A case study of satellite synthetic aperture radar signatures of spatially evolving atmospheric convection over the Western Atlantic Ocean

A case study of a particularly intense cold air outbreak over the north Atlantic Ocean extending from the northeast coast of the United States to the Gulf Stream is described. A RADARSAT satellite synthetic aperture radar (SAR) image of this outbreak dramatically illustrates the spatial evolution of convection. Nearly coincident images from the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer are used to compare many interesting features. In addition, National Weather Service rawinsonde data, National Data Buoy Center buoy data, and Woods Hole Oceanographic Institute Coastal Mixing and Optics mooring data are presented. We use these data to help describe the spatial evolution of the atmospheric boundary-layer processes involved in this outbreak. Rows of cellular convective clouds begin to appear some distance offshore and then slowly increase in horizontal diameter and wavelength in the downwind direction, with a subsequent jump in cloud diameter downwind of the Gulf Stream North Wall (GSNW). The SAR image shows a similar evolution of sea-surface footprints of these boundary-layer features. This change in boundary-layer structure is attributed to corresponding changes in static stability. About 300 km south of the GSNW in the SAR image, an even larger jump in cell diameter appears and the cells become non-uniform with bright crescents and filled semi-circles on the downwind sides of the cells. These are believed to be surface effects of gust fronts induced by the mesoscale cellular convection and enhanced by the overall northwesterly flow.