Rainfall Estimation - Introduction From the radar equation, we can get Z, the radar reflectivity factor. Recall that: (1) Z depends on the rainfall rate (RR) and the size distribution (SD) of the drops. So Z = Z(RR,SD) (2) Since Z is known, but RR and SD are unknown, (2) shows that we have one equation, but two unknowns. Solution to problem: Let's assume a specific form of the size distribution - the Marshal Palmer Drop Size Distribution: (3) Where D is the drop diameter (mm) N_{0} is just a constant = 8000/m^{3}mm l can be expressed in terms of the rainfall rate: l = 4.1R^{-0.21} (for rain) where R is the rainfall rate in mm/hr. |
So (3) becomes: (4) Hence, we can use (4) with a known rainfall rate (R) to calculate the number of drops (N_{d}) at a given diameter, (D). (see graph above) |