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Surface and Bottom Stress

The surface wind stress was assumed to be uniform over the model domain and was determined from wind measurements at the Boston Buoy (Station BB in Figure 2.2) using the Large and Pond (1981) formulation.

Although the scale of the bay is relatively small compared to most synoptic scale wind features, there is certainly some variability over the bay, caused in part by local effects such as the seabreeze. Some of the discrepancies between model and data, especially during the summer, are undoubtably the result of the idealized wind stress field. Observations were available at 1 hourly intervals from the Boston Buoy (anemometer height 13.6 m) except for the period 8/31/90 -- 10/4/90, when wind data from Logan Airport was used instead. A comparison between Boston Buoy and Logan Airport winds (Figure 2.1-1, Geyer et al, 1992) suggests that the winds are comparable during this time period, although the wind at Boston Buoy is weaker during the summer and stronger during the winter than winds at Logan. The hourly averaged winds were converted to wind stress, then averaged over 4 hourly intervals before input to the model to reduce the size of the input file.

The bottom stress is determined from the quadratic drag law applied at the point closest to the bed where there is a velocity estimate. This is the middle of the lowest grid cell. Because the distance between the the lowest velocity point and the bed varies with location, the bottom roughness length is specified at each location, and then a constant stress layer is assumed so that an effective drag coefficient can be computed at the velocity point. For this study, was assumed to be uniform throughout the domain with a value of , which is equivalent to a drag coefficient of at 10 m above the bottom (which is in turn a typical value used for depth-averaged models of coastal seas). In these runs, the bottom sigma layer thickness is 10% of the water depth, so the lowest velocity point is 5% of the water depth above the bed. Within Massachusetts Bay, the maximum distance between the lowest velocity point and the bed is 4.8 m in Stellwagen Basin, where the model water depth reaches 96 m.