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W. N. SHAW

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W. N. SHAW

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[Reprinted from the author's corrected seperate print.]

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W. N. SHAW

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W. J. Shaw and J. E. Tillman

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Fast-responding thermocouple psychrometers are often used in atmospheric boundary-layer turbulence measurements for the computation of heat and moisture fluxes. Small size, low cost, ease of interchange-ability and the use of the familiar psychrometric equations make this an ideal sensor for many applications at temperatures above freezing. However, a feature of these instruments that is frequently disregarded is that, due to wicking, the wet-bulb sensor has a frequency response that is an order of magnitude slower than the dry-bulb sensor. This difference in response time between the wet and dry sensors causes errors in the variances of humidity in one set of data as large as a factor of 5 and major errors in the shape of the humidity spectrum at high frequencies. We present a known but infrequently applied solution to this problem of sensor response differences in the hope that its simplicity, together with the reminder that a problem exists, will serve to encourage its use in the computation of humidity from fast-response psychrometric sensors.

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W. N. SHAW

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W. J. Shaw and J. H. Trowbridge

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Velocities produced by energetic waves can contaminate direct covariance estimates of near-bottom turbulent shear stress and turbulent heat flux. A new adaptive filtering technique is introduced to minimize the contribution of wave-induced motions to measured covariances. The technique requires the use of two sensors separated in space and assumes that the spatial coherence scale of the waves is much longer than the spatial coherence scale of the turbulence. The proposed technique is applied to an extensive set of data collected in the bottom boundary layer of the New England shelf. Results from the oceanic test demonstrate that the technique succeeds at removing surface-wave contamination from shear stress and heat flux estimates using pairs of sensors separated in the vertical dimension by a distance of approximately 5 times the height of the lower sensor, even during the close passage of hurricanes. However, the technique fails at removing contamination caused by internal motions that occur occasionally in the dataset. The internal case is complicated by the facts that the motions are highly intermittent; the internal-wave period is comparable to the Reynolds-averaging period; the height of the internal-wave boundary layer is on the order of the height of measurement; and, specifically for heat flux estimates, nonlinear effects are large. The presence of internal motions does not pose a significant problem for estimating turbulent shear stress, because contamination caused by them is limited to frequencies lower than those of the stress-carrying eddies. In contrast, the presence of internal motions does pose a problem for estimating turbulent heat flux, because the contamination extends into the range of the heat flux–carrying eddies.

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S. Nicholls, W. Shaw, and T. Hauf

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During the Joint Air-Sea Interaction (JASIN) experiment over the North Atlantic, three aircraft equipped to measure turbulent fluctuations of wind, temperature and humidity flew together in close formation, in order to compare results. These aircraft were the MRF C130, the NCAR Electra and the DFVLR Falcon. Most runs were made in the atmospheric boundary layer. This paper presents the results of this intercomparison exercise. Results are presented in terms of comparisons between variances and covariances which are further investigated by comparing spectra and co-spectra.

Overall, very good agreement is found between the C130 and the Electra, although small differences can be detected. However, these are negligible compared to the scatter usually observed when making measurements in the turbulent atmospheric boundary layer. The Falcon, at an earlier stage of development, also shows reasonable agreement although the amount of available data was much more limited.

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W. N. SHAW

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W. N. SHAW

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W. N. SHAW

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