Multi-parameter radar uses two polarization waves, while conventional meteorological radars use a single-polarization wave (Fig. 3a). The principle of rainfall estimation using multi-parameter radar is based on the fact that raindrop shapes change from round to flat with increases in rainfall intensity.

Structure of horizontally and vertically polarized waves

Changes in raindrop shape

Radar parameters used to estimate rainfall intensity:

	Reflectivity factor ZH: The strength of the radio waves reflected back to the radar after it hits raindrops. ZH is a parameter that is used in conventional metrological radar observations. The rainfall intensity is computed using a predetermined R- ZH relationship.
	Differential reflectivity ZDR: Defined as the ratio of the reflectivity factor for horizontal polarization ZH to that for vertical polarization ZV. ZDR is one of the important polarization parameters that MP radar can measure, and from which parameter information on rain drop size distribution can be obtained.
	Specific differential phase KDP: Defined as the difference in phase velocity per unit distance of horizontally- and vertically-polarized waves. KDP is one of the polarization parameters that MP radar can measure, and from which accurate rainfall intensity can be determined.