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Peter T. May

Abstract

The performance and limitations of the Bureau of Meteorology Research Centre's (BMRC) 50-MHz wind profiler operating at Saipan in the central western Pacific are assessed. Hourly averaged profiler estimates of horizontal wind are compared with 120 radiosonde ascents. This comparison shows the best agreement of any large profiler-sonde comparison published with rms differences of about 1.5 m s−3 at some altitudes and about 2.3 m s−1 for all altitudes below 10 km. The results appear to be almost independent of wind-speed or precipitation conditions.

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Peter T. May

Abstract

A simple model is used to illustrate the limitations of dual pulse repetition time (PRT) radars for measurements of wind fields with strong horizontal wind gradients. In such conditions, errors may occur even if the radial velocity measurements in the individual rays are not aliased. However, basic circulation and divergent signatures are seen and some improvement is possible with further postprocessing of the data. This is significant, as radars using this procedure are being used operationally for severe weather applications in Australia and Canada.

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Peter T. May

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Peter T. May and Richard G. Strauch

Abstract

It is well known that the presence of ground clutter may severely bias radar measurements of the Doppler shift, particularly with wind profilers undertaking boundary layer measurements. It is shown both qualitatively and quantitatively with simulated data that a simple detrending of the time series data is often sufficient to significantly reduce the clutter problem. Finite impulse response filters are also investigated. Improvements are seen when long records are filtered prior to spectral analysis of the time series. The results are not very sensitive to the width of the filter (within reason) as long as the filter width encompasses the clutter spectrum.

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Peter T. May and Richard G. Strauch

Abstract

A numerical model to simulate radar data is used for testing various estimators of the Doppler shift in Doppler radar echoes. The estimators are the pulse pair and poly-pulse pair algorithms in the correlation domain, a least-squares fitting to the spectral peak of the power spectra, and direct calculations of the moments from periodograms in the spectral domain. Two averaging schemes (a consensus average and a median filter) are also examined for data with poor signal-to-noise ratios. The data processing method used in Doppler radar wind profilers, which operate over a very wide range of signal to noise ratios, is examined in detail. It is shown that the direct moment calculation combined with a consensus averaging technique has the best overall performance for accuracy and the ability to use data with a very low signal-to-noise ratio.

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Christopher R. Williams and Peter T. May

Abstract

Polarimetric weather radars offer the promise of accurate rainfall measurements by including polarimetric measurements in rainfall estimation algorithms. Questions still remain on how accurately polarimetric measurements represent the parameters of the raindrop size distribution (DSD). In particular, this study propagates polarimetric radar measurement uncertainties through a power-law median raindrop diameter D 0 algorithm to quantify the statistical uncertainties of the power-law regression. For this study, the power-law statistical uncertainty of D 0 ranged from 0.11 to 0.17 mm. Also, the polarimetric scanning radar D 0 estimates were compared with the median raindrop diameters retrieved from two vertically pointing profilers observing the same radar volume as the scanning radar. Based on over 900 observations, the standard deviation of the differences between the two radar estimates was approximately 0.16 mm. Thus, propagating polarimetric measurement uncertainties through D 0 power-law regressions is comparable to uncertainties between polarimeteric and profiler D 0 estimates.

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Sergey Y. Matrosov, Peter T. May, and Matthew D. Shupe

Abstract

An attenuation-based method to retrieve vertical profiles of rainfall rate from vertically pointing Ka-band radar measurements has been refined and adjusted for use with the U.S. Department of Energy’s cloud radars deployed at multiple Atmospheric Radiation Program (ARM) test bed sites. This method takes advantage of the linear relationship between the rainfall rate and the attenuation coefficient, and can account for a priori information about the vertical profile of nonattenuated reflectivity. The retrieval method is applied to a wide variety of rainfall events observed at different ARM sites ranging from stratiform events with low-to-moderate rainfall rates (∼5 mm h−1) to heavy convective rains with rainfall rates approaching 100 mm h−1. The Ka-band attenuation-based retrieval results expressed in both instantaneous rainfall rates and in rainfall accumulations are compared to available surface data and measurements of a scanning C-band precipitation polarimetric radar located near the Darwin, Australia, ARM test bed site. The Ka-band retrievals are found to be in good agreement with the C-band radar estimates, which are based both on conventional radar reflectivity approaches and on polarimetric differential phase shift measurements. Typically, the C-band–Ka-band radar estimate differences are within the expected retrieval uncertainties. The magnitude of the Ka-band rainfall-rate estimate error depends on the retrieval resolution, rain intensity, and uncertainties in the profiles of nonattenuated reflectivity. It is shown that reasonable retrieval accuracies (∼15%–40%) can be achieved for a large dynamic range of observed rainfall rates (4–100 mm h−1) and the effective vertical resolution of about 1 km. The potential enhancements of the Ka-band attenuation-based method by including a priori information on vertical profiles of nonattenuated reflectivity and increasing the height range of the retrievals by using Ka-band polarization measurements are also discussed. The addition of the precipitation products to the suite of ARM hydrometeor retrievals can enhance the overall characterization of the vertical atmospheric column.

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Peter T. May, Tatsuhiro Adachi, Toshitaka Tsuda, and Richard J. Lataitis

Abstract

An experiment to observe the spatial distribution of radio acoustic sounding system (RASS) echo intensity and Doppler shift using the MU radar is described. Various transmitting configurations are used to confirm that the RASS signal is focused onto a diffraction limited spot approximately the size of the transmitting antenna, except when a very small transmitting array is used where turbulence acting on the acoustic wave smears the spot. The signal fades away from the central spot with values about 6-10 dB lower in intensity next to the main spot. Significant gradients of Doppler shift across the radar antenna are seen in the lower height ranges. This may result in errors as large as a degree in the RASS virtual temperature estimates when large radar antennas and a single acoustic source are used.

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Peter T. May, Chris Lucas, Richard Lataitis, and Iain M. Reid

Abstract

An experiment describing the use of a 50-MHz wind profiler Radio Acoustic Sounding System (RASS) to observe the vertical velocity, precipitation, and buoyancy structure of convection is discussed. In most cases the RASS signal was lost when convection was overhead. A theoretical argument that this is the effect of turbulence on the acoustic wave fronts is presented.

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Deepak K. Rajopadhyaya, Peter T. May, and Robert A. Vincent

Abstract

A technique is described that allows estimates of the raindrop size distribution to be obtained from the Doppler spectra measured by wind-profiling radars. The method makes no a priori assumptions regarding the shape of the drop size distributions. To test the accuracy of the technique, artificial data with realistic statistical properties have been generated and the shape of the model drop size distribution varied, The analysis technique obtains an accuracy of around 10% in the drop size range between 1 and 4 mm for data consistent with typical 50-MHz observations averaged over 5–10 min. There are limitations outside this range and the physical reasons for these are discussed. Simulations with multiple-peaked distributions show that the technique can also well resolve complicated distributions

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