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- Author or Editor: R. A. Kropfli x
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Abstract
The velocity-azimuth-display technique provides a measurement of the mean wind components above a conically scanning Doppler radar. Wind components are often computed from a least-squares fit of a sinusoid to the Doppler velocity-versus-azimuth data but it is not widely known that information about turbulence can also be obtained from such data. This paper demonstrates that the fluctuations of the measurements about the best-fit sinusoid are related to Reynolds stress components. These fluctuations, when computed about the mean from an ensemble of scans, provide estimates of stress that contain contributions from scales of motion from ∼50 m to ∼5 km. The method was tested with observations taken by the NOAA/WPL 3.2 and 0.86 cm wavelength radars in the dry, convective boundary layer in which small, naturally occurring particulates were used as tracers of the air motion. Results indicate that continuous profiles of wind and stress components call be produced from heights of about 200 m to the capping inversion (∼2000 m) during periods of strong surface heat flux that occur in Colorado during the summer.
Abstract
The velocity-azimuth-display technique provides a measurement of the mean wind components above a conically scanning Doppler radar. Wind components are often computed from a least-squares fit of a sinusoid to the Doppler velocity-versus-azimuth data but it is not widely known that information about turbulence can also be obtained from such data. This paper demonstrates that the fluctuations of the measurements about the best-fit sinusoid are related to Reynolds stress components. These fluctuations, when computed about the mean from an ensemble of scans, provide estimates of stress that contain contributions from scales of motion from ∼50 m to ∼5 km. The method was tested with observations taken by the NOAA/WPL 3.2 and 0.86 cm wavelength radars in the dry, convective boundary layer in which small, naturally occurring particulates were used as tracers of the air motion. Results indicate that continuous profiles of wind and stress components call be produced from heights of about 200 m to the capping inversion (∼2000 m) during periods of strong surface heat flux that occur in Colorado during the summer.
Abstract
Some characteristics of a scattering, turbulent region near the 3-km level were measured with radar and an instrumented aircraft. It was found that the weakly scattering region was near a thin (20 m) layer of extreme stability (0.1C m−1) which seemed to cap a region of light to moderate turbulence. For the most turbulent run lasting 110 sec, an average eddy dissipation rate was found to be 16 cm2 sec−2. Smaller intervals (7.5 see) showed ε to be as large as 45 cm2 sec−2.
An RHI photograph taken just before the measurements showed two wave-like layers similar to structures previously interpreted as Kelvin-Helmholtz waves. The wave-like structure disappeared within 15 min and only an extremely weak single layer remained after ∼30 min. In general, stronger turbulence was associated with larger temperature variations and thus with greater reflectivity.
Radar and radiosonde data were used for the first time to calculate eddy dissipation rates. These agreed to within a factor of 4 with the more dependable estimates of dissipation derived from airborne hot wire anemometer velocity spectra. Provided the radar scattering occurs within a layer ∼35 m thick (as appears reasonable from the aircraft data) rather than from the entire radar sample volume, then the eddy dissipation rates calculated from the radar and radiosonde data become consistent with those derived from velocity spectra.
Abstract
Some characteristics of a scattering, turbulent region near the 3-km level were measured with radar and an instrumented aircraft. It was found that the weakly scattering region was near a thin (20 m) layer of extreme stability (0.1C m−1) which seemed to cap a region of light to moderate turbulence. For the most turbulent run lasting 110 sec, an average eddy dissipation rate was found to be 16 cm2 sec−2. Smaller intervals (7.5 see) showed ε to be as large as 45 cm2 sec−2.
An RHI photograph taken just before the measurements showed two wave-like layers similar to structures previously interpreted as Kelvin-Helmholtz waves. The wave-like structure disappeared within 15 min and only an extremely weak single layer remained after ∼30 min. In general, stronger turbulence was associated with larger temperature variations and thus with greater reflectivity.
Radar and radiosonde data were used for the first time to calculate eddy dissipation rates. These agreed to within a factor of 4 with the more dependable estimates of dissipation derived from airborne hot wire anemometer velocity spectra. Provided the radar scattering occurs within a layer ∼35 m thick (as appears reasonable from the aircraft data) rather than from the entire radar sample volume, then the eddy dissipation rates calculated from the radar and radiosonde data become consistent with those derived from velocity spectra.
Abstract
A new method for studying the entrainment of air into clouds has been developed and tested, and initial results demonstrating the technique are presented here. This new method is superior to other chaff techniques that allow air parcels to be followed only until the chaff reflectivity becomes indistinguishable from cloud reflectivity. Frequently this is won after the chaff enters the cloud. The new technique uses circular depolarization ratio (CDR) measurements to avoid this limitation. Since the CDR of chaff is typically 20 dB greater than that of most hydrormeteors, the signature provided by CDR allows chaff to be followed into moderately reflective regions of the cloud long after the reflectivity signature of the chaff has disappeared. Circular polarization is superior to linear polarization in measurements such as these because the chaff signatures in circular depolarization do not require assumptions about chaff filament orientation, whereas interpretation of ZDR signatures (from linear polarization) does require such assumptions. In this paper, we discuss the technique, and present initial 8.6 mm wavelength radar measurements of chaff released near the base of a cumulus cloud. These measurements demonstrate that the chaff can be distinguished from its CDR signature well after its reflectivity has been lost in the reflectivity of the cloud. Possible applications as well as limitations of the technique are discussed. An extension of the technique that offers the potential for qualitatively observing electrical fields in cloud is also discussed.
Abstract
A new method for studying the entrainment of air into clouds has been developed and tested, and initial results demonstrating the technique are presented here. This new method is superior to other chaff techniques that allow air parcels to be followed only until the chaff reflectivity becomes indistinguishable from cloud reflectivity. Frequently this is won after the chaff enters the cloud. The new technique uses circular depolarization ratio (CDR) measurements to avoid this limitation. Since the CDR of chaff is typically 20 dB greater than that of most hydrormeteors, the signature provided by CDR allows chaff to be followed into moderately reflective regions of the cloud long after the reflectivity signature of the chaff has disappeared. Circular polarization is superior to linear polarization in measurements such as these because the chaff signatures in circular depolarization do not require assumptions about chaff filament orientation, whereas interpretation of ZDR signatures (from linear polarization) does require such assumptions. In this paper, we discuss the technique, and present initial 8.6 mm wavelength radar measurements of chaff released near the base of a cumulus cloud. These measurements demonstrate that the chaff can be distinguished from its CDR signature well after its reflectivity has been lost in the reflectivity of the cloud. Possible applications as well as limitations of the technique are discussed. An extension of the technique that offers the potential for qualitatively observing electrical fields in cloud is also discussed.
Abstract
We report on radar observations of a plume of microwave-reflecting chaff. The chaff was released from the top of a 300 m tower and observed as it was blown 18 km downwind through the growing boundary layer. We present the following: 1) a discussion of the advantages and disadvantages of radar-acquired transport and diffusion data; 2) our data on the horizontal and vertical spread of the plume, which we compare with other atmospheric experiments and the tank model results of Willis and Deardorff (1976); 3) axial concentration data that compare favorably with data from an oil fog release at Brookhaven National Laboratories; and 4) peak-to-average concentration ratios that show general agreement with the work of Gifford (1961). These results indicate that radar can provide useful turbulence and diffusion data that cannot be obtained by other means.
Abstract
We report on radar observations of a plume of microwave-reflecting chaff. The chaff was released from the top of a 300 m tower and observed as it was blown 18 km downwind through the growing boundary layer. We present the following: 1) a discussion of the advantages and disadvantages of radar-acquired transport and diffusion data; 2) our data on the horizontal and vertical spread of the plume, which we compare with other atmospheric experiments and the tank model results of Willis and Deardorff (1976); 3) axial concentration data that compare favorably with data from an oil fog release at Brookhaven National Laboratories; and 4) peak-to-average concentration ratios that show general agreement with the work of Gifford (1961). These results indicate that radar can provide useful turbulence and diffusion data that cannot be obtained by other means.
Abstract
No abstract available.
Abstract
No abstract available.
Abstract
It has been shown previously that the use or elliptically polarized radar signals can help to obtain polarization signatures from tenuous cirrus clouds. Estimates of particle dimension ratios and orientations can be made in situations where conventional circular and linear polarizations fail because of weak echoes in one of the received polarization channels. One way of achieving elliptical polarizations is to install a quarter wave plate before the radar transmitter. This paper introduces two new easily measurable elliptical polarization parameters. These parameters are 1) the depolarization ratio for elliptical polarizations that slightly differ from the circular polarization, and 2) the difference in quarter wave plate angular positions that provide equal returns in both received polarization channels. The use of the elliptical depolarization ratio increases echo in the “weak” channel without significant change of echo in the “main” channel. This ratio is used to estimate ice particle deformity (deviation from sphericity). The second suggested parameter can be measured during continuous rotations of the quarter wave plate. It can be used to estimate the combined effect of the degree of common alignment of the ice crystals and their deformity. Operational procedures and possible data interpretations are discussed with emphasis on cloud sensing with Ka-band radar.
Abstract
It has been shown previously that the use or elliptically polarized radar signals can help to obtain polarization signatures from tenuous cirrus clouds. Estimates of particle dimension ratios and orientations can be made in situations where conventional circular and linear polarizations fail because of weak echoes in one of the received polarization channels. One way of achieving elliptical polarizations is to install a quarter wave plate before the radar transmitter. This paper introduces two new easily measurable elliptical polarization parameters. These parameters are 1) the depolarization ratio for elliptical polarizations that slightly differ from the circular polarization, and 2) the difference in quarter wave plate angular positions that provide equal returns in both received polarization channels. The use of the elliptical depolarization ratio increases echo in the “weak” channel without significant change of echo in the “main” channel. This ratio is used to estimate ice particle deformity (deviation from sphericity). The second suggested parameter can be measured during continuous rotations of the quarter wave plate. It can be used to estimate the combined effect of the degree of common alignment of the ice crystals and their deformity. Operational procedures and possible data interpretations are discussed with emphasis on cloud sensing with Ka-band radar.
Abstract
The NOAA/WPL dual-Doppler radar System has been used to determine the three-dimensional kiematic, structure of a convective storm during its decaying Stage which grew in a weakly sheared environment. The internal flow appears Similar in many respects to the Wokingham storm described by Browning and Ludlam even though the latter existed in a strongly sheared environment. Among the similar features are an upshear tilted updraft, a surface gust front, the intrusion of middle-level cool dry air, a precipitation-filled down-draft, and a vortex pattern suggestive of obstacle flow.
Quantitative flux results are presented. Profiles of mass, water vapor, energy, momentum and vorticity fluxes were computed using the Doppler data and other supporting data from the National Hail Research Experiment surface network and upper air soundings.
Abstract
The NOAA/WPL dual-Doppler radar System has been used to determine the three-dimensional kiematic, structure of a convective storm during its decaying Stage which grew in a weakly sheared environment. The internal flow appears Similar in many respects to the Wokingham storm described by Browning and Ludlam even though the latter existed in a strongly sheared environment. Among the similar features are an upshear tilted updraft, a surface gust front, the intrusion of middle-level cool dry air, a precipitation-filled down-draft, and a vortex pattern suggestive of obstacle flow.
Quantitative flux results are presented. Profiles of mass, water vapor, energy, momentum and vorticity fluxes were computed using the Doppler data and other supporting data from the National Hail Research Experiment surface network and upper air soundings.
Abstract
The Wave Propagation Laboratory (WPL) dual-Doppler radar system has been used in METROMEX to study the effect of a large urban area (St. Louis) on air motions within the planetary mixing layer during clear conditions. It was demonstrated that aircraft-dispensed chaff could be used to trace the air motion over a several hundred square kilometer area and through the entire depth of the developed mixing layer. Data from two days having similar temperature and wind profiles have been selected for discussion. Both data sets suggest the presence of horizontal roll vortices existing throughout the duration of the observations (20–70 min) and beyond the extent of the chaff cloud (8–12 km). A comparison with the temperature field reveals that a line of convergence (∼10−3 s−1) 300 m above the surface was found to coincide with a thermal ridge of about 0.2°C measured by instruments on the University of Wyoming aircraft. Differences in the apparent roll structures on these two days may be explained in terms of the cross-roll shear and buoyancy resulting from the elongated urban heat island. Also, a persistent localized region of relatively high convergence was observed on the convergence line several kilometers downwind from an area of concentrated industrial activity at Granite City.
Abstract
The Wave Propagation Laboratory (WPL) dual-Doppler radar system has been used in METROMEX to study the effect of a large urban area (St. Louis) on air motions within the planetary mixing layer during clear conditions. It was demonstrated that aircraft-dispensed chaff could be used to trace the air motion over a several hundred square kilometer area and through the entire depth of the developed mixing layer. Data from two days having similar temperature and wind profiles have been selected for discussion. Both data sets suggest the presence of horizontal roll vortices existing throughout the duration of the observations (20–70 min) and beyond the extent of the chaff cloud (8–12 km). A comparison with the temperature field reveals that a line of convergence (∼10−3 s−1) 300 m above the surface was found to coincide with a thermal ridge of about 0.2°C measured by instruments on the University of Wyoming aircraft. Differences in the apparent roll structures on these two days may be explained in terms of the cross-roll shear and buoyancy resulting from the elongated urban heat island. Also, a persistent localized region of relatively high convergence was observed on the convergence line several kilometers downwind from an area of concentrated industrial activity at Granite City.
Abstract
Observations of boundary-layer flow within the Santa Barbara region taken on 20 September 1985 revel the presence of a wide variety of flow features, including mesoscale wind vortices sea/land breezes, and thermally driven upslope/downslope winds. Details of these features, in particular the mesoscale vortices, are documented with dual-Doppler radar, Doppler sodar, aircraft, surface mesonet, and rawinsonde data. Numerical simulations of flow in the region using a mixed-layer model show good agreement with the observations. Model simulations indicate that sea-/land-roughness differences and planetary vorticity are of minor importance in forming the midchannel eddy (MCE), an eddy that is observed in the channel during the early morning hours. MCE formation is, however, shown to be strongly dependent on the initial stratification of the atmosphere, with more intense eddies forming as the stability increases. A second independent mechanism for MCE formation appears to be the interaction of drainage flows with the large-scale flow. A daytime vortex, known as the Gaviota eddy, occurs as the result of surface heating that generates a sea-breeze flow opposing the large-scale ambient flow.
Abstract
Observations of boundary-layer flow within the Santa Barbara region taken on 20 September 1985 revel the presence of a wide variety of flow features, including mesoscale wind vortices sea/land breezes, and thermally driven upslope/downslope winds. Details of these features, in particular the mesoscale vortices, are documented with dual-Doppler radar, Doppler sodar, aircraft, surface mesonet, and rawinsonde data. Numerical simulations of flow in the region using a mixed-layer model show good agreement with the observations. Model simulations indicate that sea-/land-roughness differences and planetary vorticity are of minor importance in forming the midchannel eddy (MCE), an eddy that is observed in the channel during the early morning hours. MCE formation is, however, shown to be strongly dependent on the initial stratification of the atmosphere, with more intense eddies forming as the stability increases. A second independent mechanism for MCE formation appears to be the interaction of drainage flows with the large-scale flow. A daytime vortex, known as the Gaviota eddy, occurs as the result of surface heating that generates a sea-breeze flow opposing the large-scale ambient flow.
Abstract
A pulse Doppler radar system operating at 35 GHz and having full polarization (linear and circular) diversity capability is described. Separate antennas are used for the transmitter and the receiver because this design approach allows better overall radar sensitivity. The transmitter operates in the double-pulse mode to optimize the unambiguous Doppler velocity measurable with the system. A polarizer capable of handling about 200 kW of peak power when pressurized with sulfur hexafluoride (SF6) at 25 psi was developed for the transmitter. The radar system has built-in test sequences for checking the gain and alignment of the transmitter and receiver antennas. The dual-polarization intermediate frequency (IF) receiver has a total of six analog channels. A very flexible data acquisition and processing system has been developed to allow both coherent and incoherent dual-polarization measurements to be performed; the system includes a microprocessor-controlled pulse-pair processor and a minicomputer with associated peripherals. The radar was operated successfully in Montana during the 1981 Cooperative Convective Precipitation Experiment (CCOPE). Preliminary results on the observed variation of the circular depolarization ratio (CDR) are shown and related to the storm and cloud structure.
Abstract
A pulse Doppler radar system operating at 35 GHz and having full polarization (linear and circular) diversity capability is described. Separate antennas are used for the transmitter and the receiver because this design approach allows better overall radar sensitivity. The transmitter operates in the double-pulse mode to optimize the unambiguous Doppler velocity measurable with the system. A polarizer capable of handling about 200 kW of peak power when pressurized with sulfur hexafluoride (SF6) at 25 psi was developed for the transmitter. The radar system has built-in test sequences for checking the gain and alignment of the transmitter and receiver antennas. The dual-polarization intermediate frequency (IF) receiver has a total of six analog channels. A very flexible data acquisition and processing system has been developed to allow both coherent and incoherent dual-polarization measurements to be performed; the system includes a microprocessor-controlled pulse-pair processor and a minicomputer with associated peripherals. The radar was operated successfully in Montana during the 1981 Cooperative Convective Precipitation Experiment (CCOPE). Preliminary results on the observed variation of the circular depolarization ratio (CDR) are shown and related to the storm and cloud structure.
