ESTIMATE OF BOTTOM AND SURFACE STRESS DURING A SPRING-NEAP TIDE CYCLEBY DYNAMICAL ASSIMILATION OF TIDE-GAUGE OBSERVATIONS IN THE CHESAPEAKE BAY

Dynamical assimilation of surface elevation from tide gauges is invest igated to estimate the bottom drag coefficient and surface stress as a first step in improving modeled tidal and wind-driven circulation in the Chesapeake Bay. A two-dimensional shallow water model and an adjoi nt variational method with a limited memory quasi-Newton optimization algorithm are used to achieve this goal. Assimilation of tide gauge ob servations from 10 permanent stations in the Bay and use of a two-dime nsional model adequately estimate the bottom drag coefficient, wind st ress, and surface elevation at the Bay mouth. Subsequent use of these estimates in the circulation model considerably improves the modeled s urface elevation in the entire Bay. Assimilation of predicted tidal el evations yields a drag coefficient, defined in the hydraulic way, vary ing between 2.5 x 10(-4) and 3.1 x 10(-3) The bottom drag coefficient displays a periodicity corresponding to the spring-neap tide cycle wit h a maximum value during neap tide and a minimum value during spring t ide. From assimilation of actual tide gauge observations, it is found that the fortnightly modulation is altered during frontal passage. Fur thermore, the response of the sea surface to the wind forcing is found to be more important in the lower Bay than in the upper Bay, where th e barometric pressure effect seems to be more important.