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Phillip B. Chilson

Abstract

Range imaging (RIM) is used to describe a constrained optimization signal-processing method that can be applied to wind profilers capable of operating over a small set of distinct transmitter frequencies. The results of the signal-processing method are typically high-resolution maps of the backscattered power as a function of range. In this paper it is discussed how RIM processing can be implemented in order to additionally obtain high-resolution estimates of the Doppler velocity. The method has been demonstrated using data from a 915-MHz tropospheric profiler located in Platteville, Colorado. Examples of data collected during an experiment conducted on 10 April 2001 are presented. In this experiment the radar was operated alternately in two different modes. The cycle time for the two modes was about 50 s. The particular operation of the radar allowed comparison of radar reflectivity (η) and vertical velocity (V) measurements collected using the two modes. In the first mode, 2-μs pulses were transmitted and RIM processing was used to produce estimates of η and V on a grid with a separation of only 15 m. Without RIM processing the range resolution of the data would have been 300 m. In the second mode, 0.5-μs pulses were used, corresponding to a range resolution of 75 m. Estimates of η and V were then obtained from these data through conventional Doppler spectral processing. A conditional averaging method was used to process the reflectivity and vertical velocity data from the two modes. It is shown that the RIM-processed data can be used to resolve structures in the height profiles of η and V on scales less than those of the conventional range resolution of the radar as dictated by the pulse width.

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Carlton W. Ulbrich and Phillip B. Chilson

Abstract

A description is given of the influence of variations in the form of the hydrometeor particle size distribution and of variations in the form of the particle fallspeed law on relations between mean Doppler fallspeed vD and reflectivity factor Z. It is shown that for rain, variations in distribution shape can produce errors in vD of 20%–25%. The resultant errors in raindrop distribution parameters deduced from vD and Z are calculated. Uncertainties regarding hydrometeor phase (and thus in the particle fallspeed law) are shown to produce much larger errors in the value of vD deduced from Z of up to a factor of 4. These results are deduced theoretically, tested with experimental raindrop size spectra, and demonstrated with experimental Doppler radar data for a tropical thunderstorm. It is concluded that the use of theoretical or empirical vD-Z relations to deduce particle size distributions will produce large errors in most meteorological situations. However, in those cases where a reasonable assumption can be made about particle phase, the use of such relations may produce estimates of vertical winds with acceptable accuracy.

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Timothy A. Alberts, Phillip B. Chilson, B. L. Cheong, and R. D. Palmer

Abstract

Trends in current weather research involve active phased-array radar systems that have several advantages over conventional radars with klystron or magnetron transmitters. However, phased-array radars generally do not have the same peak transmit power capability as conventional systems so they must transmit longer pulses to maintain an equivalent average power on target. Increasing transmits pulse duration increases range gate size but the use of pulse compression offers a means of recovering the otherwise lost resolution. To evaluate pulse compression for use in future weather radar systems, modifications to a weather radar simulator have been made to incorporate phase-coding into its functionality. Data derived from Barker-coded pulses with matched and mismatched filters were compared with data obtained from uncoded pulses to evaluate the pulse compression performance. Additionally, pulse compression was simulated using data collected from an experimental radar to validate the simulated results. The data derived from both experimental and simulated methods were then applied to a fuzzy logic tornado detection algorithm to examine the effects of the pulse compression process. It was found that the fuzzy logic process was sufficiently robust to maintain high levels of detection accuracy with low false alarm rates even though biases were observed in the pulse-compressed data.

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Michihiro S. Teshiba, Phillip B. Chilson, Alexander V. Ryzhkov, Terry J. Schuur, and Robert D. Palmer

Abstract

A method is presented by which combined S-band polarimetric weather radar and UHF wind profiler observations of precipitation can be used to extract the properties of liquid phase hydrometeors and the vertical velocity of the air through which they are falling. Doppler spectra, which contain the air motion and/or fall speed of hydrometeors, are estimated using the vertically pointing wind profiler. Complementary to these observations, spectra of rain drop size distribution (DSD) are simulated by several parameters as related to the DSD, which are estimated through the two polarimetric parameters of radar reflectivity (ZH) and differential reflectivity (Z DR) from the scanning weather radar. These DSDs are then mapped into equivalent Doppler spectra (fall speeds) using an assumed relationship between the equivolume drop diameter and the drop’s terminal velocity. The method is applied to a set of observations collected on 11 March 2007 in central Oklahoma. In areas of stratiform precipitation, where the vertical wind motion is expected to be small, it was found that the fall speeds obtained from the spectra of the rain DSD agree well with those of the Doppler velocity estimated with the profiler. For those cases when the shapes of the Doppler spectra are found to be similar in shape but shifted in velocity, the velocity offset is attributed to vertical air motion. In convective rainfall, the Doppler spectra of the rain DSD and the Doppler velocity can exhibit significant differences owing to vertical air motions together with atmospheric turbulence. Overall, it was found that the height dependencies of Doppler spectra measured by the profiler combined with vertical profiles of Z, Z DR, and the cross correlation (ρHV) as well as the estimated spectra of raindrop physical terminal fall speeds from the polarimetric radar provide unique insight into the microphysics of precipitation. Vertical air motions (updrafts/downdrafts) can be estimated using such combined measurements.

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Phillip B. Chilson, Tian-You Yu, Richard G. Strauch, Andreas Muschinski, and Robert D. Palmer

Abstract

The available range resolution of pulsed radar wind profilers is usually limited by bandwidth restrictions. Range imaging (RIM) has recently been developed as a means of mitigating these limitations by operating the wind profilers over a small set of distinct transmitter frequencies. A constrained optimization method can then be used to generate high-resolution maps of the reflectivity field as a function of range. This paper presents a description of how the RIM technique has been recently implemented on the Platteville 915-MHz tropospheric profiler, the first such implementation at UHF. Examples of data collected during a two-part experiment on 10 April 2001 using the Platteville 915-MHz tropospheric profiler are presented. In the first part, an intercomparison was made involving measurements from RIM and standard radar techniques. It is shown that available frequency bandwidth can be very effectively utilized through the RIM processing. In the second part of the experiment, RIM was applied to radar observations collected with a short (0.5 µs) transmit pulse. The resulting data include observations of a thin, persistent scattering layer attributed to a subsidence inversion and billows from a Kelvin–Helmholtz instability. Estimates of the width of the layer were found to be as small as 12 m.

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Andreas Muschinski, Phillip B. Chilson, Stefan Kern, Jost Nielinger, Gerhard Schmidt, and Thomas Prenosil

Abstract

The spatiotemporal distribution of the vertical velocity at synoptic and subsynoptic scales is key to the patterns of weather and climate on earth. On these scales, the vertical velocity is on the order of one to a few centimeters per second, typically about three orders of magnitude smaller than typical horizontal wind velocities. Because of the smallness of large-scale vertical velocities relative to typical horizontal velocities, a direct observation of the large-scale vertical air velocity is extremely difficult.

In a case study on observational material obtained during a 68-h experiment using the SOUSY very high frequency (VHF) radar in the Harz Mountains in Germany, the authors present the first intercomparison between three different sources of physical information that can provide large-scale vertical wind velocities: (i) the Doppler shifts observed with a vertically pointing VHF radar; (ii) the rates of change of the altitudes of refractive-index discontinuities as identified with frequency-domain interferometry (FDI), which is still a relatively unexplored technique in meteorology; and (iii) the output of a regional numerical weather prediction model (NWPM), which has been set up to model the meteorological situation during the observational period.

There are several phenomena that have been known to possibly cause significant biases in mean vertical velocities retrieved from the Doppler shifts measured with vertically pointing clear-air VHF radars: (i) stationary or nonstationary gravity waves with vertical-velocity amplitudes up to the order of 1 m s−1; (ii) stationary or horizontally advected tilted refractive-index discontinuities that are aspect sensitive in the VHF regime; and (iii) a correlation between the radar-reflectivity fluctuations and the vertical-velocity fluctuations within a vertically propagating gravity wave.

On the basis of an intercomparison between the vertical velocities retrieved from (i) “standard Doppler” VHF radar observations, (ii) VHF FDI observations, and (iii) the NWPM output, the authors present first evidence that, under ideal conditions, VHF FDI can be used to directly monitor large-scale vertical motion.

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Danny E. Scipión, Phillip B. Chilson, Evgeni Fedorovich, and Robert D. Palmer

Abstract

The daytime atmospheric convective boundary layer (CBL) is characterized by strong turbulence that is primarily caused by buoyancy forced from the heated underlying surface. The present study considers a combination of a virtual radar and large eddy simulation (LES) techniques to characterize the CBL. Data representative of a daytime CBL with wind shear were generated by LES and used in the virtual boundary layer radar (BLR) with both vertical and multiple off-vertical beams and frequencies. To evaluate the virtual radar, a multiple radar experiment (MRE) was conducted using five virtual radars with common resolution volumes at two different altitudes. Three-dimensional wind fields were retrieved from the virtual radar data and compared with the LES output. It is shown that data produced from the virtual BLR are representative of what one expects to retrieve using a real BLR and the measured wind fields match those of the LES. Additionally, results from a frequency domain interferometry (FDI) comparison are presented, with the ultimate goal of enhancing the resolution of conventional radar measurements. The virtual BLR produces measurements consistent with the LES data fields and provides a suitable platform for validating radar signal processing algorithms.

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Charlotte E. Wainwright, Phillip M. Stepanian, Phillip B. Chilson, Robert D. Palmer, Evgeni Fedorovich, and Jeremy A. Gibbs

Abstract

A sodar simulator capable of producing time series data emulating sodar signals has been developed and tested. The atmospheric fields used to populate the sodar simulator are taken from output of a large-eddy simulation code. The characteristics of the sodar (number and zenith angle of beams, beamwidth, transmit frequency, range resolution, etc.) are defined by the user to allow emulation of existing systems. The range of the reflected acoustic signal is calculated based upon a temperature-dependent speed of sound. Realistic acoustic background noise is simulated using filtered white noise. The raw acoustic time series data are processed using a Fourier transform to yield acoustic Doppler spectra, from which the radial velocities are calculated. The design of the simulator allows for the testing of and comparisons between various signal-processing techniques and averaging periods. An example case of feeding the sodar simulator with large-eddy simulation data representative of a developing convective boundary layer is presented and discussed.

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Valery M. Melnikov, Dusan S. Zrnić, Richard J. Doviak, Phillip B. Chilson, David B. Mechem, and Yefim L. Kogan

Abstract

Sounding of nonprecipitating clouds with the 10-cm wavelength Weather Surveillance Radar-1988 Doppler (WSR-88D) is discussed. Readily available enhancements to signal processing and volume coverage patterns of the WSR-88D allow observations of a variety of clouds with reflectivities as low as −25 dBZ (at a range of 10 km). The high sensitivity of the WSR-88D, its wide velocity and unambiguous range intervals, and the absence of attenuation allow accurate measurements of the reflectivity factor, Doppler velocity, and spectrum width fields in clouds to ranges of about 50 km. Fields of polarimetric variables in clouds, observed with a research polarimetric WSR-88D, demonstrate an abundance of information and help to resolve Bragg and particulate scatter. The scanning, Doppler, and polarimetric capabilities of the WSR-88D allow real-time, three-dimensional mapping of cloud processes, such as transformations of hydrometeors between liquid and ice phases. The presence of ice particles is revealed by high differential reflectivities and the lack of correlation between reflectivity and differential reflectivity in clouds in contrast to that found for rain. Pockets of high differential reflectivities are frequently observed in clouds; maximal values of differential reflectivity exceed 8 dB, far above the level observed in rain. The establishment of the WSR-88D network consisting of 157 polarimetric radars can be used to collect cloud data at any radar site, making the network a potentially powerful tool for climatic studies.

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Steven E. Koch, Martin Fengler, Phillip B. Chilson, Kimberly L. Elmore, Brian Argrow, David L. Andra Jr., and Todd Lindley

Abstract

The potential value of small unmanned aircraft systems (UAS) for monitoring the preconvective environment and providing useful information in real time to weather forecasters for evaluation at a National Weather Service (NWS) Forecast Office are addressed. The general goal was to demonstrate whether a combination of fixed-wing and rotary-wing UAS can provide detailed, accurate, and useful measurements of the boundary layer important for determining the potential for convection initiation (CI). Two field operations were held: a validation study in which the UAS data were compared with collocated measurements made by mobile rawinsondes and ground-based remote sensing systems and a real-time experiment held to evaluate the potential value of the UAS observations in an operationally relevant environment. Vertical profile measurements were made by the rotary-wing UAS at two mesonet sites every 30 min up to 763 m (2500 ft) AGL in coordination with fixed-wing UAS transects between the sites. The results showed the ability of the fixed-wing UAS to detect significant spatial gradients in temperature, moisture, and winds. Although neither of two different types of rotary-wing UAS measurements were able to strictly meet the requirements for sensor accuracy, one of the systems came very close to doing so. UAS sensor accuracy, methods for retrieving the winds, and challenges in assessing the representativeness of the observations are highlighted. Interesting mesoscale phenomena relevant to CI forecasting needs are revealed by the UAS. Issues needing to be overcome for UAS to ever become a NOAA operational observing system are discussed.

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