Showing papers in "Journal of Atmospheric and Oceanic Technology in 1998"
TL;DR: In this article, the authors present a reference guide to users wishing to make use of the Tropical Rainfall Measuring Mission data, which covers each of the three primary rainfall instruments: the passive microwave radiometer, the precipitation radar, and the Visible and Infrared Radiometer System on board the spacecraft.
Abstract: This note is intended to serve primarily as a reference guide to users wishing to make use of the Tropical Rainfall Measuring Mission data. It covers each of the three primary rainfall instruments: the passive microwave radiometer, the precipitation radar, and the Visible and Infrared Radiometer System on board the spacecraft. Radiometric characteristics, scanning geometry, calibration procedures, and data products are described for each of these three sensors.
2,317 citations
TL;DR: In this paper, an improved objective analysis method that takes into account along-track correlated errors is developed, which is used to allow an efficient correction of long-wavelength errors while estimating the oceanic signal.
Abstract: Objective analysis of altimetric data (sea level anomaly) usually assumes that measurement errors are well represented by a white noise, though there are long-wavelength errors that are correlated over thousands of kilometers along the satellite tracks. These errors are typically 3 cm rms for TOPEX/Poseidon (T/P), which is not negligible in low-energy regions. Analyzing maps produced by conventional objective analysis thus reveals residual long-wavelength errors in the form of tracks on the maps. These errors induce sea level gradients perpendicular to the track and, therefore, high geostrophic velocities that can obscure ocean features. To overcome this problem, an improved objective analysis method that takes into account along-track correlated errors is developed. A specific data selection is used to allow an efficient correction of long-wavelength errors while estimating the oceanic signal. The influence of data selection is analyzed, and the method is first tested with simulated data. The me...
823 citations
TL;DR: In this article, the accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in a 17m flume using an ADV and a laser DOF.
Abstract: Accuracy of the acoustic Doppler velocimeter (ADV) is evaluated in this paper. Simultaneous measurements of open-channel flow were undertaken in a 17-m flume using an ADV and a laser Doppler velocimeter. Flow velocity records obtained by both instruments are used for estimating the true (“ground truth”) flow characteristics and the noise variances encountered during the experimental runs. The measured values are compared with estimates of the true flow characteristics and values of variance (〈u′2〉, 〈w′2〉) and covariance (〈u′w′〉) predicted by semiempirical models for open-channel flow. The analysis showed that the ADV sensor can measure mean velocity and Reynolds stress within 1% of the estimated true value. Mean velocities can be obtained at distances less than 1 cm from the boundary, whereas Reynolds stress values obtained at elevations greater than 3 cm above the bottom exhibit a variation that is in agreement with the predictions of the semiempirical models. Closer to the boundary, the measure...
566 citations
TL;DR: In this paper, various errors and influences leading to differences between tower- and aircraft-measured fluxes are surveyed, and strategies for obtaining adequate flux sampling using repeated aircraft passes and grid patterns are outlined.
Abstract: Various errors and influences leading to differences between tower- and aircraft-measured fluxes are surveyed. This survey is motivated by reports in the literature that aircraft fluxes are sometimes smaller than tower-measured fluxes. Both tower and aircraft flux errors are larger with surface heterogeneity due to several independent effects. Surface heterogeneity may cause tower flux errors to increase with decreasing wind speed. Techniques to assess flux sampling error are reviewed. Such error estimates suffer various degrees of inapplicability in real geophysical time series due to nonstationarity of tower time series (or inhomogeneity of aircraft data). A new measure for nonstationarity is developed that eliminates assumptions on the form of the nonstationarity inherent in previous methods. When this nonstationarity measure becomes large, the surface energy imbalance increases sharply. Finally, strategies for obtaining adequate flux sampling using repeated aircraft passes and grid patterns are outlined.
482 citations
TL;DR: In this paper, the extinction-to-backscatter ratios for the Nd:YAG wavelengths are calculated for continental, maritime, and desert aerosols; the corresponding aerosol components are varied within the expected natural variabilities of the particle number mixing ratios.
Abstract: An adequate estimation of the aerosol extinction-to-backscatter ratio S is important for solving the underdetermined single scattering lidar equation and for investigating the climate impact of aerosols. In this study, the extinction-to-backscatter ratios for the Nd:YAG wavelengths are calculated for continental, maritime, and desert aerosols; the corresponding aerosol components are varied within the expected natural variabilities of the particle number mixing ratios. For continental aerosol, S increases with the relative humidity f from 40 to 80 sr. For maritime aerosol, the extinction-to-backscatter ratios lie between 15 and 30 sr for 355 and 532 nm and between 25 and 50 sr for 1064 nm. The desert aerosol exhibits a weak dependence of S on f and ranges between 42 and 48 sr for 355 nm and between 17 and 25 sr for 532 and 1064 nm. For practical applications, the calculated values of S are fitted by a power series expansion with respect to their dependence on f.
312 citations
TL;DR: In this paper, two methods for computing direct covariance fluxes from anemometers mounted on moving platforms at sea are described. But neither of these methods involve the use of either a strapped-down or gyro-stabilized system that are used to compute terms that correct for the instantaneous tilt of the anemometer due to the pitch, roll, and heading variations of the platform.
Abstract: This paper describes two methods for computing direct covariance fluxes from anemometers mounted on moving platforms at sea. These methods involve the use of either a strapped-down or gyro-stabilized system that are used to compute terms that correct for the 1) instantaneous tilt of the anemometer due to the pitch, roll, and heading variations of the platform; 2) angular velocities at the anemometer due to rotation of the platform about its local coordinate system axes; and 3) translational velocities of the platform with respect to a fixed frame of reference. The paper provides a comparison of fluxes computed with three strapped-down systems from two recent field experiments. These comparisons shows that the direct covariance fluxes are in good agreement with fluxes derived using the bulk aerodynamic method. Additional comparisons between the ship system and the research platform FLIP indicate that flow distortion systematically increases the momentum flux by 15%. Evidence suggests that this correction is appropriate for a commonly used class of research vessels. The application of corrections for both motion contamination and flow distortion results in direct covariance flux estimates with an uncertainty of approximately 10%‐20%.
299 citations
TL;DR: The double-fence intercomparison reference (DFIR) was the reference standard used at all stations in the WMO Solid Precipitation Measurement Intercomparisons (SPMI) project as discussed by the authors.
Abstract: The standard 8" nonrecording precipitation gauge has been used historically by the National Weather Service (NWS) as the official precipitation measurement instrument of the U.S. climate station network. From 1986 to 1992, the accuracy and performance of this gauge (unshielded or with an Alter shield) were evaluated during the WMO Solid Precipitation Measurement Intercomparison at three stations in the United States and Russia, representing a variety of climate, terrain, and exposure. The double-fence intercomparison reference (DFIR) was the reference standard used at all the intercomparison stations in the Intercomparison project. The Intercomparison data collected at different sites are compatible with respect to the catch ratio (gauge measured/DFIR) for the same gauges, when compared using wind speed at the height of gauge orifice during the observation period. The effects of environmental factors, such as wind speed and temperature, on the gauge catch were investigated. Wind speed was found t...
282 citations
TL;DR: The Nevzorov liquid water content and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid content of clouds as mentioned in this paper.
Abstract: The Nevzorov liquid water content (LWC) and total water content (TWC) probe is a constant-temperature, hot-wire probe designed for aircraft measurements of the ice and liquid water content of clouds. The probe consists of two separate sensors for measurements of cloud liquid and total (ice plus liquid) water content. Each sensor consists of a collector and a reference winding. The reference sensors are shielded from impact with cloud particles, specifically to provide an automatic compensation for convective heat losses. This results in a potentially improved sensitivity over uncompensated probes such as the King LWC probe. The Nevzorov probe has been used in four Canadian field experiments on the National Research Council (NRC) Convair580 since 1994. Intercomparison of Nevzorov LWC, TWC, King, and two PMS Forward Scattering Spectrometer Probes show good agreement in liquid clouds, although the Nevzorov probe displays distinct advantages in low-LWC situations due to a more stable baseline. The se...
220 citations
TL;DR: In this paper, it was shown that the velocity error, associated with the evaluation of horizontal density or pressure gradients on a sigma coordinate grid, prognostically disappeared, leaving behind a small and physically insignificant distortion in the density field.
Abstract: In a recent paper by Mellor et al., it was found that, in two-dimensional ( x, z) applications with finite horizontal viscosity and zero diffusivity, the velocity error, associated with the evaluation of horizontal density or pressure gradients on a sigma coordinate grid, prognostically disappeared, leaving behind a small and physically insignificant distortion in the density field. The initial error is numerically consistent in that it decreases as the square of the grid increment size. In this paper, we label this error as a sigma error of the first kind. In three-dimensional applications, the authors have encountered an error that did not disappear and that has not been understood by us or, apparently, others. This is a vorticity error that is labeled a sigma error of the second kind and is a subject of this paper. Although it does not prognostically disappear, it seems to be tolerably small. To evaluate these numerical errors, the authors have adopted the seamount problem initiated by Beckman and Haidvogel. It represents a stringent test case, as evidenced by their paper, wherein the model is initialized with horizontal isopycnals, zero velocity, and no forcing; then, any velocities that develop must be considered errors. Two appendices are important adjuncts to the paper, the first providing theoretical confirmation and understanding of the numerical results, and the second delving into additional errors related to horizontal or isosigma diffusion. It is, however, shown that satisfactory numerical solutions are obtained with zero diffusivity.
204 citations
TL;DR: In this paper, a method for using tide gauge sea levels to monitor time-dependent drift in satellite altimetric measurements of sea surface height is described, and a recently discovered algorithm error, which resulted in a slow drift in the TOPEX sea surface heights, is exploited to evaluate the success of the tide gauge drift estimation.
Abstract: A method is described for using tide gauge sea levels to monitor time-dependent drift in satellite altimetric measurements of sea surface height. The method depends on a careful assessment of the quality of the tide gauge measurements available for this application and also takes into account the degree of independence between the altimeter minus tide gauge differences in order to construct an optimal drift estimate and an accurate error estimate for it. The method is applied to the TOPEX altimeter measurements, and a recently discovered algorithm error, which resulted in a slow drift in the TOPEX sea surface heights, is exploited to evaluate the success of the tide gauge drift estimation. It is important to note that the tide gauge analysis was done without any prior knowledge of this error. The result is that the tide gauge analysis reproduces the drift due to the algorithm error to within 6 mm rms, which is comparable to the 5–6-mm internal estimate of the uncertainty of the drift analysis. Th...
171 citations
TL;DR: In this paper, a wave-induced bias can be diminished substantially by differencing measurements obtained from two velocity sensors separated by a distance larger than the correlation scale of the turbulence but small in comparison to the inverse wavenumber of the surface waves.
Abstract: Surface waves can produce large biases in estimates of turbulent shear stress obtained from single-sensor measurements of velocity if there is even a small uncertainty in the orientation of either the velocity sensor or the principal axes of the wave-induced velocity field. The wave-induced bias can be diminished substantially by differencing measurements obtained from two velocity sensors separated by a distance larger than the correlation scale of the turbulence but small in comparison to the inverse wavenumber of the surface waves. If the scale separation is sufficiently large, then minus the density times half of the covariance between horizontal and vertical velocity differences is a nearly wave-free estimate of the average of the turbulent shear stresses at the two sensors. A theoretical analysis determines the bias associated with this technique under simplified conditions, in which waves and turbulence are uncorrelated and the waves are weakly nonlinear and narrow-banded in both frequency...
TL;DR: The first Australian C-band polarimetric/Doppler radar system (C-POL) was developed by the Australian Bureau of Meteorology (BOM), the Commonwealth Scientific and Industrial Research Organisation of Australia, and the National Center for Atmospheric Research.
Abstract: The development of the first Australian C-band polarimetric/Doppler meteorological radar system (C-POL) is described. Motivated by the need to obtain improved rainfall estimation and the vertical profile of hydrometeors, C-POL was developed jointly by the Bureau of Meteorology (BOM), the Commonwealth Scientific and Industrial Research Organisation of Australia, and the National Center for Atmospheric Research. C-POL is based on a standard operational C-band radar employed by the BOM but modified to be capable of transmitting linear horizontal and vertical polarizations and receiving the co- and cross polarizations on a pulse-to-pulse basis. Standard variables extracted include horizontal reflectivity (ZHH), radial velocity (Vr), spectral width (συ), differential reflectivity (ZDR), differential phase shift (ΦDP), and zero lag correlation coefficient [ρHV(0)]. With the addition of a second receiver chain, the linear depolarization ratio will soon be available. Initial results with the radar are di...
TL;DR: In this paper, the authors considered the theory of diffraction image formation of spherical particles and peculiarities of particle sizing by discrete imaging probes and derived several methods of size correction of individual droplets and droplet ensembles.
Abstract: This paper considers the theory of diffraction image formation of spherical particles and peculiarities of particle sizing by discrete imaging probes. The diffraction images of spherical water droplets are approximated by Fresnel diffraction by an opaque disc. The approach developed in the paper is applicable to all types of array and matrix imaging probes. The analysis measurement accuracy is performed for the PMS Optical Array Prove (OAP)-2D-C and OAP-2Dgray probes. It is shown that a 25-mm resolution PMS OAP-2D-C probe can both oversize and undersize droplets smaller than approximately 100 mm in diameter, and oversize droplets larger than approximately 100 mm. The errors in droplet sizing increase with decreasing size. The discrete manner of particle image registration also leads to losses of particles with sizes smaller than 100 mm. For the ideal case with zero photodiode response time, these losses reach 70% for 25-mm droplets. A nonzero response time will increase these losses. These findings help explain discrepancies observed in the overlap region of the PMS FSSP and OAP droplet spectra. A variety of calculated digital images for PMS OAP-2D-C and OAP-2Dgray probes is presented. Different methods of particle image sizing are discussed. Several methods of size correction of individual droplets and droplet ensembles are suggested. Correction algorithms for these effects are derived, and distortion and correction retrieval matrices are calculated. Several examples of actual and measured size distributions are presented.
TL;DR: In this paper, the authors discussed the basics of single particle measurements and illustrated with measurements of the droplet size distribution with an optical spectrometer, the Forward Scattering Spectrometer Probe (FSSP), and its improved version, the Fast-FSSP.
Abstract: The basics of single particle measurements are discussed and illustrated with measurements of the droplet size distribution with an optical spectrometer, the Forward Scattering Spectrometer Probe (FSSP), and its improved version, the Fast-FSSP. The various sources of uncertainties are successively analyzed: the statistical significance of an incomplete sampling of the particle population, counting losses due to coincidence of particles and electronic dead time of the counter, artificial broadening of the size distributions by the coincidences, inhomogeneities of the sensitive volume of the probe, ambiguities of the Mie scattering curve, errors on the sampled volume, and, finally, the uncertainties on the size calibration of the instrument. It is demonstrated with examples of data collected with the Fast-FSSP that additional parameters, such as the pulse duration and the interarrival times between detections, are crucial for improving the sizing of the particles and the retrieval of the spatial ev...
TL;DR: In this paper, a numerical technique sensitive to both spectral and spatial aspects of sea surface wind measurements is introduced to transform the irregularly sampled satellite-based scatterometer data into regularly gridded wind fields.
Abstract: A numerical technique sensitive to both spectral and spatial aspects of sea surface wind measurements is introduced to transform the irregularly sampled satellite-based scatterometer data into regularly gridded wind fields. To capture the prevailing wavenumber characteristics (power-law dependence) of sea surface wind vector components, wavelet coefficients are computed from the scatterometer measurements along the satellite tracks. The statistics of the wavelet coefficients are then used to simulate high-resolution wind components over the off-track regions where scatterometer data are not available. Using this technique, daily wind fields with controlled spectral features have been produced by combining the low-wavenumber wind fields from ECMWF analyses with the high-wavenumber measurements from the ERS-1 scatterometer. The resulting surface wind fields thus reflect nearly all available measurements affecting surface wind, including the synoptic surface pressure. The new surface wind forces a b...
TL;DR: In this paper, the authors compare the performance of the real-vector EOF and the complex EOF analysis on Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993.
Abstract: Empirical orthogonal function (EOF) analysis has been widely used in meteorology and oceanography to extract dominant modes of behavior in scalar and vector datasets. For analysis of two-dimensional vector fields, such as surface winds or currents, use of the complex EOF method has become widespread. In the present paper, this method is compared with a real-vector EOF method that apparently has previously been unused for current or wind fields in oceanography or meteorology. It is shown that these two methods differ primarily with respect to the concept of optimal representation. Further, the real-vector analysis can easily be extended to threedimensional vector fields, whereas the complex method cannot. To illustrate the differences between approaches, both methods are applied to Ocean Surface Current Radar data collected off Cape Hatteras, North Carolina, in June and July 1993. For this dataset, while the complex analysis ‘‘converges’’ in fewer modes, the real analysis is better able to isolate flows with wide cross-shelf structures such as tides.
TL;DR: In this article, the authors proposed a time-stepping scheme for the Bryan-Cox-Semtner ocean model based on a control volume form of the advection equation, which can be used with the leapfrog-Euler forward time-stpping schemes used by the ocean model.
Abstract: Leonard’s widely used QUICK advection scheme is, like the Bryan–Cox–Semtner ocean model, based on a control volume form of the advection equation. Unfortunately, in its normal form it cannot be used with the leapfrog–Euler forward time-stepping schemes used by the ocean model. Farrow and Stevens overcame the problem by implementing a predictor–corrector time-stepping scheme, but this is computationally expensive to run. The present paper shows that the problem can be overcome by splitting the QUICK operator into an O(δx2) advective term and a velocity dependent biharmonic diffusion term. These can then be time-stepped using the combined leapfrog and Euler forward schemes of the Bryan–Cox–Semtner ocean model, leading to a significant increase in model efficiency. A small change in the advection operator coefficients may also be made leading to O(δx4) accuracy. Tests of the improved schemes are carried out making use of a global eddy-permitting ocean model. Results are presented from cases where th...
TL;DR: In this article, the correlation coefficient between horizontally and vertically polarized backscatter signals and local variability of the total differential phase can be used as a good identifier of ground clutter-contaminated data.
Abstract: Rainfall estimation from specific differential phases in meteorological situations with significant anomalous propagation (AP) is discussed. It is shown that the correlation coefficient between horizontally and vertically polarized backscatter signals and local variability of the total differential phase can be good identifiers of ground clutter–contaminated data. Further, it is suggested how to estimate rainfall in regions of ground clutter caused by AP.
TL;DR: The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally in this paper, and it is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs.
Abstract: The calibration and accuracy of the Eppley precision infrared radiometer (PIR) is examined both theoretically and experimentally. A rederivation of the fundamental energy balance of the PIR indicates that the calibration equation in common use in the geophysical community today contains an erroneous factor of the emissivity of the thermopile. If a realistic value (0.98) for the emissivity is used, then this leads to errors in the total flux of 5–10 W m−2. The basic precision of the instrument is found to be about 1.5% of the total IR irradiance when the thermopile voltage and both dome and case temperatures are measured. If the manufacturer’s optional battery-compensated output is used exclusively, then the uncertainties increase to about 5% of the total (20 W m−2). It is suggested that a modern radiative transfer model combined with radiosonde profiles can be used as a secondary standard to improve the absolute accuracy of PIR data from field programs. Downwelling IR fluxes calculated using the ...
TL;DR: In this paper, the authors examined the solar radiative heating errors in buoy-mounted, naturally ventilated air temperature sensors and found that the errors are at times larger than the difference between the air and sea surface temperatures.
Abstract: Solar radiative heating errors in buoy-mounted, naturally ventilated air temperature sensors are examined. Data from sensors with multiplate radiation shields and collocated, fan-aspirated air temperature sensors from three buoy deployments during TOGA COARE (Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment) and the Arabian Sea Mixed Layer Dynamics Experiment are used to describe the errors in the naturally ventilated measurements. The naturally ventilated sensors have mean daytime errors of 0.27°C and maximum instantaneous errors of 3.4°C. The errors are at times larger than the difference between the air and sea surface temperatures. These errors lead to mean daytime biases in sensible and latent heat fluxes of 1–4 W m−2 and instantaneous errors up to 22 W m−2. The heating errors increase with increasing shortwave radiation and diminish with increasing wind speed. The radiative heating is also found to be a function of sun elevation with maximum heating errors occur...
TL;DR: A new approach is presented to account for a simultaneous solution of the three wind components from at least a pair of Doppler radar observations, which could remove potential drawbacks of an iterative (nonsimultaneous) solution of Cartesian dual-Doppler analysis techniques.
Abstract: A new approach is presented to account for a simultaneous solution of the three wind components from at least a pair of Doppler radar observations, which could remove potential drawbacks of an iterative (nonsimultaneous) solution of Cartesian dual-Doppler analysis techniques. The multiple-Doppler synthesis and continuity adjustment technique (MUSCAT) is derived from the extended overdetermined dual-Doppler (EODD) variational formalism that contains the basis for a simultaneous (noniterative) solution of a dual- or multiple-equation system and a mass continuity equation. Necessary accommodations are discussed, including the solutions for a plane-to-plane synthesis (as in EODD) instead of a fully three-dimensional and computationally intensive analysis, owing to the three-dimensional character of the continuity equation. The evaluation of MUSCAT is carried out by first considering real data and then performing numerical tests based on simulated radar observations. The comparative study with EODD ap...
TL;DR: In this paper, the authors investigate the effect of nonuniform filling of the rain radar antenna beam on the retrieved rain rate profile and find that the path-integrated attenuation and the resulting path-averaged rain rate are overestimated.
Abstract: The Tropical Rainfall Measuring Mission (TRMM) will carry the first spaceborne radar for rainfall observation. Because the TRMM Precipitation Radar (PR) footprint size of 4.3 km is greater than the scale of some convective rainfall events, there is concern that nonuniform filling of the PR antenna beam may bias the retrieved rain-rate profile. The authors investigate this effect theoretically and then observationally using data from the NASA Jet Propulsion Laboratory Airborne Rain Mapping Radar (ARMAR), acquired during Tropical Oceans Global Atmosphere Coupled Ocean Atmosphere Response Experiment in early 1993. The authors' observational approach is to simulate TRMM PR data using the ARMAR data and compare the radar observables and retrieved rain rate from the simulated PR data with those corresponding to the high-resolution radar measurements. The authors find that the path-integrated attenuation and the resulting path-averaged rain rate are underestimated. The reflectivity and rain rate near the top of the rainfall column are overestimated. The near-surface reflectivity can be overestimated or underestimated, with a mean error very close to zero. The near-surface rain rate, however, is usually underestimated, sometimes severely.
TL;DR: In this article, a cloud detection algorithm for a low power micropulse lidar is presented that attempts to identify all of the significant power returns from the vertical column above the lidar at all times.
Abstract: A cloud detection algorithm for a low power micropulse lidar is presented that attempts to identify all of the significant power returns from the vertical column above the lidar at all times. The main feature of the algorithm is construction of lidar power return profiles during periods of clear sky against which cloudy-sky power returns are compared. This algorithm supplements algorithms designed to detect cloud-base height in that the tops of optically thin clouds are identified and it provides an alternative approach to algorithms that identify significant power returns by analysis of changes in the slope of the backscattered powers with height. The cloud-base heights produced by the current algorithm during nonprecipitating periods are comparable with the results of a cloud-base height algorithm applied to the same data. Although an objective validation of algorithm performance on high, thin cirrus is lacking because of no truth data, the current algorithm produces few false positive and fals...
TL;DR: In this paper, the behavior of the cup anemometer rotor in turbulent atmospheric flow is discussed in terms of a general equation of motion, which equates the rate of change s of the rotation rate s of a rotor to a forcing F(s, h, w), which is proportional to the torque and a function of s and of the total horizontal and the vertical wind velocity components, h and w, respectively.
Abstract: The behavior of the cup anemometer rotor in turbulent atmospheric flow is discussed in terms of a general equation of motion. This equates the rate of change s of the rotation rate s of the rotor to a forcing F(s, h, w), which is proportional to the torque and a function of s and of the total horizontal and the vertical wind velocity components, h, and w, respectively. To determine the so-called overspeeding, it is necessary to carry out first-and second-order perturbation calculations around the response curve obtained in a laminar flow. From this curve, which for the purpose of this paper can be considered linear, five constraints are derived between the first and second partial derivatives of F. These constraints provide sufficient information for deriving an expression for the overspeeding to which four distinctly different biases contribute—one for each of the velocity components and one from the covariance between streamwise velocity components ũ and w. A phenomenological model of...
TL;DR: In this article, the authors describe and compare several different measurements of boundary layer depth, and find good agreement between the different types of measurement but see that the profiler peak reflectivity is slightly raised above cloud base in the presence of boundary layers clouds.
Abstract: The depth of the atmospheric boundary layer is of interest in several different areas, such as chemistry, pollutant studies, and global modeling. In this research the authors describe and compare several different measurements of boundary layer depth. First, the authors use the standard measurement from radiosondes to confirm the validity of wind-profiler measurements, which use humidity gradients to estimate the boundary layer depth. A method for obtaining meaningful cloud-base altitudes is then presented, and the results are compared to the wind-profiler boundary layer heights. The authors find good agreement between the different types of measurement but see that the profiler peak reflectivity is slightly raised above cloud base in the presence of boundary layer clouds. This may be due to increased humidity gradients at the top and edges of clouds or to increased turbulence within the cloud. Calculation of the boundary layer height using the bulk Richardson number is commonly used in computer ...
TL;DR: In this article, an instrument for the measurement of bubble size distributions is described, which exploits the free-flooding resonator design of Medwin with modifications to overcome the limitations in the original implementation, especially those due to a sensitivity to ambient pressure fluctuations in the surrounding medium.
Abstract: An instrument for the measurement of bubble size distributions is described. The sensing element exploits the free-flooding resonator design of Medwin with modifications to overcome the limitations in the original implementation, especially those due to a sensitivity to ambient pressure fluctuations in the surrounding medium. A mathematical model of the resonator provides insight into the factors affecting its performance and motivates application of appropriate signal processing algorithms. Comparison of different bubble size calculation methods shows the direct approach of Commander and MacDonald to be most successful. The stability of this new implementation of the resonator facilitates accurate measurement of the complex dispersion relation. Comparison of the real and imaginary components then leads to the definition of a measurement quality factor that may be calculated for each sample. Practical considerations are discussed for implementation of autonomous battery-powered resonator arrays f...
TL;DR: The turbulent eddy profiler (TEP) as discussed by the authors is a volume-imaging, UHF radar wind profiler designed for clear-air measurements in the atmospheric boundary layer on scales comparable to grid cell sizes of large eddy simulation models.
Abstract: This paper describes the turbulent eddy profiler (TEP), a volume-imaging, UHF radar wind profiler designed for clear-air measurements in the atmospheric boundary layer on scales comparable to grid cell sizes of large eddy simulation models. TEP employs a large array of antennas—each feeding an independent receiver—to simultaneously generate multiple beams within a 28° conical volume illuminated by the transmitter. Range gating provides 30-m spatial resolution in the vertical dimension. Each volume image is updated every 2–10 s, and long datasets can be gathered to study the evolution of turbulent structure over several hours. A summary of the principles of operation and the design of TEP is provided, including examples of clear-air reflectivity and velocity images.
TL;DR: In this paper, the authors present the measurements of the directional spreading of fetch-limited waves and support the conclusion of I. R. Young et al. that the spreading function is bimodal at high frequencies.
Abstract: The directional spreading of waves is important for both theoretical and practical reasons. Enough measurements have now been made to draw conclusions about the behavior of wave spreading at sites in different climatic regimes. The measurements presented here of the directional spreading of fetch-limited waves agree in general with those of M. A. Donelan et al., but additional evidence is also found to support the conclusion of I. R. Young et al. that the spreading function is bimodal at high frequencies. The spreading factor ϕ is defined to be the square root of the in-line variance ratio defined by R. E. Haring and J. C. Heideman. This spreading factor gives an integrated measure of the degree of directional spreading in the wave spectrum and predicts the reduction in the in-line particle velocities under waves due to direction spreading. The value of ϕ is 1 for unidirectional waves and 0.707 for omnidirectional waves. For fetch-limited conditions, ϕ is essentially constant at 0.906. Results fr...
TL;DR: A new version of the DieCAST ocean circulation model to be referred to as CANDIE (Canadian Diecast) are considered, which differs from DieCAST in the use of a standard C grid formulation and a reduction in the magnitude of the time truncation error associated with the implicit treatment of the Coriolis force.
Abstract: The development and verification of a new version of the DieCAST ocean circulation model to be referred to as CANDIE (Canadian Diecast) are considered. Both CANDIE and DieCAST have many features in common with the well-known Modular Ocean Model (MOM) of the Geophysical Fluid Dynamics Laboratory. Of particular relevance to the present study are the rigid-lid approximation and the use of standard Cartesian coordinates. The DieCAST formulation in terms of the surface pressure, rather than the volume transport streamfunction, is also used in CANDIE to reduce numerical sensitivity to ocean depth variations. The major difference between MOM and DieCAST is the use of a mixed C and A grid formulation in DieCAST rather than the B grid formulation used in MOM. CANDIE differs from DieCAST in the use of a standard C grid formulation and a reduction in the magnitude of the time truncation error associated with the implicit treatment of the Coriolis force. The implementation of the rigid-lid approximation is r...
TL;DR: A new method for estimating moments from wind measurement devices that measure Doppler spectra as a function of range that relies on mathematical analyses, fuzzy logic synthesis, and global image processing algorithms to mimic the human expert.
Abstract: A new method for estimating moments from wind measurement devices that measure Doppler spectra as a function of range is presented. Quite often the spectra are contaminated by a wide variety of sources, including (but not limited to) birds, aircraft, velocity and range folding, radio frequency interference, and ground clutter. These contamination sources can vary in space, time, and even in their basic characteristics. Human experts analyzing Doppler spectra can often identify the desired atmospheric signal among the contamination. However, it is quite difficult to build automated algorithms that can approach the skill of the human expert. The method described here relies on mathematical analyses, fuzzy logic synthesis, and global image processing algorithms to mimic the human expert. Fuzzy logic is a very simple, robust, and efficient technique that is well suited to this type of feature extraction problem. These new moment estimation algorithms were originally designed for boundary layer wind profilers; however, they are quite general and have wide applicability to any device that measures Doppler spectra as a function of range (e.g., lidars, sodars, and weather radars).