and under certain wind conditions. Observations of the diurnal cycle have motivated the development of ocean models to represent the diurnal cycle. This can be achieved in one of two ways: the first approach is to provide sufficient vertical resolution (O(1m)) in the near surface to explicitly resolve the diurnal cycle (2017; 2018) which models the bulk SST. The second approach is to use coarser vertical resolution but provide a sub-model within the mixed layer scheme which parameterises the bulk and skin temperature (2016). Both approaches agree well with the observed diurnal amplitude of the bulk SST (equivalent to 1 m). There is a subtle difference in the approaches which is taken into account when calculating surface fluxes from bulk formulae; (2016) were able to parameterise the skin temperature of the ocean while (2018) resolve the bulk SST with a 1 m vertical resolution and three hourly coupling. Ref. (2016) show differences in the air-sea fluxes of up to 10 W/m\({}^{2}\) in the Tropics when the diurnal cycle of SST is included. The diurnal cycle of solar radiation can lead to stabilization of the surface ocean during daytime and to nighttime convective mixing. As a consequence, when winds are weak, daytime solar warming can create near-surface temperature stratification, referred to as a \"warm layer\", with skin temperatures up to 2C (or more) warmer than the foundation SST below the warm layer. This stratification and the peak skin temperature, however, cannot be simulated in an ocean model forced by daily-averaged air-sea fluxes (2016). Critically, the very large daytime surface heat fluxes associated with solar radiation, acting on the thin daytime surface mixed layer, result in a rectified SST that is warmer than the SST produced by the averaged fluxes. Without the diurnal cycle coupling, the skin temperature would be equivalent to the bulk foundation SST. The rectified SST can then feedback to affect the air-sea fluxes. Ref. (2016) show that, when foundation SST is used instead of the diurnally varying skin temperature,the computed net surface heat flux can differ by more than 5-10 W/m\({}^{2}\) in parts of the tropics when averaged over the 10-year period. The afternoon near-surface stratification can also cause trapping of wind-momentum resulting in a thin diurnal jet [263] and enhanced surface freshening associated with daytime rainfall under light winds [264]. The resulting rectified SST, enhanced near-surface stratification, shear, and advection can lead to a cascade of errors if not properly represented. The benefits of coupling a dynamical ocean to the ECMWF medium-range forecasts have been evaluated in a set of dedicated forecast sensitivity experiments and from June 2018 all forecasting systems running operationally now include the ocean model (NEMO v3.4) in the ORCA025Z75 configuration with 75 levels in the vertical and a quarter of degree horizontal resolution to represent the 3D ocean and includes a dynamical sea-ice model (LIM2) and a wave-model (EC-WAM). These coupled forecasts, in which the ocean interactively responds to the meteorology, are better performing particularly for the prediction of tropical cyclones and their intensity [265].