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- Author or Editor: C. H. Gibson x
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Abstract
Vertical fluxes of momentum and sensible heat have been measured above the sea surface by the direct dissipation method. Measurements were made over the open ocean from the Scripps Floating Instrument Platform (FLIP) during the Barbados Oceanographic and Meteorological Experiment (BOMEX). The results are compared with simultaneous measurements of the fluxes by the profile, dissipation, and eddy correlation methods.
The momentum flux was inferred from the rate of viscous dissipation ε above the sea surface. The dissipation was determined by integrating the velocity derivative spectra after correcting the spectra for filter response. The friction velocity (u *) corrected for diabatic effects was 17.4 cm sec−1, corresponding to a shear stress τ=0.35 dyn cm−2. Profile measurements by the University of Washington gave the same value of u * in agreement with the present results. Measurements of momentum flux by Oregon State University (OSU) and the University of British Columbia using dissipation and eddy correlation methods gave somewhat higher values. Correction of the Kolmogoroff inertial subrange constant used in the OSU dissipation calculations gives fluxes in good agreement with the present work.
The sensible heat flux was inferred from the rate of dissipation χ of temperature variance. The temperature derivative spectra were corrected for instrument response and integrated to obtain values of χ. The average value of the sensible heat flux was 0.74 mW cm−2, in reasonable agreement with the profile and eddy correlation measurements. A value of sensible beat flux of 2.8 mW cm−2 has been reported by OSU using the dissipation technique. Correction of the temperature inertial subrange constant used by OSU lowered their heat flux to 1.1 mW cm−2.
Abstract
Vertical fluxes of momentum and sensible heat have been measured above the sea surface by the direct dissipation method. Measurements were made over the open ocean from the Scripps Floating Instrument Platform (FLIP) during the Barbados Oceanographic and Meteorological Experiment (BOMEX). The results are compared with simultaneous measurements of the fluxes by the profile, dissipation, and eddy correlation methods.
The momentum flux was inferred from the rate of viscous dissipation ε above the sea surface. The dissipation was determined by integrating the velocity derivative spectra after correcting the spectra for filter response. The friction velocity (u *) corrected for diabatic effects was 17.4 cm sec−1, corresponding to a shear stress τ=0.35 dyn cm−2. Profile measurements by the University of Washington gave the same value of u * in agreement with the present results. Measurements of momentum flux by Oregon State University (OSU) and the University of British Columbia using dissipation and eddy correlation methods gave somewhat higher values. Correction of the Kolmogoroff inertial subrange constant used in the OSU dissipation calculations gives fluxes in good agreement with the present work.
The sensible heat flux was inferred from the rate of dissipation χ of temperature variance. The temperature derivative spectra were corrected for instrument response and integrated to obtain values of χ. The average value of the sensible heat flux was 0.74 mW cm−2, in reasonable agreement with the profile and eddy correlation measurements. A value of sensible beat flux of 2.8 mW cm−2 has been reported by OSU using the dissipation technique. Correction of the temperature inertial subrange constant used by OSU lowered their heat flux to 1.1 mW cm−2.
Abstract
Anomalous results concerning the micrometeorological temperature field in the boundary layer over the ocean have been obtained in many recent experiments. These include lack of an inertial-convective subrange in temperature spectra, unusually large values for the scalar universal subrange constant, underestimation of the sensible heat flux by the bulk aerodynamic formula, gross imbalance of dissipation and production terms in the temperature variance budget equation, and dissimilarities of the temperature and humidity statistics and time traces. Empirically it has been observed that such results occur for unstable conditions when the temperature time series is characterized by a peculiar waveform, termed a “cold spike”, which has no counterpart in the humidity field and has not been observed over land.
To explain these results, it is proposed that surfaces of the small temperature sensors (thermistors, thermocouples and resistance wires) commonly used in marine boundary layer experiments become contaminated with salt spray when used over the ocean. Under typical ocean conditions (relative humidity > 70%), the results of Twomey (1953) indicate that the spray will exist as saline drops on the probe surfaces. Water will evaporate from or condense on the saline drops as the humidity around the sensor decreases or increases, respectively. The latent heat of vaporization associated with the evaporation and condensation processes will cool and heat the sensor, and therefore generate erroneous temperature signals. Evidence is presented that most of the anomalous temperature results observed over the ocean, including “cold spikes”, may be due to the spray-induced humidity sensitivity of such temperature sensors.
Abstract
Anomalous results concerning the micrometeorological temperature field in the boundary layer over the ocean have been obtained in many recent experiments. These include lack of an inertial-convective subrange in temperature spectra, unusually large values for the scalar universal subrange constant, underestimation of the sensible heat flux by the bulk aerodynamic formula, gross imbalance of dissipation and production terms in the temperature variance budget equation, and dissimilarities of the temperature and humidity statistics and time traces. Empirically it has been observed that such results occur for unstable conditions when the temperature time series is characterized by a peculiar waveform, termed a “cold spike”, which has no counterpart in the humidity field and has not been observed over land.
To explain these results, it is proposed that surfaces of the small temperature sensors (thermistors, thermocouples and resistance wires) commonly used in marine boundary layer experiments become contaminated with salt spray when used over the ocean. Under typical ocean conditions (relative humidity > 70%), the results of Twomey (1953) indicate that the spray will exist as saline drops on the probe surfaces. Water will evaporate from or condense on the saline drops as the humidity around the sensor decreases or increases, respectively. The latent heat of vaporization associated with the evaporation and condensation processes will cool and heat the sensor, and therefore generate erroneous temperature signals. Evidence is presented that most of the anomalous temperature results observed over the ocean, including “cold spikes”, may be due to the spray-induced humidity sensitivity of such temperature sensors.
