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Earl E. Gossard

Abstract

Two theoretical models of shear layers in the atmosphere are examined. The conditions for their dynamic stability are found and their predictions of wavelength to layer-thickness ratio are compared with classical models and with available observational data. Although the models are only rigorously applicable to in-compressible fluids, it is suggested that they also represent conditions in the atmosphere, and clear-air returns published by Katz from the high-power pulse radar at Wallops Island are especially emphasized. Model 2 appears to be able to account for the narrow band characteristic of many of the observed events and also to explain better than other models the observed wavelength to layer-thickness ratios.

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Earl E. Gossard

Abstract

The advent of Doppler clear-air radars for wind-height profiling opens the way for their use in a variety of other applications. This paper uses knowledge of the clear-air Doppler spectrum from a zenith-pointing radar together with the measured water droplet Doppler vertical velocity spectrum to calculate spectra of drop number density through clouds of droplets having substantial fall velocity. The method has been applied by Japanese scientist to measure drop-size distributions of precipitation particles from data acquired at the VHF MU radar facility. Here the method is applied to records obtained with a 915 MHz wind profiler located at Denver, Colorado, and the resulting spectra are presented and compared with the spectra that would have been obtained if the clear-air information were ignored. From the number density drop-size distribution, the corresponding liquid water distribution can be calculated. It is concluded that failure to take into account turbulence in the medium can result in order-of-magnitude errors in number density and liquid water. The requirements and limitations of a radar remote sensing drop spectrometer are discussed.

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Earl E. Gossard

Abstract

A new technique is examined for using Doppler radars to extract information about the size spectrum of cloud droplets too small to have terminal velocities large enough to be resolvable by the radar. If the drops are very small, motions of the drops are dominated by turbulent fluctuations in the medium rather than their fall velocity. Their motion is then the convolution of the terminal velocity with the turbulent velocity probability density function, and size information about the population can be obtained only by deconvolving the spectra. Doppler radars can extract this velocity and size information, as well as cloud liquid and liquid flux, using a surprisingly simple and accurate technique assuming some functional form (e.g., gamma) for the drop number density spectrum. The method also allows Doppler radars to extract drop size information independent of up-/downdrafts in the medium in which they are embedded. Various gamma and lognormal functions are compared, and finally, a “Stokes range” of drop sizes is added and found to he important. Examples are shown and errors are discussed.

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Earl E. Gossard

Seasonal charts of air-sea difference in refractive index are presented for the Mediterranean and Southeast Asia areas. The charts are discussed briefly in terms of the climatic regimes of the area, and applications to extended radar coverage are suggested.

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Earl E. Gossard
and
A. Shelby Frisch

Abstract

The relationship between the variances of temperature and vertical velocity fluctuations is examined experimentally and theoretically. Comparison of the variance data and the mean gradient data recorded on the 300 m tower at the Boulder Atmospheric Observatory leads to the conclusion that the remotely sensed ratio of the temperature and velocity variances offers hope of measuring gradients of temperature and radar refractive index from ground-based acoustic or radar clear-air sounders. Relationships in which temperature gradient depends only on the ratio of the variances of temperature and vertical velocity are found both from the flux equation and from the energy budget/temperature variance equations. From the two independent relations, a theoretical expression for Prandtl number versus Richardson number is found for a limited range of Richardson numbers. Finally, the character and magnitude of the influence of the stress and conductivity terms are estimated from the linearized problem, and solutions are found in terms of eddy viscosity and conductivity.

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Earl E. Gossard
and
Richard G. Strauch

Abstract

When long-wavelength radars are used to observe the atmosphere, there are occasions when radar return from a volume of cloud is unexpectedly large relative to that predicted by the classical incoherent scatter from individual cloud droplets. The assumption of incoherence predicts the scattered power to be proportional to the inverse fourth power of the wavelength. The observed weaker wavelength dependence could result from Bragg-coherent scatter from the ensemble of droplets or it could result from an enhancement by the cloud of inhomogeneities in the dielectric constant of the gaseous medium within the cloud. Both mechanisms are discussed and compared with data acquired in a forward-scatter mode by two 3 cm wavelength radars of NOAA's Wave Propagation Laboratory. Observed differences between the in-cloud and out-of-cloud refractive index spectra are discussed and conclusions are suggested.

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Earl E. Gossard
and
William R. Moninger

Abstract

The dynamic instability and the kinematics of a multi-layer, shear model of a convective boundary layer are analyzed. Important features of the model include a capping temperature inversion that may or may not be accompanied by a wind discontinuity, a surface-based superadiabatic layer, and a statically stable upper atmosphere. It is shown that the capping inversion can result in a relatively narrow band of dynamically unstable wavenumbers that depend on shear layer thickness, implying a strong selection of scale in growing disturbances. The influence of the various model parameters on selection of the “most unstable” scales is shown and their corresponding propagation velocities are calculated.

A simple form of the model is also used to examine the characteristics of the convectively unstable modes. It is found that two-dimensional disturbances aligned transverse to the wind shear are most dynamically unstable, whereas two-dimensional disturbances parallel to the wind shear are most convectively unstable.

The vorticity and general kinematics of the disturbances are affected in an important way by the presence of a critical level within the height range occupied by the disturbance.

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Earl E. Gossard
,
Russell B. Chadwick
,
Thomas R. Detman
, and
John Gaynor

Abstract

Radars and acoustic sounding systems sense properties of the turbulence structure of the atmosphere. If atmospheric turbulence can be related to the mean gradient parameters, Doppler radars and acoustic sounders can provide information about height profiles of quantities such as temperature and refractive index as well as wind in stable regions of the atmosphere. In this paper turbulent and mean quantities were measured on the 300 m meteorological tower at the Boulder Atmospheric Observatory near Erie, Colorado, and the relationships between the turbulent and mean gradient quantities were examined in order to evaluate hypotheses for simplifying the kinetic energy balance and refractive index variance equations. FM-CW radar measurements of backscattered power and Doppler spectral width were also made for comparison with tower-measured refractive index spectra and Doppler velocity spectra. Height distributions of the turbulent dissipation rate within stable layers are shown and viscous cutoff radar wavelengths calculated.

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B. Boba Stankov
,
Earl E. Gossard
,
Bob L. Weber
,
Richard J. Lataitis
,
Allen B. White
,
Daniel E. Wolfe
,
David C. Welsh
, and
Richard G. Strauch

Abstract

An algorithm to compute the magnitude of humidity gradient profiles from the measurements of the zeroth, first, and second moments of wind profiling radar (WPR) Doppler spectra was developed and tested. The algorithm extends the National Oceanic and Atmospheric Administration (NOAA)/Environmental Technology Laboratory (ETL) Advanced Signal Processing System (SPS), which provides quality control of radar data in the spectral domain for wind profile retrievals, to include the retrieval of humidity gradient profiles. The algorithm uses a recently developed approximate formula for correcting Doppler spectral widths for the spatial and temporal filtering effects. Data collected by a 3-beam 915-MHz WPR onboard the NOAA research vessel Ronald H. Brown (RHB) and a 5-beam 449-MHz WPR developed at the ETL were used in this study. The two datasets cover vastly different atmospheric conditions, with the 915-MHz shipborne system probing the tropical ocean atmosphere and the 449-MHz WPR probing continental winter upslope icing storm in the Colorado Front Range. Vaisala radiosonde measurements of humidity and temperature profiles on board the RHB and the standard National Weather Service (NWS) radiosonde measurements at Stapleton, Colorado, were used for comparisons. The cases chosen represent typical atmospheric conditions and not special atmospheric situations. Results show that using SPS-obtained measurements of the zeroth, first, and second spectral moments provide radar-obtained humidity gradient profiles up to 3 km AGL.

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