Search Results
You are looking at 1 - 10 of 16 items for
- Author or Editor: R. J. Doviak x
- Refine by Access: All Content x
Abstract
Probert-Jones' radar equation assumes receiver bandwidth large compared to the reciprocal of the transmitted pulse width τ. The advent of coherent radars with precise transmitter frequencies allows consideration of receiver bandwidth “matched” to and sometimes smaller than τ−1 in order to enhance measurement signal-to-noise ratio.
An extension to the radar equation has been made to show explicitly the dependence of echo power on the product of transmitter pulse width and receiver bandwidth. When receiver bandwidth is less than twice τ−1, there is significant loss in echo power. This should be accounted for when estimating reflectivities.
Considerable improvement in Doppler velocity estimation can often be obtained by matching range resolution to the angular one and this has implications of practical importance when moderately sensitive dual-Doppler radars are used to map the mesoscale wind in clear air.
Abstract
Probert-Jones' radar equation assumes receiver bandwidth large compared to the reciprocal of the transmitted pulse width τ. The advent of coherent radars with precise transmitter frequencies allows consideration of receiver bandwidth “matched” to and sometimes smaller than τ−1 in order to enhance measurement signal-to-noise ratio.
An extension to the radar equation has been made to show explicitly the dependence of echo power on the product of transmitter pulse width and receiver bandwidth. When receiver bandwidth is less than twice τ−1, there is significant loss in echo power. This should be accounted for when estimating reflectivities.
Considerable improvement in Doppler velocity estimation can often be obtained by matching range resolution to the angular one and this has implications of practical importance when moderately sensitive dual-Doppler radars are used to map the mesoscale wind in clear air.
Abstract
First results derived from an NSSL effort to develop a real-time Plan Position Indicator (PPI) display of Doppler isotachs are described. The mean Doppler velocity is estimated for multiple range locations by measuring the phase change of the complex echo envelope over time intervals equal to the pulse repetition period. This technique, phase change per pulse pair, provides velocity estimates directly from samples of the Doppler time record to circumvent spectrum computation.
Abstract
First results derived from an NSSL effort to develop a real-time Plan Position Indicator (PPI) display of Doppler isotachs are described. The mean Doppler velocity is estimated for multiple range locations by measuring the phase change of the complex echo envelope over time intervals equal to the pulse repetition period. This technique, phase change per pulse pair, provides velocity estimates directly from samples of the Doppler time record to circumvent spectrum computation.
Abstract
In this note we propose a technique to uncouple the tie between unambiguous Doppler velocity and range. Transmission of orthogonally polarized waves in pairs of pulses increases a weather Doppler radar's capability to unambiguously resolve velocity spectrum mean and variance at simultaneous range locations sampled in real time.
Abstract
In this note we propose a technique to uncouple the tie between unambiguous Doppler velocity and range. Transmission of orthogonally polarized waves in pairs of pulses increases a weather Doppler radar's capability to unambiguously resolve velocity spectrum mean and variance at simultaneous range locations sampled in real time.
Abstract
A perturbation method is applied to spheroidal oscillating drops to determine the effect of fundamental axisymmetric oscillations on the power spectra and polarimetric measurements. It is shown that vibration magnitudes that produce less than 10% change in equilibrium axial ratios are insufficient to produce measurable changes in the differential reflectivity or differential phase constant, yet they can lead to a detectable increase in sidebands of the power spectra. Larger oscillations do produce measurable effects on the polarimetric variables.
Abstract
A perturbation method is applied to spheroidal oscillating drops to determine the effect of fundamental axisymmetric oscillations on the power spectra and polarimetric measurements. It is shown that vibration magnitudes that produce less than 10% change in equilibrium axial ratios are insufficient to produce measurable changes in the differential reflectivity or differential phase constant, yet they can lead to a detectable increase in sidebands of the power spectra. Larger oscillations do produce measurable effects on the polarimetric variables.
Abstract
Methods of statistical regression have been applied to single-radar radial velocity fields to map certain mesoscale (20–100 km) kinematic properties (e.g., divergence) of the convective boundary layer (CBL). Several methods, previously proposed, were found to produce estimates that were biased or whose variances were too large. When wind fields are linear on the meso- or larger scale, then single-Doppler velocity accuracies allow the estimation of horizontal divergence with an accuracy of about 4 × 10−5 s−1 and a resolution of ∼30 km, which may be sufficient to sense pre-thunderstorm convergence
A case study for 19 June 1980 suggests that single-Doppler weather radars of modest sensitivity can map the mesoscale divergence patterns within the cloud-free CBL. For this day, convergence zones to the northeast seem to precede cloud development by 1–2 h, and to the west precede thunderstorms by 3–4 h.
Abstract
Methods of statistical regression have been applied to single-radar radial velocity fields to map certain mesoscale (20–100 km) kinematic properties (e.g., divergence) of the convective boundary layer (CBL). Several methods, previously proposed, were found to produce estimates that were biased or whose variances were too large. When wind fields are linear on the meso- or larger scale, then single-Doppler velocity accuracies allow the estimation of horizontal divergence with an accuracy of about 4 × 10−5 s−1 and a resolution of ∼30 km, which may be sufficient to sense pre-thunderstorm convergence
A case study for 19 June 1980 suggests that single-Doppler weather radars of modest sensitivity can map the mesoscale divergence patterns within the cloud-free CBL. For this day, convergence zones to the northeast seem to precede cloud development by 1–2 h, and to the west precede thunderstorms by 3–4 h.
Abstract
A case-study comparison is made of simultaneous airborne gust probe and dual-Doppler radar measurements of motions associated with roll vortices in the optically clear planetary boundary layer. Inter-comparison of the cross-roll component of motion is emphasized. Some similarities and some differences in the data obtained with the two systems are discussed. Considering the differences in measurement techniques, agreement is good between the independent depictions of the roll structure and quantitative determinations of the intensities and predominant scales of eddy motion. The observed roll vortices fit descriptions and cause-effect relationships from certain models and other observations.
Abstract
A case-study comparison is made of simultaneous airborne gust probe and dual-Doppler radar measurements of motions associated with roll vortices in the optically clear planetary boundary layer. Inter-comparison of the cross-roll component of motion is emphasized. Some similarities and some differences in the data obtained with the two systems are discussed. Considering the differences in measurement techniques, agreement is good between the independent depictions of the roll structure and quantitative determinations of the intensities and predominant scales of eddy motion. The observed roll vortices fit descriptions and cause-effect relationships from certain models and other observations.
Abstract
Dual Doppler-radars are used to obtain momentum flux and turbulence intensities in the cloudless convective boundary-layer (CBL) containing roll vortices. Momentum flux from small scales unresolved by the radar is determined from the difference between the measured flux and the flux implied by the velocity defect profile. While the small-scale flux indicates a reasonable eddy diffusivity profile, the measured flux is of the opposite sign expected from K theory. Two terms in the turbulent kinetic energy budget, viz, the shear production and the divergence of the energy flux, are calculated for measurable scales. It is proposed that the tilting of convective roll circulations by cross-shear produces the observed flux in the upper part of the CBL.
Abstract
Dual Doppler-radars are used to obtain momentum flux and turbulence intensities in the cloudless convective boundary-layer (CBL) containing roll vortices. Momentum flux from small scales unresolved by the radar is determined from the difference between the measured flux and the flux implied by the velocity defect profile. While the small-scale flux indicates a reasonable eddy diffusivity profile, the measured flux is of the opposite sign expected from K theory. Two terms in the turbulent kinetic energy budget, viz, the shear production and the divergence of the energy flux, are calculated for measurable scales. It is proposed that the tilting of convective roll circulations by cross-shear produces the observed flux in the upper part of the CBL.
Abstract
Data show that Doppler shift of storm echoes and the range extent of severe storm systems encompass a span exceeding the unambiguous resolving capability of centimetric radars. Echo coherency which is related to Doppler spectrum width and radar pulse repetition frequency (PRF) places limits on pulse-Doppler radar's unambiguous range and velocity measurements. Statistics on severe storm Doppler velocities and spectrum widths are given to evaluate limits for radars operating at different wavelengths.
The maximum range to which coherent Doppler measurements can be obtained is related to shear, turbulence and radar parameters. Data suggest that either eddy dissipation is high (ε > 1 m2 s−3) in large (20%) portions of severe storms or that many of the measured Doppler spectrum widths arise from vortices of scales small compared to the pulse volume but outside the equilibrium range of turbulence scales. Rangescrambled echoes are most annoying because they obscure observation, whereas velocity ambiguities can usually be resolved. The extent to which overlaid echoes obscure observation of mesocyclones is estimated for scattered and squall line thunderstorms. Staggered PRF techniques are shown to increase the unambiguous range and velocity limits of pulse-Doppler radars.
This study concludes that, aside from attenuation considerations, the biggest obstacle to shorter wavelength Doppler radar observation of severe storms is the larger occurrence of overlaid echoes resulting from the smaller unambiguous range necessary to meet echo coherency requirements.
Abstract
Data show that Doppler shift of storm echoes and the range extent of severe storm systems encompass a span exceeding the unambiguous resolving capability of centimetric radars. Echo coherency which is related to Doppler spectrum width and radar pulse repetition frequency (PRF) places limits on pulse-Doppler radar's unambiguous range and velocity measurements. Statistics on severe storm Doppler velocities and spectrum widths are given to evaluate limits for radars operating at different wavelengths.
The maximum range to which coherent Doppler measurements can be obtained is related to shear, turbulence and radar parameters. Data suggest that either eddy dissipation is high (ε > 1 m2 s−3) in large (20%) portions of severe storms or that many of the measured Doppler spectrum widths arise from vortices of scales small compared to the pulse volume but outside the equilibrium range of turbulence scales. Rangescrambled echoes are most annoying because they obscure observation, whereas velocity ambiguities can usually be resolved. The extent to which overlaid echoes obscure observation of mesocyclones is estimated for scattered and squall line thunderstorms. Staggered PRF techniques are shown to increase the unambiguous range and velocity limits of pulse-Doppler radars.
This study concludes that, aside from attenuation considerations, the biggest obstacle to shorter wavelength Doppler radar observation of severe storms is the larger occurrence of overlaid echoes resulting from the smaller unambiguous range necessary to meet echo coherency requirements.
Abstract
The theory of internal nonlinear waves in a motionless medium is extended to waves in sheared flow to provide a basis for the interpretation of atmospheric solitary waves. It is shown that if the Scorer parameter is zero in a semi infinite upper region, the integro-differential equation [i.e., the Benjamin-Davis-Ono (BDO) equation] defining the evolution of nonlinear waves in sheared flow is independent of shear and stability in the upper layer. A new type of evolution model is presented, based on the numerical solution of the BDO equation, for the generation of solitary waves by a thunderstorm moving at supercritical speeds. The results of a thorough study of observations of a thunderstorm-generated solitary wave are presented in detail. These observations show that some of the storm's outflow, which was denser than the environment through which the wave propagated, was trapped in the interior of the wave. It is hypothesized that the Coriolis force then caused this denser air to flow along the axis of the wave away from the storm's southern flank for distances in excess of 100 km. Analysis reveals that the temporarily trapped recirculating air in the leading solitary wave is gradually deposited along the ground, forming an advancing shallow layer of denser air behind the wave. The derived wave parameters are then compared with theory. A new type of analysis, based on the separation of the wind change due to vertical transport of horizontal momentum from the observed wind perturbations, results in improved agreement between weakly nonlinear theory and observations. This analysis supports the deduction that wave energy propagates along straight rays, parallel to the plane of recirculating flow, but oblique to the curved wave front. The failure of weakly nonlinear theory to account for all the observed wave characteristics is shown to be caused by the presence of recirculating flow. Comparison with numerical results for strongly nonlinear waves shows reasonably good agreement for all the wave characteristics, except wave speed, which is significantly less than that predicted when wave amplitudes are large. Finally, relations are developed that govern the minimum wave amplitude threshold required for the propagation of solitary waves in waveguides bordered by weakly stratified sheared flow.
Abstract
The theory of internal nonlinear waves in a motionless medium is extended to waves in sheared flow to provide a basis for the interpretation of atmospheric solitary waves. It is shown that if the Scorer parameter is zero in a semi infinite upper region, the integro-differential equation [i.e., the Benjamin-Davis-Ono (BDO) equation] defining the evolution of nonlinear waves in sheared flow is independent of shear and stability in the upper layer. A new type of evolution model is presented, based on the numerical solution of the BDO equation, for the generation of solitary waves by a thunderstorm moving at supercritical speeds. The results of a thorough study of observations of a thunderstorm-generated solitary wave are presented in detail. These observations show that some of the storm's outflow, which was denser than the environment through which the wave propagated, was trapped in the interior of the wave. It is hypothesized that the Coriolis force then caused this denser air to flow along the axis of the wave away from the storm's southern flank for distances in excess of 100 km. Analysis reveals that the temporarily trapped recirculating air in the leading solitary wave is gradually deposited along the ground, forming an advancing shallow layer of denser air behind the wave. The derived wave parameters are then compared with theory. A new type of analysis, based on the separation of the wind change due to vertical transport of horizontal momentum from the observed wind perturbations, results in improved agreement between weakly nonlinear theory and observations. This analysis supports the deduction that wave energy propagates along straight rays, parallel to the plane of recirculating flow, but oblique to the curved wave front. The failure of weakly nonlinear theory to account for all the observed wave characteristics is shown to be caused by the presence of recirculating flow. Comparison with numerical results for strongly nonlinear waves shows reasonably good agreement for all the wave characteristics, except wave speed, which is significantly less than that predicted when wave amplitudes are large. Finally, relations are developed that govern the minimum wave amplitude threshold required for the propagation of solitary waves in waveguides bordered by weakly stratified sheared flow.
Abstract
Velocity-azimuth display (VAD) products (i.e., wind, divergence, deformations, etc.) and a Lagrangian advective scheme applied to the reflectivity factor field are jointly used to estimate vertical profiles of vorticity. The reflectivity field is assumed to be conserved over the time period between radar observations. The vorticity selected gives the best least squares fit between observed and predicted reflectivity fields. These results suggest that vorticity is retrieved with a precision better than about 1×10−1 s−1 for scales on the order of 100 km and larger. The vorticity profiles are consistent with U.S. National Weather Service's analysis. The surface value of vorticity, extrapolated from the estimated profiles, is also compared with the vorticity calculated from Oklahoma mesonet data. The agreement is within the estimated standard error.
Abstract
Velocity-azimuth display (VAD) products (i.e., wind, divergence, deformations, etc.) and a Lagrangian advective scheme applied to the reflectivity factor field are jointly used to estimate vertical profiles of vorticity. The reflectivity field is assumed to be conserved over the time period between radar observations. The vorticity selected gives the best least squares fit between observed and predicted reflectivity fields. These results suggest that vorticity is retrieved with a precision better than about 1×10−1 s−1 for scales on the order of 100 km and larger. The vorticity profiles are consistent with U.S. National Weather Service's analysis. The surface value of vorticity, extrapolated from the estimated profiles, is also compared with the vorticity calculated from Oklahoma mesonet data. The agreement is within the estimated standard error.
