Differential Phase (PhiDP) and Specific Differential Phase (Kdp)

In the case of phase measurement, the speed of propagation is also affected by the asymmetry of the larger drops. Because of the longer dimension of the horizontal axis of drops, the medium is more dense for horizontal than for vertical polarization. This causes the horizontal wavelength to propagate slightly slower (more phase cycles per unit distance) in comparison with the vertical wavelength, which leads to a phase difference between horizontal and vertical.

This difference is called the differential phase shift, PhiDP. Phase difference accumulates (PhiDP increases) with range as a function of the relative difference in density that the horizontal and vertical microwaves must propagate through. An important concept to understand about PhiDP is that the measurement value is not indicative of what is present at a particular range, but what the microwaves have travelled through to get to that point (and back).

The range derivative of PhiDP, that is, the change of phase per unit distance, is called specific differential phase (K_dp). K_dp is almost directly proportional to the rainfall rate, so it has the potential to improve precipitation rate measurements compared to traditional Z-R relationship measurements, which can be highly inaccurate.