Sea Surface Temperature (SST) is a key climate variable that holds an important role in the exchange of heat, gas and momentum between the ocean and the atmosphere. During day time and under favourable conditions of low wind and solar heating, the upper few meters of the oceanic layer may experience an increase of temperature that can reach up to several degrees. This is most intense in the first few millimetres of the water column; the part observable from microwave and infra-red sensors on space-borne platforms.

Diurnal warming studies are limited by the availability of SST observations with a high spatio-temporal resolution; in situ studies offer high temporal resolution but are limited in spatial extent. As the diurnal cycle of the SST is still not properly understood, most atmospheric and climate models only use one SST field per day, representative of night-time conditions. However, the water column is well mixed during the night and no diurnal warming is present. This simplification of the SST has been reported to cause biases in the estimated surface fluxes while strong SST diurnal signals can complicate the assimilation of SST fields in ocean and atmospheric models, the derivation of atmospheric correction algorithms for satellite radiometers and the merging of satellite SST from different sensors. Not accounting for the daily SST signal can cause biases in the scatterometer derived ocean wind fields and biases in the estimated net flux of CO2, as the out-flux of oceanic CO2 is positively correlated with the increase of SST.

Thus, the project SSTDV: R.EX.- IM.A.M. focuses on two important topics related with the diurnal variability of SST. Characterizing and quantifying the regional diurnal warming from the MSG/SEVIRI hourly SST fields is important in order to identify regions prone to diurnal warming. Investigating the impact of the diurnal cycle of SST on atmospheric modelling will be achieved by increasing the SST temporal resolution in the model WRF and model outputs will be evaluated in terms of modelled 10m winds and surface heat fluxes.

Within this context, 3 main tasks have been identified. The first task includes the validation and inter-comparison of SEVIRI and AATSR data, the construction of the night-time foundation temperature fields and the characterization of the regional diurnal warming.

The second task focuses on modelling the diurnal SST variability using the General Ocean Turbulence Model (GOTM). The activities within this task include sensitivity tests on the GOTM set-up, comparison of GOTM, SEVIRI and buoys in point locations and a focus in the North Sea/Baltic Sea with comparisons of GOTM, SEVIRI and 3 diurnal paremeterisation schemes.

Assessing the perturbations in an atmospheric model associated with the daily SST cycle is the prime goal of the third and final task. This will be performed by a) increasing the temporal resolution of the SST initial conditions in WRF and b) by evaluating the already existing WRF diurnal scheme. Impact assessment will include comparison of the modelled 10-m wind fields against the ESA's Envisat ASAR 10-m winds, routinely retrieved at DTU, the MetOp-A ASCAT winds and in situ measurements from meteorological masts located offshore. Heat flux error estimates will be assessed and compared with the SEVIRI SSI and SLI products.

The proposed project aims at expanding the scientific background for understanding the spatial and temporal variability of key climate variables and their representativity in atmospheric and oceanic models. ESA SENTINEL-1 and SENTINEL-3 are highly anticipated as new satellite observations for 10-m winds and SST and will be useful for the continuation of this research project. ESA's future ADM-Aeolus mission will provide satellite observations of vertical wind profiles, offering the possibility of investigating the potential signature of diurnal SST signals at higher atmospheric levels and validating the modelled wind fields at various heights.