Viewing a cold air outbreak using satellite‐based synthetic aperture radar and advanced very high resolution radiometer imagery

Abstract

In this phenomenological study we compare nearly simultaneous visible‐wavelength advanced very high resolution radiometer (AVHRR) and synthetic aperture radar (SAR) images of a cold air outbreak that occurred off 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 linear features, known as cloud streets. These streets are spaced about 5 km apart, 5 times the depth of the boundary layer. By 200 km offshore, the visible image shows nascent, closed‐cell, mesoscale cellular convection (MCC), with connected disks and smears of white clouds surrounded 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, broadly defined spatial pattern but with a substantial increase in detail uncorrelated with the AVHRR image. In the area of the cloud streets shown in the AVHRR image are two‐dimensional, elongated regions of enhanced radar backscatter, called SAR streaks, with the same cross‐wind spacing as the cloud streets. This suggests the presence of quasi‐two‐dimensional roll vortices in the atmospheric boundary layer. Downstream, in the area of proto‐MCC, are broadly spaced expanses of enhanced backscatter that form two‐dimensional patterns, such as connected arcs that have structure and scales similar to the spaces between the clouds. The spatial evolution of the streaks and streets broadly conforms to the patterns of downdrafts and updrafts, respectively, exhibited in modeling studies of cold air outbreaks. However, the comparison with modeling studies is more successful for the cloud streets than for the SAR streaks. We suggest that this difference is because the clouds in the AVHRR images give a time‐integrated signal of large‐scale, quasi‐two‐dimensional atmospheric turbulence, whereas mesoscale radar patterns of surface roughness give a relatively instantaneous view of fundamentally three‐dimensional structure within the atmospheric boundary layer.