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Carl A. Friehe

Abstract

The effects of sound speed fluctuations on various measurements made with sonic anemometers are derived. Corrections due to temperature-generated sound speed fluctuations on measured statistics are estimated from the 1968 AFCRL Kansas data for different stability conditions. For most statistics, the corrections were not large: the maxima were −3.5% to the measured horizontal velocity, vertical velocity covariance (uw, proportional to the Reynolds stress) and −8.2% to the measured horizontal velocity, temperature covariance (, proportional to the horizontal heat flux) for unstable conditions. It is also shown that the measured cospectrum of uw is contaminated by the cospectrum of ; in the inertial subrange of frequencies, the measured uw cospectra for the AFCRL data for unstable conditions were calculated to be high by 5–11%.

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Michael Tjernström and Carl A. Friehe

Abstract

A “radome gust probe” system was installed on a twin-jet aircraft for the purpose of boundary-layer research. This system provided a useful relatively low-cost method for air motion and turbulence measurements on an aircraft already equipped with an inertial navigation system (INS) and a data acquisition system. An error analysis was made for the wind measurements and gave the limitations for the present system with an unmodified airliner-type INS, The major factors that limit the precision of the horizontal wind are the resolutions and accuracy of the aircraft ground speed components and the true heading. A simple method was devised to improve the heading resolution. From in-flight maneuvers, it was determined that the mean horizontal airspeed vector was accurate to <0.5 m s−1—limited by the long-term drift and oscillation errors from the INS-and that pitch and yaw contamination of the wind was less than 5%. The in-flight data indicate that there probably are unknown time lags within the INS, which degrade the wind vector measurement for anything but straight and level flight. Some intercomparisons were obtained from fly-by(s) past an instrumented 140-m tower. These showed general agreement between mean, variance and high-frequency spectral measurements of the velocity components and temperature. Flux or covariance comparisons were not as good, probably due both to the short flight tracks and a complex boundary layer structure at the tower.

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Carl A. Friehe and Djamal Khelif

Abstract

Three aircraft temperature sensors were compared in clear-air conditions on the NCAR King Air: a standard Rosemount nondeiced, fast-response flight test probe, the NCAR K probe, and a modified Rosemount probe with the platinum wire element replaced with a small thermistor bead. Responses to transient temperature changes were compared from soundings through sharp inversions. High-frequency spectral comparisons were obtained from level runs in the marine boundary layer. All three probes followed a two-time-constant response. The response of the thermistor-modified Rosemount probe was, however, much closer to a one-time-constant model than the two others. Following previous results and analyses, it appears that the longer time constant in the Rosemount probe is largely due to the contact of the platinum wire element, which is wound around mica supports. The long unsupported wire elements in the NCAR K probe do produce a superior high-frequency response, but low-frequency response is anomalous, perhaps due to the large plastic body placed upstream of the wires to separate out particles. The two-time-constant temperature response was compared for the three probes by developing expressions for the time derivative and time integral of the normalized temperature that separated the relative contributions of the sensor element and its support.

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Carl A. Friehe and S. E. Pazan

Abstract

Surface meteorological data from a large instrumented buoy moored in the North Pacific were analyzed and estimates of air-sea interaction fluxes of momentum, sensible heat, latent heat and radiation were made. Good agreement was obtained for one month of buoy operation from redundant sensors for wind speed, air temperature, sea temperature and barometric pressure. Flux estimates were made using the bulk aerodynamic formulas. The buoy data, sampled once per hour, were low-pass filtered to compare with the 6 and 12 h surface analyses of the U.S. Navy Fleet Numerical Weather Central (FNWC) for the area surrounding the buoy. Good agreement was obtained between the buoy data and the independent FNWC analysis.

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Robert L. Grossman and Carl A. Friehe

Abstract

The vertical structure of the low-level wind jet over the Arabian Sea during the southwest monsoon is modeled by using the geostrophic wind shear (thermal wind) and a two-layer boundary layer model. We show that geostrophic wind shear is dominant above the low-level wind maximum while turbulent momentum exchange is dominant below. Both effects combine to produce a low-level wind jet. Model results compare well with observations obtained from a research aircraft during the 1979 Summer Monsoon Experiment.

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Carl A. Friehe and Henry M. Stommel

Andrew F. Bunker was a research scientist at the Woods Hole Oceanographic Institution (WHOI) for over 30 years until his death in 1979. He was interested in the energy exchanges between the atmosphere and ocean, and devised techniques for their measurement from aircraft. He participated in many of the major oceanic field experiments of the 1960s and 1970s with Woods Hole research aircraft. His work in air-sea interaction culminated in a climatological atlas of air-sea fluxes for the Atlantic Ocean. We document here his career and scientific work in his memory.

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Carl A. Friehe and Kurt F. Schmitt

Abstract

The parameterizations of the sensible heat and moisture fluxes by the bulk aerodynamic formulas are determined from a compilation of existing data, together with some new results. The data set comprised 152 determinations of the sensible heat flux and 30 of the moisture flux from experiments in which the fluxes were measured directly over water with suitable turbulence instrumentation. Least-square-error fits were performed on the data. The moisture flux (and therefore the latent heat flux) is adequately described by the bulk formula with a coefficient of 1.32 × 10−3. The parameterization of the sensible heat flux is complicated, for the data show 1) a small positive heat flux for zero temperature difference between the air and sea surface, 2) the coefficient for stable conditions is smaller than for unstable conditions, and 3) the coefficient appears to increase at high wind speeds, as shown by the data of Smith and Banke (1975). Separate bulk formulas are presented for the sensible heat flux for the different conditions of the temperature field.

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Kurt F. Schmitt, Carl A. Friehe, and Carl H. Gibson

Abstract

Measurements of turbulent wind velocity, humidity and temperature spectra for stable and unstable stratification in the atmospheric surface layer obtained during an experiment over the North Pacific Ocean are presented. The velocity field appears to be in a state of local isotropy as measured by the ratio of vertical to streamwise velocity spectra S u(n>/ S u(n> at the measurement height of 29 m above the sea surface. Using Monin-Obukhov scaling, spectral shapes for humidity are similar to those for overland temperature. Evidence is presented which suggests that previous departures of marine temperature measurements from Monin-Obukhobzv similarity may be due to humidity sensitivity of salt-spray-contaminated temperature probes. Overland humidity data from the AFCRL-UCSD 1973 Minnesota Experiment (Champagne et al., 1977) were analyzed and also found to exhibit Monin-Obukhov similarity.

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David P. Rogers, Douglas W. Johnson, and Carl A. Friehe

Abstract

Observations of the mean and turbulent structure of the marine atmospheric boundary layer (MABL). obtained using the U.K. Meteorological Research Flight C-130 Hercules aircraft are used to investigate the momentum balance over the Irish Sea when warm air is advected offshore. The marine boundary layer is made up of two layers: a strongly stable internal boundary layer (IBL). and a stable residual layer located between the top of the IBL and the base of the planetary boundary layer inversion.

Measurements obtained near the upwind coast indicate that the flow is highly ageostrophic. Downwind of the Irish coast, there is a transition toward equilibrium between the geostrophic, Coriolis. and friction components of the flow along part of the flight track. However, another segment of the flight track indicates an imbalance between the pressure gradient and the other measured terms, which may be attributable to gravity waves affecting the adjustment process. This is more apparent in the leg perpendicular to the coast where the pressure gradient is balanced by the observed acceleration with negligible contributions from the Coriolis and friction terms.

Gravity waves associated with mountain lee waves propagate along the direction of the mean wind shear in the IBL, which is directed to the right of the wind measured along the flight track perpendicular to the coast at 30-m altitude. The dominant wavelength is about 19 km, which corresponds with the buoyancy frequency of the MABL new the Irish coast and is supported by satellite images of the cloud structure. Farther downstream the buoyancy frequency increases, but the longer wavelength signal remains dominant. An important result of the gravity waves is the modification of the wind field and wind stress within the IBL. The largest effect is observed in the stress direction, but large changes in magnitude are also observed. The results indicate that the direction of the wind stress corresponds to a large degree with the direction of the mean horizontal wind sheer.

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Qingtao Song, Tetsu Hara, Peter Cornillon, and Carl A. Friehe

Abstract

Simulations, made with the fifth-generation Pennsylvania State University (PSU)–National Center for Atmospheric Research (NCAR) Mesoscale Model (MM5), of the response of the marine atmospheric boundary layer (MABL) as air moves over a sharp SST front are compared with observations made during the Frontal Air–Sea Interaction Experiment (FASINEX) in the North Atlantic subtropical convergence zone. The purpose of undertaking these comparisons was to evaluate the performance of MM5 in the vicinity of an SST front and to determine which of the planetary boundary layer (PBL) parameterizations available best represents MABL processes. FASINEX provides an ideal dataset for this work in that it contains detailed measurements for scenarios at the two extremes: wind blowing from warm to cold water normal to a 2°C SST front and the converse, wind blowing from cold to warm water.

For the wind blowing from warm to cold water, there is a pronounced modification of the near-surface wind field over the front, in both model results and aircraft observations. The decrease of near-surface wind speed and stress is due to a stable internal boundary layer (IBL) induced by the SST front, restricting exchange of mass and momentum between the surface and upper part of the MABL. For the cold-to-warm case, the relatively strong vertical mixing through the entire MABL over warm water dampens the response of the near-surface winds and surface stress to the SST front. The properties observed by the aircraft are simulated quite well in both cases, suggesting that MM5 captures the appropriate boundary layer physics at the mesoscale or regional scale.

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