COMPARISON BETWEEN SAR STREAKS AND AVHRR SPATIAL AND TEMPORAL EVOLUTION OF CLOUD STREETS TO DERIVE ROLLS ASPECT RATIO MAPS

摘要

Combination of spaceborne Synthetic Aperture Radar (SAR) and Advanced Very High Resolution Radiometer (AVHRR) images are used to investigate the planetary roll vortex and cloud street formation. Consequently, rolls aspect ratio maps are derived and the PBL depth estimated. As case studies one midlatitude ERS-1 SAR image, acquired on 30 March 2000 offshore Genoa gulf, is presented. Evidence of the occurrence of the characteristic streaks due to the presence of roll vortices are analysed [2]. In correspondence with the area covered by the SAR image all the available AVHRR (channel 2 and channel 4) images for the day are shown in a sequence to highlight the spatial and temporal evolution of the evident streak formations and give indication of how streets are spaced. Two AVHRR images are then analysed to compute the cloud top heights over the area of interest, and by assuming a constant cloud thickness, the cloud base height is derived. By applying Fourier analysis to the properly low-pass filtered and subsampled SAR subimage, the rolls horizontal wavelength is measured while, from the AVHRR images, the cloud top and bottom heights are derived. By matching the time and scale resolution of the two kinds of satellite images, 6 km resolution rolls aspect ratio maps are derived and the planetary bounday layer (PBL) depth estimated.