Showing papers in "Boundary-Layer Meteorology in 1978"

Theories for the interaction of electromagnetic and oceanic waves — A review

Abstract: This paper reviews analytical methods in electromagnetic scattering theory (i.e., geometrical and physical optics, perturbation, iteration, and integral-equation) which are applicable to the problems of remote sensing of the ocean. In dealing with Earth's surface (in this case, the weakly non-linear ocean), it is not possible to have a complete and exact description of its spatial and temporal statistics. Only the first few moments are generally available; and in the linear approximation the statistics are assumed homogeneous, stationary and Gaussian. For this case, the high-frequency methods (geometrical and physical optics) and perturbation (Rayleigh-Rice), or a combination of them, provide tractable analytical results (i.e., the specular-point, the slightly-rough Bragg scattering and the composite-surface models). The applicability and limitations of these models are discussed. At grazing incidence and for higher frequencies, other scattering mechanisms become significant; and shadowing, diffraction and trapping must be considered. The more exact methods (integral-equation and Green's function) have not been as successful in yielding tractable analytical solutions, although they have the potential to provide improved theoretical scattering results in the future.

The two-frequency microwave technique for measuring ocean-wave spectra from an airplane or satellite

Abstract: The two-frequency microwave technique at slanting incidence for the measurement of ocean wave spectra, first proposed by Rucket al. (1972), is investigated in more detail with respect to its applicability in aircraft and space vehicles. It is shown that by carrying out signal processing in the frequency domain the system-inherent signal-to-noise ratio can be increased considerably, making the operation of the system from air- and space-borne platforms feasible.

Radar backscatter from the ocean - Dependence on surface friction velocity

Abstract: From the mid 1960s to the present, the normalized radar cross-section (NRCS) of the ocean has been measured using airborne radars operating over a frequency range of 0.4 to 14 GHz. Analyses of these data have shown that the NRCS was proportional to the ocean surface wind speed raised to some power, but the values of the exponent remained in dispute. This paper extends previous work and uses these NRCS measurements to demonstrate that to the first order, the NRCS is a function of only the friction velocity at the ocean's surface. Further analyses characterize the dependence of the NRCS on radar variables such as frequency, incidence angle, polarization, etc. Finally, recommendations are made for using Ku-band radars at large incidence angles for remote sensing of the wind friction velocity vector.

Turbulent flow over a very rough, random surface

Abstract: A knowledge of the nature of turbulent flow over very rough surfaces is important for an understanding of the environment of crops, forests, and cities. For this reason, a wind-tunnel investigation was carried out on the variations in mean velocity, Reynolds shear-stress, and other turbulence quantities in a deep turbulent flow over a rough surface having a fair degree of randomness in the shapes, sizes, and positions of its elements.

Measurement of ocean temperature and salinity via microwave radiometry

Abstract: Sea-surface temperature with an accuracy of 1 °C and salinity with an accuracy of 1‰ were measured with a 1.43 and 2.65 GHz radiometer system after correcting for the influence of cosmic radiation, intervening atmosphere, sea-surface roughness, and antenna beamwidth. The radiometers are a third-generation system using null-balancing and feed-back noise injection. Flight measurements from aircraft over bay regions and coastal areas of the Atlantic resulted in contour maps with spatial resolution of 0.5 km.

Bulk characteristics of heat transfer in the unstable, baroclinic atmospheric boundary layer

Abstract: Field data for the unstable, baroclinic, atmospheric boundary layer over land and over the sea are considered in the context of a general similarity theory of vertical heat transfer. The dependence of δθ/θ*upon logarithmic functions of hczTand stability (through the similarity function C) is clearly demonstrated in the data. The combined data support the conventional formulation for the heat transfer coefficient δθ/θ* when, (a) the surface scaling length is zT(« z0), the height at which the surface temperature over land is obtained by extrapolation of the temperature profile (b) the height scale is taken as the depth of convective mixing hc (c) the temperature profile equivalent of the von Karman constant is taken as 0.41 (d) areal average, rather than single point, values of δθ are employed in strongly baroclinic conditions. No significant effect of baroclinity or the height scale ratio as proposed in the general theory is found. Variations in C about a linear regression relation against stability are most probably due to uncertainties in the areal surface temperature and to experimental errors in general temperature measurements.

The computation of the friction velocity u * and the temperature scale T * from temperature and wind velocity profiles by least-square methods

Abstract: A method is given to calculate the surface layer parameters: u * (friction velocity) and T * (temperature scale) from wind speed and temperature profiles The problem is formulated as a minimization of a least-square function, which is constructed from the difference between the measured profiles and the well-known Kansas profile relations The wind speed and temperature profiles are treated simultaneously in this procedure All the available wind speed and temperature measurements are used in order to reduce the effect of measurement errors Estimates of the goodness of fit and confidence limits on the estimated parameters are discussed The method has been applied to data obtained during experiments in a wide variety of conditions: Project Prairie Grass, experiments over Lake Flevo and experiments at the meteorological tower at Cabauw, the last two in the Netherlands

Modelling waving crops in a wind tunnel

Abstract: Analysis of movie films of a field of barley, combined with observations of the motions of individual plants, show that single stalks oscillate at a well-defined natural frequency even when stimulated by turbulent winds. Treating single stalks as resonant cantilevers allows the use of standard engineering methods to determine their elastic properties. Armed with these values, the application of similarity analysis to the equation of motion of a single stalk leads to criteria for aeroelastic modelling of wheat plants in the wind tunnel. A representative value for the spacing of stalks in a small section of model wheat field was calculated by referring to published data on momentum absorption in a variety of real and model canopies. Preliminary measurements of first and second moments of velocity in the model appear to confirm the importance of including elastic properties in wind-tunnel simulations of airflow in flexible crops.

Daytime variations of ozone eddy fluxes to maize

Abstract: The vertical fluxes of ozone, momentum and heat in the atmospheric surface layer have been measured by eddy correlation above both mature and senescent maize canopies. Aerodynamic formulae are applied to find that the bulk canopy surface resistancerc to ozone uptake and destruction varies between 4.0 and 0.5 s cm−1 during the daytime. Apparently, surface properties tend to control the removal of ozone at the surface of the earth. For a lush canopy, the stomatal diffusion resistance is the most important property, while changes in surface temperature have little effect. Destruction at the soil and exterior plant surfaces appears to account for 20–50% of the total loss if leaf mesophyll resistances are assumed to be very small. Free water at leaf surfaces may at times inhibit ozone removal by both senescent and healthy plants.

Detection of ocean waves by microwave radar; The modulation of short gravity-capillary waves

Abstract: Short gravity-capillary waves, the predominant radar scatterers under many oceanic and radar-viewing conditions, are modulated in amplitude, velocity and orientation by the larger-scale motions of the ocean surface. These modulations render the larger scales observable to microwave radar. The high data rate and advanced technology of modern radar systems make it possible to measure these modulations and, in some cases, to display them as images of the ocean surface. While the modulation of orientation and velocity are straightforward to understand, the amplitude modulation is a dynamic response of the equilibrium short gravity-capillary waves to larger scale driving forces including straining by orbital velocities of large waves. Microwave studies of the growth and equilibrium of short wind-generated waves are reviewed. It is shown that the response of these waves to straining can produce modulations in radar cross-section greater than those due to tilting but which are wind-speed dependent. The net modulation depends strongly on the direction as well as the magnitude of the wind speed. Quantitative determination of ocean wave-height spectra from measured modulations will therefore be a complicated procedure. Measurements of ocean waves with CW Doppler and Synthetic Aperture radars are discussed in the light of these findings.

Analysis of canopy index values for various canopy densities

Abstract: Canopy wind profiles can often be represented by an exponential function such that wind-speeds in these vegetative canopies are a function of height and the attenuation coefficient of this wind profile relationship. To be more precise, canopy flow is a function of canopy density, element flexibility, and height. An index of canopy flow, therefore, can be defined as a conservative measure of the gross flow response to the presence of various types of roughness elements. For this study, windspeed profile data of two quite different canopy density experiments — field and wind tunnel - have been analyzed based on least-square fittings. The results indicate that the two sets of index values of canopy flow behave in a similar manner with maxima occurring for optimum densities of one-third the potential full array of roughness elements. These index values also differ by some 0.2, but are still compatible when one accounts for the respective levels of turbulence within these dissimilar canopies.

Time-dependence of sea-ice concentration and multiyear ice fraction in the Arctic Basin

Abstract: Microwave images of sea ice obtained by Nimbus-5 and the NASA CV-990 airborne laboratory are used to determine the time variation of the sea-ice concentration and multiyear ice fraction within the pack ice in the Arctic Basin. The images, constructed from data acquired from the electrically scanned microwave radiometer, are analyzed for four seasons during 1973-1975. Observations indicate significant variations in the sea-ice concentration in the spring, late fall, and early winter. Sea-ice concentrations as low as 50% were detected in large areas in the interior of the Arctic polar sea-ice pack. The applicability of passive-microwave remote sensing for monitoring the time dependence of sea-ice concentration is considered.

Accuracy of moments of velocity and scalar fluctuations in the atmospheric surface layer

Abstract: A detailed accuracy analysis is presented for moments, up to order four, of both velocity (horizontal u and vertical w) and scalar (temperature θ and humidity q) fluctuations, as well as of the products uw, wθ and wq, in the atmospheric surface layer. The high-order moments and integral time scales required for this analysis are evaluated from data obtained at a height of about 5 m above the ocean surface under stability conditions corresponding to Z/L \- −0.05. Measured moments and probability density functions of some of the individual fluctuations show departures from Gaussianity, but these are sufficiently small to enable good estimates to be obtained using Gaussian instead of measured moments. For the products, the assumption of joint Gaussianity for individual fluctuations provides a reasonable, though somewhat conservative, estimate for the integration times required. The concept of Reynolds number similarity implies that differences in integration time requirements for flows at different Reynolds numbers arise exclusively from differences in integral time scales. A first approximation to the integral time scales relevant to atmospheric flows is presented.

HF radio oceanography — A review

Abstract: The understanding and utilization of HF radar sea-echo have enjoyed steady progress since the experimental discovery of the underlying radar/sea interaction process over two decades ago. The agreement of theory with measured data confirms the correctness of currently accepted explanations for both the first-order and second-order portions of the sea-echo Doppler spectrum in terms of the wave-height directional spectrum. Furthermore, experiments have shown that any currents present near the surface produce a readily distinguishable Doppler shift on the wave-scattered echo which is directly related to the current velocity.

A simplified one-dimensional theoretical description of the vegetation-atmosphere interaction

Abstract: Shuttleworth (1976b) demonstrated that it is possible to write a ‘multi-layer’ model of the vegetation-atmosphere interaction in analytically continuous form and create a one-dimensional description, of general applicability, with evaporation described by a combination equation similar to the Penman-Monteith equation. The present paper develops and simplifies that analysis in an attempt to provide a more practical description. It is shown that the generalized combination equation can be rewritten in a form which is identical to the Penman-Monteith equation in the single-source limit providing that the canopy resistance is redefined in two alternative ways, according to whether there is large-scale variation in surface wetness. Both of these definitions reduce to the Monteith (1965) form in dry conditions, and indicate zero surface resistance in wet conditions. The version applicable to situations involving large-scale variation in surface wetness, which is considered the general description in rainfall conditions, is consistent with published data in partially wet conditions (Shuttleworth, 1976a). This version allows a separation of the ‘interception loss’ and ‘transpiration loss’ components and, although approximate, has potential value in that it allows an experimental test of speculative models of interception loss, and provides the means whereby such models can be merged with Penman-Monteith models of transpiration loss, to provide a model of evapotranspiration. A preliminary test of a simple, single-source model of interception loss shows satisfactory agreement with experimental data.

Modelling surface turbulent fluxes in stable conditions

Abstract: Recently published observational studies of the stable surface layer have produced several quite different prescriptions for the universal functions required by the Monin-Obukhov similarity hypothesis for the fully turbulent surface layer. Three such formulations are described and the different relationships between the surface turbulent fluxes and standard (model) profile variables implied by them are illustrated and compared. The results provide insight to help discriminate between the choices available to the modeller. It is argued that the formulation of Hicks (1976) is that best suited to modelling the surface turbulent fluxes compatible with both our current understanding of the stable boundary layer and the need in a numerical model to estimate fluxes representative over the area of a grid-box or element. A full description of the general procedure for determining surface-layer bulk transfer coefficients from Monin-Obukhov similarity functions is given.

Microwave Measurements Over the North Sea

Abstract: As reported in earlier IUCRM meetings, we investigate radar remote sensing as a tool for the control and study of the Dutch part of the North Sea. This is done in a program which comprises ground- based microwave measurements, and flights with a real-aperture digital SLAR. This program is called Project Noordwijk. We investigate the average (noncoherent) backscatter of the sea as a function of incidence angle, wind speed, and look direction, but also the modulation of the backscatter by sea waves and other phenomena such as: bottom topography, currents, eddies, and oil films. Some results of this program are reported.

Detection of centripetal heat-island circulations from tower data in St. Louis

Abstract: Hourly averaged meteorological data gathered by a 25-tower network about St. Louis during 1976 are used in a search for centripetal circulations generated by the urban heat island. Considering data collected when the network resultant speed was less than 1.5 m s-1, two data classes of several hundred hours each are formed. One class is associated with weak heat islands, daytime hours, and convective instability, while the other class is associated with strong heat islands, nighttime hours, and extreme rural stability. Mean centripetal flows are clearly discernible from data of both classes, but the convergence is stronger for the flows associated with the weaker heat islands. This unexpected result is explained in terms of the ease with which sustained vertical motions can be generated over the city by the available forcing under different stability regimes. The detectability of the heat-island influence diminishes very rapidly with increasing speed of the large-scale flow.

A review of applications of microwave radiometry to oceanography

Abstract: The emissivity of sea ice and atmospheric precipitation was investigated. Using the above physics, the data from the Electrically Scanning Microwave Radiometers (ESMR's) on the Nimbus-5 and Nimbus-6 satellites operating at wavelengths of 1.55 cm and 8mm, respectively, can be interpreted in terms of rain rate, ice coverage, and first year versus multi-year ice determination. The rain rate data is being used to establish a climatology of rainfall over the oceans. Both ice and rain data sets have been generated for the Global Atmospheric Research Project Data Systems Test.

The influence of water vapor fluctuations on turbulent fluxes

Abstract: The influence of water vapor fluctuations on vertical turbulent fluxes is examined. It is shown that effects on density and, consequently, buoyancy are insignificant. However, because specific heat of air is a function of specific humidity, these fluctuations are found to influence sensible heat flux significantly. The critical parameter is the Bowen ratio, and a formulation relating the heat flux assuming dry air to the true value is given. The implications of this analysis to flux-gradient relationships in the surface layer are commented on.

Microwave remote sensing of sea ice in the AIDJEX Main Experiment

Abstract: A microwave remote sensing program of sea ice in the Beaufort Sea was conducted during the Arctic Ice Dynamics Joint Experiment (AIDJEX). Several types of both passive and active sensors were used to perform surface and aircraft measurements during all seasons of the year. In situ observations were made of physical properties (salinity, temperature, density, surface roughness), dielectric properties, and passive microwave measurements were made of first-year, multiyear, and first-year/multiyear mixtures. Airborne passive microwave measurements were performed with the electronically scanning microwave radiometer while airborne active microwave measurements were performed by synthetic aperture radar, X- and L-band radar, and a scatterometer.

On fitting the log-linear model to wind speed and temperature profiles over a melting glacier

Abstract: The profile structure of wind speed and temperature in katabatic flow over a melting glacier is analyzed within the log-linear framework. Similarity between windspeed and temperature profiles is indicated but the log-linear model should be restricted to heights within 1.5 m of the ice. Marked deviation from the model occur at greater heights, probably due to the effects of flux divergence. Unlike results from other stable atmospheres, a decreases with increasing stability. This may arise from the use of the Obukhov length L. When the height H at which the wind speed reaches its maximum value is used instead, α does not vary with stability. It has mean values of 4.5 and 4.3 for windspeed and temperature profiles, respectively.

A wind-tunnel boundary-layer study of the effects of a surface roughness change: Rough to smooth

Abstract: The changes imposed on mean velocities and turbulence statistics in the lower atmosphere by an abrupt change in surface roughness, from very rough to smooth, were modelled in a wind tunnel. The influence of a change in the effective surface level, which often accompanies such a variation in surface roughness, was also studied. A deep, turbulent flow was generated upstream of the change, which had a logarithmic mean velocity profile and constant shear-stress for approximately 200 mm above the floor, except for a region near the surface which was influenced by the three-dimensional nature of the random rough surface.

Aerodynamic roughness as a function of wind direction over asymmetric surface elements

Abstract: Wind speed and temperature profiles to a height of 8 m were recorded for 30-, 60-, and 90-min averaging times over a striated snow surface at the geographic South Pole during the austral winter of 1975. A gradient Richardson number was calculated for each averaging time to determine conditions of neutral stability under which the logarithmic wind law would hold. A log-linear regression technique was used to determine values of aerodynamic roughness height (Z0) for those profile averages recorded in conditions of neutral stability. A plot of Z0 as a function of average wind direction revealed a variation in Z0 of almost three orders of magnitude, from 0.01 to 7 cm, over 120 deg of wind direction. A simple model is presented to justify the fact that aerodynamic roughness is a function of wind direction and erosion history.

Observation of the temperature structure function parameter, C T 2, over the ocean

Abstract: Shipboard measurements of temperature fluctuations, mean wind, temperature, and humidity permit comparisons to be made of experimental and empirical estimates of ct2, the temperature structure function parameter. Surface flux estimates are obtained from bulk aerodynamic formulae. Temperature fluctuation data are selected to minimize a salt-contamination effect which causes increases in temperature variance. Predictions for CT2 based on surface flux scaling agree within 20%, except for near neutral and large unstable conditions where disagreement can be attributed to measurement problems.

Comparison of in situ and remotely sensed ocean waves off Marineland, Florida

Abstract: Some early results from an oceanographic experiment staged off Marineland, Florida, in December 1975 are presented, viz., intercomparisons between the X-band and L-band imagery obtained by the Environmental Research Institute of Michigan's (ERIM) dual-wavelength, dualpolarization multiplexed radar. This radar allows direct comparison since the images are produced simultaneously. The wave data obtained from the radar imagery are compared with surface measurements of waves obtained with a pitch- and-roll buoy. The conclusions are only applicable to medium and low wind and wave conditions encountered during the Marineland test. The results indicate that X-band images provide superior quality wave imagery and more useful Fourier Transforms compared to L-band under equivalent signal-to-noise ratios and resolution. Optimum wave imagery is seen when waves propagate in the range direction. Comparisons betweenin situ measurements and X-band imagery of the same area indicate that the dominant wave direction can be obtained from imagery to within a few degrees. A one-dimensional spectrum obtained from X-band imagery compares favorably with an equivalent wave frequency spectrum obtained from the pitch- and-roll buoy after suitable transformation asing linear wave theory.

Processing of ocean wave data from a synthetic aperture radar

Abstract: The usual operation of a synthetic-aperture radar (SAR) assumes that the sensor platform moves at a constant velocity along a straight line and that objects to be imaged are stationary. Moving ocean waves perturb the Doppler frequencies in the SAR phase histories, and when processed in a conventional manner, they produce images of waves that are dispersed and thus defocused in the azimuth (along-track) direction. This defocusing can be compensated in the processor by readjusting the azimuth focus by an amount proportional to the velocity of the wave. The relationship among the parameters of the radar system, the SAR processor, and the relative target velocity is, theoretically determined for radars that operate at both X- and L-band frequencies. Experimental observations support these calculations. The effect of varying spectrum sampling, range focus, and number of coherent averages or looks is also observed.

Boundary-layer evolution and nocturnal inversion dispersal—Part I

Abstract: The evolution of boundary layers capped by nocturnal inversions has been studied with an instrumented aircraft. A large sample of the original data obtained on two of the four observation days is presented. Profiles of temperature, humidity and sensible and latent heat flux are compared with the results of numerical models. The sensitivity of simple models for the prediction of nocturnal inversion dispersal is discussed in relation to certain measured input parameters.

Modulation of short waves by long waves

Abstract: Wave-tank experiments were performed to investigate the cyclic short-wave energy changes, related in phase to an underlying long wave, which occur during active generation of the short-wave field by wind. Measurements of time series of the short-wave slope were made by a laser-optical system, where the basic long-wave parameters were controlled and wind speeds were accurately reproducible. The short-wave slope variances were found to exhibit cyclic variations that are related to the phase of the long wave. The variations result from two combined effects: (1) the short wave frequency is varied by the long-wave orbital velocity; (2) the energy of the short waves is modulated by the actions of aerodynamic and hydrodynamic couplings that operate on the short waves in a manner related to the long-wave phase.

Ocean wave heights measured by a high resolution pulse-limited radar altimeter

Abstract: Radar measurements of wave height are compared with independent measurements made during the JONSWAP-2 experiment by Waverider and pitch-roll buoys, a shipborne wave recorder and a laser profilometer The radar data were recorded by a Naval Research Laboratory (NRL) nanosecond-pulse X-band radar altimeter flown in a NASA C-54 aircraft at 3-km altitude under various wind and sea conditions Averages of 800 pulses of the pulse-limited altimeter data were used to calculate maximum-likelihood estimates of significant wave height (SWH) and skewness of the sea-surface height distribution The mean values of the radar-estimated SWH were in good agreement with the other measurements The standard deviation of the values of the radar measurements was typically 10% of the average wave height A two-dimensional computer simulation of the sea surface indicates that the major portion of the observed standard deviation is attributable to the relatively small sea-surface area illuminated by the radar (125 m × 900 m) rather than to instrumental error Increasing the number of pulses averaged reduced the variance in the estimates without changing the means The mean value of the skewness parameter was generally near zero but the standard deviation was typically 025 The estimate of SWH did not change when the skewness parameter was constrained to zero