An airborne, real aperture radar study of the Chesapeake Bay outflow plume

An airborne, real aperture radar (RAR) has been used to study the fronts as sociated with the Chesapeake Bay outflow plume during spring outflow condit ions. The RAR produced images of the ocean surface with a range resolution of 10 m, an azimuthal resolution of approximately 30 m, and an image size o f 2.5 km x 24 km. Two sampling strategies were utilized: one to synopticall y map the entire mouth of the Chesapeake Bay at roughly hourly intervals; a nd a second to capture the rapid evolution of particular features. In addit ion, flight times were chosen such that over the course of the entire exper iment, data were collected over all phases of the semidiurnal tidal cycle. Three distinct frontal signatures were observed in the imagery. A primary f ront extended from inside the estuary along the Chesapeake Channel to an an ticyclonic turning region east of Cape Henry, and then extended southward a long the coast toward Cape Hatteras. This is the classic expression of the plume front, inertial turning region, and coastal jet. A second front with a north-south orientation was observed approximately 20 km east of the bay mouth. This secondary front appears to mark the residual offshore density g radient. A third front was identified east and south of Cape Henry, within 2 km of the coast. This front appears to mark the inshore edge of the plume and has not been documented previously. Time sequences of the imagery indi cate that when moving in a clockwise sense around the primary front, the fr ontal translation speed varies systematically from 20 cm/s in the northern section to 50 cm/s in the south. The position of the primary front and the locations and trajectories of small-scale frontal cusps suggest that bathym etry may be both a significant determinant of the front location as well as a source of along-front variability. These observations are possible due t o the airborne RAR's ability to collect high-frame rate image sequences, a capability that,is not shared by present space-based radar systems.