Towards a methodology for the validation of low spatial resolution remote sensing data and products. The Valencia Anchor Station

The SCALES (SEVIRI {& GERB Cal/Val Area for Large-scale field ExperimentS}) Project has specifically been defined to assist in the validation of new radiation budget and cloud products provided by the GERB (Geostationary Earth Radiation Budget) instrument on board the first European METEOSAT Second Generation geostationary satellite (MSG-1). The special character of remote sensing measurements to correspond to area integrated values, obliges independent in situ measurements to be representative of zones of a minimum number of pixels of the sensor under consideration. The large GERB pixel size (around 50 x 50 km²) and its high frequency sampling (every 15 min) makes it necessary to develop a new specific validation methodology to carry out independent measurements over large extended areas. Basically, the methodology proposed here leans on the use of a robust reference meteorological station (Valencia Anchor Station) set up on a reasonably homogeneous area, around which 3D high resolution meteorological fields are obtained from the MM5 Meteorological Model, and mobile stations and transect measurements help to account for non-homogeneities. During the GERB Commissioning Period, and in the framework of the GIST (GERB International Science Team), two field campaigns have so far been carried out where CERES instruments onboard Terra and Aqua NASA satellites provided additional radiance measurements specifically obtained in the PAPS mode (Programmable Azimuth Plane Scanning) over the Valencia Anchor Station to support validation efforts. Besides, continuous ground sun photometer and GPS precipitable water content measurements were also obtained, as well as radiosounding ascents, exactly launched at the station site and at CERES overpassing times. The large data set gathered so far, together with other higher resolution data available from instruments such as LANDSAT, MODIS and SEVIRI, provide a wealth of conditions under which the methodology may be progressively established and tested.