James B. Mead

A Volume-Imaging Radar Wind Profiler for Atmospheric Boundary Layer Turbulence Studies

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.

Surface-Based Polarimetric C-Band Scatterometer for Field Measurements of Sea Ice

A portable surface-based polarimetric C-band scatterometer for field deployment over sea ice is presented. The scatterometer system, its calibration, signal processing, and near-field correction are described. The near-field correction is shown to be effective for both linear polarized and polarimetric backscatter. Field methods for the scatterometer are described. Sample linear polarized and polarimetric backscatter results are presented for snow-covered first-year sea ice (FYI), multiyear hummock ice, and rough melt pond water on FYI. The magnitude of backscatter signature variability due to system effects is presented, providing the necessary basis for quantitative analysis of field data.

A Mobile Rapid-Scanning X-band Polarimetric (RaXPol) Doppler Radar System

AbstractA novel, rapid-scanning, X-band (3-cm wavelength), polarimetric (RaXPol), mobile radar was developed for severe-weather research. The radar employs a 2.4-m-diameter dual-polarized parabolic dish antenna on a high-speed pedestal capable of rotating the antenna at 180° s−1. The radar can complete a 10-elevation-step volume scan in about 20 s, while maintaining a 180-record-per-second data rate. The transmitter employs a 20-kW peak-power traveling wave tube amplifier that can generate pulse compression and frequency-hopping waveforms. Frequency hopping permits the acquisition of many more independent samples possible than without frequency hopping, making it possible to scan much more rapidly than conventional radars. Standard data products include vertically and horizontally polarized equivalent radar reflectivity factor, Doppler velocity mean and standard deviation, copolar cross-correlation coefficient, and differential phase. This paper describes the radar system and illustrates the capabilities ...

Polarization Diversity Pulse-Pair Technique for Millimeter-Wave Doppler Radar Measurements of Severe Storm Features

The Polarization Diversity Pulse-Pair (PDPP) technique can extend simultaneously the maximum unambiguous range and the maximum unambiguous velocity of a Doppler weather radar. This technique has been applied using a high-resolution 95-GHz radar to study the reflectivity and velocity structure in severe thunderstorms. This paper documents the technique, presents an analysis of the first two moments of the estimated mean velocity, and provides a comparison of the results with experimental data, including PDPP images of high-vorticity regions in supercell storms.

95-GHz Polarimetric Radar Measurements of Orographic Cap Clouds

Abstract The use of millimeter-wavelength radars for cloud microphysical research was investigated in experiments at the Elk Mountain Observatory near Laramie, Wyoming, between April 1990 and March 1992. The 95-GHz polarimetric radar used in these experiments is a portable, high-power, dual-polarization radar capable of characterizing the complex scattering matrix in two pulses. The scatterers polarimetric response is characterized in terms of the Mueller matrix, a form that is seen to be convenient for computing the response of a scatterer for any arbitrary combination of transmit and receive antenna polarizations. This paper summarizes the results of recent experiments carried out at the Elk Mountain Observatory. Polarimetric data from orographic cap clouds are found to be sensitive to ice-particle orientation and composition. Comparison of radar-observed reflectivities with those computed from in situ images shows good agreement if the volume fraction of ice in ice-air mixtures is taken into account.

A 225 GHz polarimetric radar

An incoherent 225-GHz polarimetric radar capable of measuring the Mueller matrix of point and distributed targets is described. The transmitter employs an extended interaction oscillator that transmit 60-W pulses of 50- to 600-ns duration. Incoherent measurements of the Mueller matrix are achieved by transmitting four linearly independent polarizations and measuring the scattered wave using a dual-polarized receiver. A novel calibration technique that requires a single in-scene reflector is presented. Polarimetric measurements are presented of a dihedral corner reflector and foliage which are the first polarimetric measurements reported at this wavelength. The foliage measurements indicate a pronounced sensitivity of the polarimetric data to fine-scale surface structure. >

Polarimetric observations and theory of millimeter-wave backscatter from snow cover

Polarimetric radar measurements carried out at 95 and 225 GHz are presented for fresh and refrozen snow cover. These data indicate that the Mueller matrix for snow cover consisting of spherical ice particles has a relatively simple form, with 10 of the 16 elements approximately zero. Measurements of new-fallen snow consisting of predominantly nonspherical snow crystals are also presented. The anisotropic structure of such snow cover results in a more complex Mueller matrix, fitting the general form for natural surfaces. An analytic expression for the Mueller matrix of isotropic snow cover is derived by computing the response of a semi-infinite layer of scatterers that are insensitive to the orientation of the incident polarization. This matrix is shown to accurately predict the polarimetric response of the snow cover comprised of spherical ice particles based solely on copolarized and cross-polarized radar cross-section measurements. >

Remote Sensing of Clouds and Fog with a 1.4-mm Radar

Abstract A recently developed 1.4 mm wavelengh incoherent radar has potential for remote sensing of low reflectivity atmospheric targets for ranges up to several kilometers. Power output of 60 W is achieved using an Extended Interaction Oscillator (EIO). Preliminary reflectivity measurements of clouds and fog for ranges between 36 and 1900 meters are believed to be the first such measurements at this wavelength Limitations on the accuracy of the reflectivity measurements are discussed, highlighting uncertainties due to highly variable attenuation.

Polarimetric backscatter from fresh and metamorphic snowcover at millimeter wavelengths

This paper presents 35, 95, and 225 GHz polarimetric radar backscatter data from snowcover. It compares measured backscatter data with detailed in situ measurements of the snowcover including microstructural anisotropies within the snowpack. Observations of backscatter mere made during melt-freeze cycles, and measurable differences in the normalized radar cross section between older metamorphic snow and fresh low-density snow were observed. In addition, these data show that the average phase difference between the copolarized terms of the scattering matrix, S/sub vv/and S/sub hh/, is nonzero for certain snow types. This phase difference was found to be related to snowpack features including anisotropy, wetness, density, and particle size. A simple backscatter model based on measured particle size and anisotropy is found to predict the Mueller matrix for dry snowcover with reasonable accuracy.

Detection of Ice Hydrometeor Alignment Using an Airborne W-band Polarimetric Radar

Abstract This paper presents airborne W-band polarimetric radar measurements at horizontal and vertical incidence on ice clouds using a 95-GHz radar on the University of Wyoming King Air research aircraft. Coincident, in situ measurements from probes on the King Air make it possible to interpret polarimetric results in terms of hydrometeor composition, phase, and orientation. One of the key polarimetric measurements recently added to those possible with the W-band radar data system is the copolar correlation coefficient ρHV. A discussion of the relation between cloud scattering properties and ρHV covers a test for isotropy of the distribution of observed hydrometeors in the plane of polarization and qualitative evaluation of the possible impact of Mie (resonant) scattering on ρHV measurements made at W band. Prior measurements of ρHV at S band and Ku band are compared with the W-band results. The technique used to measure ρHV, including the real-time and postprocessing steps required, is explained, with a...