Synthetic Aperture Radar Observations of Mesoscale Atmospheric Phenomena

Satellite-borne synthetic aperture radar (SAR) provides the capability of mapping over-water surface wind speed fields at resolutions of a kilometer or better. Thus, SAR provides an exceptionally high-resolution tool for examining the mesoscale phenomena of the marine atmospheric boundary layer. Mesoscale atmospheric phenomena commonly seen in SAR images include those associated with fronts, convection, gravity waves, and barrier effects. SAR observations allow one to identify these phenomena in otherwise data-sparse maritime regions and to study both their kinematic structure and their relationship to synoptic and topographic forcing features. Frontal features observed on SAR images include the lobe and cleft instability of gravity currents, shear-driven vortices with scale depending largely on frontal type, and shear-driven gravity waves on the frontal inversion with orientation depending largely on frontal type. Observation of these features can thus provide information on frontal type and behavior that would not otherwise be available in data-sparse marine regions. Convection, both boundary layer and deep, causes downdrafts and surface outflows, which are readily apparent in SAR images. Thus, SAR images can allow one to determine the boundary layer stability even when the lower atmosphere if hidden by upper-level clouds. Moreover, SAR images can be used to measure the size, shape, orientation, and gust intensity of the surface outflows from deep precipitating convection, again irrespective of the overlying cloud. Mesoscale barrier effects in both the boundary layer and free troposphere strongly affect the surface wind speed field, so SAR images contain the signatures of mountain lee waves, island wakes, gap flow, and barrier jets. Because these phenomena often coexist offshore of mountainous coasts, SAR images can be used to study their interaction, for example the modulation of island wakes and gap flow by mountain waves in the Kennedy Entrance region of the Alaskan coast.