After World War II, when the use of radars for rainfall observation was in its infancy, high expectations were placed on compact, low-cost X-band radars for their use in hydrology. However, after it became known that X-band radars were greatly affected by rain attenuation due to their short wavelength, and that they were unsuitable for quantitative measurement, such expectations drastically faded. Multi-parameter radars are unaffected by rain attenuation, as they use KDP information instead of a reflectivity factor. The R-KDP method has thus enabled quantitative rainfall estimates, which were conventionally difficult to obtain.

Classic method (R-Z relationship)
(a) Radar reflectivity factor of rain induced by Typhoon T0111. (b) Distribution of rainfall intensity estimated from radar reflectivity factor. Heavy rainfall within the area shown in the dotted-line circle is not being detected due to attenuation caused by the strong rain band near the radar.
New method (R-KDP relationship)
(a) Specific differential phase KDP of rain induced by Typhoon T0111. (b) Distribution of rainfall intensity estimated from the KDP. The heavy rainfall area, which was not detected in Fig. 11, is being detected.