VIEWING A COLD-AIR OUTBREAK USING SATELLITE-BASED SYNTHETIC-APERTURE RADAR AND ADVANCED VERY HIGH-RESOLUTION RADIOMETER IMAGERY

In this phenomenological study we compare nearly simultaneous visible- wavelength advanced very high resolution radiometer (AVHRR) and synthe tic aperture radar (SAR) images of a cold air outbreak that occurred o ff the ice pack in the Bering Sea. The visible image shows the spatial evolution of the cloud structure from the ice edge to several hundred kilometers offshore. Near the ice edge, the clouds form regular linea r features, known as cloud streets. These streets are spaced about 5 k m apart, 5 times the depth of the boundary layer. By 200 km offshore, the visible image shows nascent, closed-cell, mesoscale cellular conve ction (MCC), with connected disks and smears of white clouds surrounde d by arcs and rings of dark, cloud-free regions. The diameters of the disks range between 10 and 15 km. The SAR image shows the same, broadl y defined spatial pattern but with a substantial increase in detail un correlated with the AVHRR image. In the area of the cloud streets show n in the AVHRR image are two-dimensional, elongated regions of enhance d radar backscatter, called SAR streaks, with the same cross-wind spac ing as the cloud streets. This suggests the presence of quasi-two-dime nsional roll vortices in the atmospheric boundary layer. Downstream, i n the area of proto-MCC, are broadly spaced expanses of enhanced backs catter that form two-dimensional patterns such as connected arcs that have structure and scales similar to the spaces between the clouds. Th e spatial evolution of the streaks and streets broadly conforms to the patterns of downdrafts and updrafts, respectively, exhibited in model ing studies of cold air outbreaks. However, the comparison with modeli ng studies is more successful for the cloud streets than for the SAR s treaks. We suggest that this difference is because the clouds in the A VHRR images give a time-integrated signal of large-scale, quasi-two-di mensional atmospheric turbulence, whereas mesoscale radar patterns of surface roughness give a relatively instantaneous view of fundamentall y three-dimensional structure within the atmospheric boundary layer.