Intercomparison of radar measurements of rain by single‐ and dual‐wavelength techniques

Abstract

We are experimenting with three different techniques of rain rate estimation to assess their appropriateness by analyzing the data on simultaneous rain measurements with a C band weather radar and a dual-wavelength (3-and 0.86-cm) radar scatterometer. These techniques are (1) a conventional singlewavelength technique based on an empirical relation between a reflectivity factor and a rain rate; (2) a dual-wavelength technique based on radar wave attenuation at one wavelength, where it is assumed that the raindrops sampled are Rayleigh scatterers at both wavelengths; and (3) an alternative dualwavelength technique using the method of least squares to minimize the difference between measurement-derived quantities and their corresponding quantities derived from a radar equation. The rain rate estimates from the above three techniques are compared with each other, and the causes of their differences are discussed. The rain rate estimates obtained with each technique and gauge rain rates measured near the radar site are also compared to evaluate their absolute accuracy. The advantages and disadvantages of each rain rate estimation technique are discussed for specific situations and applications. As a result of investigation the following conclusions are obtained: The single-wavelength technique gives fairly good rain rate estimates if an external radar calibration is made accurately. The dual-wavelength technique based on radar wave attenuation needs verification of the estimates by auxiliary means to avoid unrealistic estimates, and the dual-wavelength technique by means of a least squares method is useful for an airborne or a spaceborne application where an external calibration is difficult to perform. 65 66 FUJITA ET AL.: RADAR MEASUREMENTS OF RAIN FUJITA ET AL.: RADAR MEASUREMENTS OF RAIN 67

Computer processing for deriving drop‐size distributions and vertical air velocities from VHF Doppler radar spectra

“…Intensive efforts have been paid to improve dual-polarization radars as reviewed by Ulbrich [1986], since they have an advantage of deriving two parameters of a target volume with a single antenna, while dual-wavelength radars may require two antennas in order to match the size of target volumes measured by the two wavelengths. Recent advances in data process-ing schemes of dual-polarization radars and of dualwavelength radars [e.g., $achidananda and Zrnid, 1987; Fujita et al, 1989] have shown substantial improvements in their comparisons of rainfall rates measured by the radars and by ground-based rain gauges.…”

“…In parallel to these developments in microwave weather radars it has been found that a sensitive VHF Doppler radar can detect precipitation echoes simultaneously with echo from the ambient atmosphere [Fukao et al, 1985]. Wakasugi et al [1986Wakasugi et al [ , 1987 investigated a computer algorithm to derive parameters of drop-size distribution as well as the mean vertical velocity and fluctuations due to turbulence from the radar spectra.…”