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Wendell S. Brown and Richard P. Trask

Abstract

A Method for inferring an area-averaged bottom stress and energy dissipation rate in a tidal estuarine channel is presented. The one-dimensional continuity and momentum relations are developed using simplifying assumptions appropriate for a well-mixed shallow and narrow estuary. The finite-difference form of these relations is derived for a section of the Great Bay Estuary, New Hampshire, an estuary which has been shown to have a relatively large energy dissipation rate. A set of current, bottom-pressure and sea-level measurements from the Estuary is used to estimate time series of all important first- and second-order terms in the momentum equation. Except near slack water, we find that the instantaneous first-order balance must be between the surface-slope-induced pressure gradient and bottom-stress forces. Important second-order contributions to the balance come from the inertial and convective acceleration terms. Time series of bottom stress are inferred by summing the estimated terms. For this study site the 14-day rms bottom stress is 45.1 ± 4,5 dyn cm−2 with a corresponding rms and mean dissipation rate of 3526 ± 420 and 2478 ± 297 ergs cm−2 s−1, respectively. The role of the first-order tidal motion and non-linearities in the mean second-order force balance is discussed.

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Robert A. Weller, Daniel L. Rudnick, Richard E. Payne, Jerome P. Dean, Nancy J. Pennington, and Richard P. Trask

Abstract

An array of five surface moorings was set in the subtropical convergence zone southwest of Bermuda with spacings of 16 to 53 km. Meteorological instrumentation on each of the surface buoys recorded wind velocity, barometric pressure, solar radiation, air temperature, sea temperature, and relative humidity. One objective of the deployment was to look for horizontal variability in the meteorological fields on the scale of the array. In support of that objective, both a high data return from the instruments and a quantitative evaluation of the quality of the measurements were sought. To maximize data return rates, two meteorological instruments were placed on each buoy. To determine the accuracy of the measurements, careful predeployment and post-deployment calibrations of all instruments were carried out, and, during the experiment, meteorological data were collected from ships stationed near the buoys. From the two redundant instruments it was possible to construct one complete dataset for each mooring. The results of the calibrations and intercomparisons provided estimates of the errors in the measurements. Significant horizontal variability was occasionally observed in some of the surface meteorological variables and in the wind stress and air-sea heat flux fields. More often, observed spatial gradients in the meteorological fields were not significantly larger than the experimental uncertainty in those gradients. Larger than anticipated errors were encountered in measuring wind speed and barometric pressure, and the preformance of anemometers, barometers, relative humidity sensors, and other sensors for use on buoys could be improved.

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Robert A. Weller, Daniel L. Rudnick, Nancy J. Pennington, Richard P. Trask, and James R. Valdes

Abstract

Measurements of upper ocean variability were made in the subtropical convergence zone southwest of Bermuda from an array of five surface moorings set with spacings of 16 to 53 km. The intent was to observe oceanic fronts and to quantify the spatial gradients associated with them. Vector Measuring Current Meters (VMCMS) and Vector Averaging Current Meters (VACMS) were attached to the mooring lines beneath the surface buoys to measure velocities and temperatures. Modifications were made to the VMCMs in an attempt to improve data return. The performance and accuracy of these moored instruments are examined. Predeployment and postdeployment calibrations were carried out; and other sources of error, such as mooring motion, are considered. A number of oceanic fronts passed through the moored array during the experiment, and the horizontal gradients observed in the velocity and temperature fields were significantly larger than the uncertainties in measuring those gradients.

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