An Analysis of Exit-Flow Drainage Jets over the Chesapeake Bay

Abstract

Synthetic aperture radar has shown great promise in detecting surface roughness patterns generated by atmospheric and oceanic features. Those roughness patterns that are the result of sea surface wind stress may be analyzed and related to characteristics of the atmospheric boundary layer. Previously reported examples of detectable atmospheric signatures include gravity waves and Rayleigh?Benard convection in cold-air outbreaks. In this paper, the results from an analysis of an image that contains the signatures of nocturnal-drainage-flow-forced exit jets along the western shore of Chesapeake Bay is presented. A regression analysis is performed that links the length of the surface stress patterns associated with these exit jets to the geometry of their source basins. This analysis differs from previous drainage-flow studies in that a population of drainage flows of varying sizes is studied under identical synoptic conditions. This large sample size provides a unique opportunity to examine the role that topography plays in forcing this kind of flow. To complement the observational study, a two-dimensional, shallow-fluid model is developed to simulate the drainage-flow exit jets once they leave their source basins. This model allows simulation of the behavior of these flows over the entire range of forcing values observed in the image. This kind of analysis provides physical insight into the dynamics of these hybrid flows and a basis for the development of a similarity theory that relates the physically significant forcing parameters to the characteristic length and speed scales of this phenomenon. The lack of in situ observations unfortunately prevents a direct comparison between model results and observations; however, the model is shown to give characteristic jet length scales that are in reasonable agreement with values obtained from the image analysis.