Communications: directive radio wave systems and devices (e.g. – Radar for meteorological use
Patent
1998-06-26
2000-05-09
Pihulic, Daniel T.
Communications: directive radio wave systems and devices (e.g.,
Radar for meteorological use
G01S 1395
Patent
active
060610131
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for determining the precipitation rate and reflectivity factor of a hydrometeor using pulsed radar operating at a wavelength which is attenuated by hydrometeors, and to a dual polarization meteorological radar implementing this method.
2. Discussion of the Background
In meteorology, it is necessary to observe rainfall and other hydrometeors, in particular for the purpose of short term forecasting. To this end, consideration has for a number of years been given to using radar for quantitatively determining the precipitation rate, this being a characteristic parameter of these phenomena. To do this, use is made of methods based on measuring the reflectivity of the rain or hydrometeors when illuminated with a radar beam. However, for most of these methods, attenuation by the hydrometeors biases the measurements, and it is therefore necessary to operate at wavelengths which are attenuated as little as possible, and are therefore relatively high. This leads to working difficulties and high cost.
Another, more attractive method has been proposed: that of attenuation for two wavelengths and a single polarization, and this has the advantage of being independent of the calibration of the radar. However, this method leads to certain technical difficulties in obtaining measurements with the required precision. This is because problems are encountered in making the beams at the two wavelengths coincide and in the measurements being simultaneous.
All the studies which have been carried out demonstrate the benefit of using radars with shorter wavelength, making it possible to choose more compact and less expensive equipment. However, this would entail problems of correcting attenuation which are very difficult to solve, in particular when there is heavy precipitation.
SUMMARY OF THE INVENTION
The invention relates to a method which overcomes these drawbacks in spite of the use of short wavelengths which lead to relatively high attenuation of the electromagnetic waves on the return path through the absorbent medium which the hydrometeors (rain, hail, clouds) constitute.
The invention is based on the use of a dual polarization radar to measure the reflectivities in the two polarization components H and V while correcting the effects of attenuation on the obtained values by virtue of the differential measurements which are taken.
The invention therefore provides a method for determining the precipitation rate and reflectivity factor of a hydrometeor using pulsed radar operating at a wavelength which is attenuated by the said hydrometeor, the said radar being a dual polarization radar delivering a double series of measurements of the attenuated reflectivity factor (Z.sub.Ha (r), Z.sub.Va (r)) for the various range gates according to each of the polarizations, the said method being characterized in that it comprises the following steps: precipitation rate (R.sub.s) on the basis of the reflectivity factor measured for a single polarization, by a combination of the equations connecting the reflectivity factor with the precipitation rate and the attenuation coefficient for a single polarization with the precipitation rate; obtaining an estimated value of the unattenuated differential reflectivity factor (Z.sub.DRs); reflectivity factor (Z.sub.DRs) and the radar measurement of the attenuated differential reflectivity factor (Z.sub.DRa), deducing an estimated value of the differential polarimetric attenuation (A.sub.DP); and the precipitation rate estimated in step A for a single polarization of the radar and the integral (I.sub.2 (r, R)) of the precipitation rate deduced from the said differential polarimetric attenuation (A.sub.DP), by iterative adjustment of one of the parameters (.alpha., .beta.) in the equation connecting the reflectivity factor with the precipitation rate, so as to make it possible to extract the corrected values of the unattenuated reflectivity factor (Z.sub.Hs (r)) and the precipitation rate (R.sub.s (r)).
REFERENCES:
Journal of Atmospheric and Oceanic technology, vol. 11, 1994, "A Robust Estimator of Rainfall Rate Using Differential Reflectivity", Eugenio Gorgucci, et al.(pp. 586-592).
Radio Science, vol. 19, 1984, "Assessment of the Contribution of Differential Polarization to Improved Rainfall Measurements", Carlton W. Ulbrich, (pp. 49-57).
Scarchilli et al.; Self-Consistency of Polarization Diversity Measurement of Rainfall; IEEE Transactions on Geoscience and Remote Sensing; vol.: 34 1, pp. 22-26, Jan. 1995.
Smirnov et al.; Estimation of rain rate by microwave radiometry and active radar during Cleopatra 92; International Geoscience and Remote Sensing Symposium, '93. Better Understanding of Earth Environment; pp. 1244-1246 vol. 3, Aug. 1993.
Beguin Daniel
Devinne Regis
Sauvageot Henri
"Thomson-CSF"
Pihulic Daniel T.
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