Combined use of SAR and scatterometer for the study of the atmospheric boundary layer

The paper illustrates a possible way of combining SAR images over the sea with scatterometer derived wind fields, to study the spatial characteristics of the marine atmospheric boundary layer (MABL). Both instruments are active microwave radars, relying upon the same physical mechanisms of electromagnetic wave-sea surface interaction. Sensing the sea surface with different spatial resolutions, they make possible to investigate the horizontal and vertical structure of the MABL from the mesoscale (< 100 km) down to the intermediate (< 10 km) and small (< 2 km) scales. An example of the multi-scale description of the MABL is provided analysing overlapping SAR image and scatterometer wind field in the Mediterranean Sea. Environmental conditions of this case study were characterised by moderate wind (8 to 10 m s super(-1)) and unstable air-sea conditions. The radar backscatter modulations exhibited by the SAR image reveal the presence of large-scale atmospheric variations, orographic wind distortions and wind rolls. The majority of the large-scale modulations of radar backscatter are obviously related to the wind speed; some other large-scale feature, which does not seem directly related to the wind, could be explained by the weak Ekman vertical velocity, derived from the scatterometer wind field. The analyses of the wind rolls have been performed through the bi-dimensional spectrum of the SAR image, which shows a two-scale orthogonal wave system with wavelengths of 7 km and 1.5 km. The paper points out the importance of other parameters, such as the sea and air temperatures, in the interpretation of the SAR images and in the full exploitation of the scatterometer winds.