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A. G. Enriquez and C. A. Friehe

Abstract

Aircraft measurements during the winter 1989 Shelf Mixed Layer Experiment (SMILE) and summer 1982 Coastal Ocean Dynamics Experiment (CODE) were used to characterize the spatial variation of the low-level wind and wind stress over the northern California shelf. The curl of the wind stress was calculated from directly measured turbulent stress components. The accuracy of the computed curl was estimated to be adequate to map the spatial structure. Wintertime measurements showed a concentration of large positive curl [over 1 Pa (100 km)−1] west of Point Arena, regardless of wind direction, due to the effects of the coastal topography on the wind fields. Results from summertime measurements showed a similar local maximum of positive curl west of Point Arena. Larger curl values [over 3.5 Pa (100 km)−1], however, were observed across a hydraulic jump propagating from Stewarts Point for highly supercritical marine boundary-layer flow.

A two-layer, vertically integrated numerical model of coastal upwelling was used to assess the relative importance of the stress curl to the stress-driven transport. The nonzero stress curl altered the thickness of the upper layer considerably after a day of integration, expanding the horizontal extent of upwelling offshore. The greatest effects were around areas of high positive curl, increasing coastal upwelling for downcoast winds and decreasing downwelling for upcoast winds. The effect of the stress curl, however, was attenuated near the coast as compared to the maximum possible deep water values. The validity of the numerical model was verified by comparison with an analytical solution of a simplified set of one-dimensional, frictionless equations of motion.

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C. E. Dorman, A. G. Enriquez, and C. A. Friehe

Abstract

The structure of the lower atmosphere over the northern California coastal ocean upwelling area was studied during the Shelf Mixed Layer Experiment in the winter of 1989. Surface data were collected at seven automated coastal stations and six buoys. Boundary layer soundings were made using balloons at the coast and a research aircraft over the ocean. The aircraft was also used to map the low-level (30 m) mean and flux fields over the 80 km × 120 km shelf area.

The wintertime coastal weather conditions were more variable than in summer and were observed to fit into three categories: strong northerly (downcoast) winds, strong southerly (upcoast) winds, and weak winds. The variability was caused by the passage of wintertime cyclones interspersed with periods of small pressure gradients. The strong wind cases had small diurnal variations, whereas the diurnal variations were large for the weak wind case.

The vertical structure of the coastal boundary layer was more uniform compared to that in summer, with weak or nonexistent temperature inversions. Winds below 600 m were not correlated with those above 1.5 km except during strong alongshore winds. The presence of a coastal mountain ridge suppresses low-level cross-shore flow. The horizontal structure over the ocean shelf measured by the low-level aircraft tracks showed an area of large positive wind stress curl [over 1 Pa (100 km)−1] west of Point Arena for both directions of the strong wind cases. This implies positive Ekman pumping of the shelf waters in this area regardless of wind direction.

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G. R. Stegen, C. H. Gibson, and C. A. Friehe

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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.

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K. F. Schmitt, C. A. Friehe, and C. H. Gibson

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.

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G. A. Payne, C. A. Friehe, and D. K. Edwards

Abstract

The heat transfer characteristics of an aircraft-mounted resistance-wire atmospheric temperature sensor are modeled to determine the time and frequency responses. The sensor element (Rosemount 102E4AL) consists of a 25-μm-diameter platinum wire wound around a cruciform mica support with approximately 143 diameters of wire between contacts with the mica. A longitudinally distributed, radially lumped capacitance model provided for the convective heat transfer to the wire and the transient heat conduction along it. Similarly, the temperature gradient across the thin dimension of the mica support was neglected, and a radially distributed model provided for the convective heat transfer to the mica and the transient conduction within it. The two solutions are coupled by the boundary conditions at the wire-mica contact. The equations were solved to produce the temperature distribution along the wire and in the mica support as a function of the frequency of a free-stream sinusoidal temperature fluctuation. The frequency response transfer function was determined and fit to a two-time-constant transfer function by regression analysis. The two-time-constant model fits the general solution very well. The small (fast response) time constant is essentially determined by the wire itself. The larger (slow response) time constant is due to conduction into and out of the mica supports. The model predicts that the effects of the mica supports are important for frequencies greater than about 0.1 Hz. The responses to five different temperature waveform inputs (sinusoid, step, pulse, ramp, and ramp level) are derived using the two-time-constant model with Laplace transform techniques for both infinite-length wire (no mica support effects) and the finite-length wire of the 102 probe. The actual temperature signals are distorted by the larger time constant of the mica supports, especially for the pulse and ramp inputs that are typical of aircraft measurements of thermals and inversions.

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C. A. Friehe, R. L. Grossman, and Y. Pann

Abstract

An improved calibration technique for an airborne Lyman-alpha hygrometer is presented. Like previous methods, it relies upon simultaneous measurement of absolute humidity determined from a slower response hygrometer. We show that a substantial improvement in the Lyman-alpha calibration is obtained by accounting for the time lag of the slower instrument.

To show our technique we use data from Lyman-alpha and thermoelectric devices on the NCAR Electra during an investigation of the nearly neutral boundary layer over the Arabian Sea as part of the WMO/ICSU Summer Monsoon Experiment. We also show that for near-neutral conditions the eddy-correlation water vapor flux can be adequately estimated using the fast response vertical velocity data from a gust probe and slower response data from the thermoelectric device, which has been properly advanced to account for the time lag.

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P. L. Fuehrer, C. A. Friehe, and D. K. Edwards

Abstract

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John C. Wyngaard, Larry Rockwell, and Carl A. Friehe

Abstract

We use a Taylor series expansion technique to calculate the effects of mean-flow distortion on turbulence measurements ahead of an axisymmetric body. The approach is valid when the integral scale of the turbulence is large compared to the maximum body diameter, which is usually the case for energy-containing turbulence statistics measured ahead of aircraft and the towed bodies used in oceanography. Using a 5:1 ellipsoid as a representative body, we show the nature of the attenuation and crosstalk error terms which the flow distortion induces in the measured turbulence components. The contours of the resulting errors in velocity covariances measured ahead of the ellipsoid and a paraboloid of revolution reveal that, in general, the errors in turbulent Roynolds stress can be the most severe. For sufficiently small distortion effects, which typically means more than one diameter away from the nose, the distortion matrix can easily be inverted analytically, giving explicit expressions for the true covariances in terms of the measured ones. We assess the effects of averaging measurements taken in different directions along the same path, as is sometimes done with aircraft data, and of nonzero angle of attack.

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P. L. Fuehrer, C. A. Friehe, and D. K. Edwards

Abstract

An analytical study was conducted of the thermal frequency response of an atmospheric temperature probe consisting of a thermistor bead with two lead wires soldered to thin support posts. Such probes are used in aircraft temperature sensors and for surface-layer turbulence studies. The results show the effects of the lead wires on the frequency response (amplitude and phase) of the probe for two end conditions of the lead wires: 1) fixed temperature at the mean free-stream value, and 2) adiabatic. For the smallest commercially available thermistor bead of approximately 200-µm diameter and for 20-µm-diameter platinum lead-wire lengths of about 0.8 mm, the conduction to the supports was found to be minimal for both end conditions. It was determined, however, that the lead wires themselves act as heat transfer fins and actually improve the frequency response over that of an ideal isolated bead. Model calculations show that the inclusion of multiple lead wires (four and six) connected mechanically, but not electrically or thermally, to supports would further improve the response. The thermal analysis is also applied to small type-E thermocouple junctions made of 12.5-, 25-, and 50-µm- diameter wires, and the results show that the lead wires also improve the frequency response.

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E. N. Brown, C. A. Friehe, and D. H. Lenschow

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An air-motion sensing technique is described for measurement of attack and sideslip angles and dynamicpressure. The sensing probe consists of an array of five pressure holes in the standard radome of a twin-jetresearch aircraft. Comparisons are made with air motion measurements (angle of attack and dynamic pressure) obtained from a conventional differential pressure flow angle sensor at the tip of a nose boom 1.5fuselage diameters ahead of the aircraft body. The results indicate that the radome system works well downto scale sizes slightly larger than the fuselage diameter. (Finer scale measurements were limited by pressuretransducer response.) An insitu calibration technique is described for the determination of the empiricalradome angle-pressure difference sensitivity factor k, as a function of aircraft Mach number. The value ofk, so determined at low Mach numbers, is in approximate agreement with that calculated for potential flowfor a spherical radome. The in-situ technique applied to the conventional nose boom sensor indicates thatthe value of k based on wind tunnel calibrations may not apply for the present installation.

The time response of the conventional pressure system on the NCAR Sabreliner twin-jet aircraft is estimated on the basis of an in-flight comparison between the conventional pressure probe and a fast-responsegust probe flown together on a nose boom. Comparison of the power spectra of the conventional and radomeangles of attack for a traverse in boundary-layer turbulence indicates that the response of the radome systemis superior to the conventional system due to the shorter pressure lines that can be used.

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