Carl A. Friehe

Flux Measurements, Flux Estimation Techniques, and Fine-Scale Turbulence Measurements in the Unstable Surface Layer Over Land

Abstract An AFCRL-UCSD joint experiment in Minnesota in 1973 has provided a comparison of direct and indirect measurements of the surface-layer fluxes of momentum, heat and moisture under unstable conditions. The direct momentum and heat flux measurements of the two groups agreed well, and also agreed well with values inferred by the direct dissipation technique. The moisture flux estimates from the inertial-dissipation technique also agreed well with the directly measured values. Several of the important terms in the budgets of turbulent kinetic energy and turbulent scalar variances were evaluated directly. The imbalance (or pressure transport) term in the energy budget was estimated, and the ratio of the imbalance term to the dissipation term determined from the present experiment agrees well with the Kansas results. The dissipation rate of temperature variance exceeded its production rate, in contrast with the Kansas results, implying an imbalanced temperature variance budget. Several possible contribu...

Surface-layer fluxes, profiles, and turbulence measurements over uniform terrain under near-neutral conditions

Abstract An atmospheric surface-layer experiment over a nearly uniform plowed field was performed to determine the constants in the flux-profile similarity formulas, particularly the von Karman constant. New instruments were constructed to minimize flow distortion effects on the turbulence measurements and to provide high-resolution gradient measurements. In addition, a hot-wire anemometer directly measured the turbulent kinetic energy dissipation rate. An average value of the von Karman constant of 0.365 ± 0.015 was obtained from 91 runs (31 h) in near-neutral stability conditions. However, four near-neutral runs when snow covered the ground gave an average value of 0.42. This result suggests that the von Karman constant depends on the roughness Reynolds number, which may resolve some of the differences in previous determinations over different surfaces. The one-dimensional Kolmogorov inertial subrange constant was found to have a value of 0.54 ± 0.03, slightly larger than previous results. The flux-prof...

The Use of Pressure Fluctuations on the Nose of an Aircraft for Measuring Air Motion

Abstract 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 con...

Temperature Ramps in the Atmospheric Surface Layer

Abstract A review of the evidence for the organized temperature structure observed in both the atmospheric surface layer and the laboratory boundary layer reveals similar features between the two turbulent flows. This similarity suggests that the atmospheric temperature ramp may be interpreted as the signature of an organized large-scale motion rather than a necessary consequence of the presence of buoyant plumes. An experiment was conducted in which the translation velocity Ut of the sharp edge of the temperature ramp is determined from the transit time of the ramp between two thermistors placed at the same height in the marine surface layer but separated in a direction parallel to the wind. Ut was found to be in more nearly constant ratio to the local velocity than to the friction velocity. Velocities determined from the phase angle of the temperature cross spectrum and from the optimum temperature cross correlation obtained from the two thermistors are in reasonable agreement with Ut. Cross correlation...

Effects of temperature and humidity fluctuations on the optical refractive index in the marine boundary layer

The variance and power spectrum of atmospheric optical refractive-index fluctuations are shown to be composed of three terms: the variance and power spectra of the temperature and humidity fluctuations and the correlation and cospectrum of the temperature and humidity fluctuations, respectively. Humidity fluctuations are found to be significant because of the correlation term. The signs of the temperature–humidity correlation and cospectrum can be positive or negative, and therefore can add to or subtract from refractive-index fluctuations caused by only temperature fluctuations. The results of two atmospheric boundary-layer experiments are reported, which show the large effect of the temperature–humidity correlation term. For cold air blowing over warm ocean water, the correlation term was positive and accounted for 17% of the total refractive-index variance. For dry hot desert air blowing over the cold Salton Sea, the correlation was −268% of the total, effectively cancelling the contribution due to temperature variance.

Structure of sheared turbulent fields

Small scale turbulent temperature and velocity measurements were made in a variety of turbulent shear flows in the atmosphere and laboratory, in air and in water. Nonzero skewness values of the streamwise temperature gradient were observed in every case, with a sign equal to that of the gradient dotted with the cross product of the mean temperature gradient and mean shear vectors. Large temperature jumps were found across sharp shear zones at the outer boundaries of large scale eddies rotating with the mean vorticity of the shear flow, giving a distinctive ramp‐like structure to the temperature signals which seems to be a characteristic feature of scalar fields mixed by sheared turbulence. The streamwise gradient of the vertical velocity in the atmospheric boundary layer over the ocean was found to have positive skewness. Thus, both velocity and temperature fields are locally anisotropic in the same sense as the anisotropic mean quantities.

The Stable Internal Boundary Layer over a Coastal Sea. Part I: Airborne Measurements of the Mean and Turbulence Structure

Abstract It has been recognized that progress toward understanding the mesoscale structure of the ocean requires more knowledge of the small-scale structure of the marine atmosphere and the processes that couple the air and the sea. The coastal ocean is characterized by large variations in sea surface temperature and surface roughness that affect the structure of the atmosphere. Stable layers are frequent features of the coastal marine atmospheric boundary layer. Their main effect is the formation of a discontinuity between the sea surface and the upper part of the boundary layer that supports gravity waves, wind speed jets, and large wind shear. The general structure of a stable internal boundary layer (IBL) that forms over the sea, downstream of a warm landmass, is discussed. Aircraft data are presented from the Internal Boundary Layer Experiment (IBLEX) conducted over the Irish Sea in 1990. With the airflow from the land to the sea, thermodynamic profiles were obtained perpendicular to the coast to inv...

Humidity Sensitivity of Atmospheric Temperature Sensors by Salt Contamination

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 c...

Structure of Marine Surface Layer Turbulence

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 Su(n>/ Su(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.

The Stable Internal Boundary Layer over a Coastal Sea. Part II: Gravity Waves and the Momentum Balance

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 perpendicula...