Vaisala IRIS - Geometry; Figure 101 Dbz Values for Various Phenomena

Figure 101 dBZ Values for Various Phenomena
For hydrology, we need an equation to combine radar reflectivity to rainfall rate. All of these
equations are empirical and approximate, since Z is proportional to D6 and precipitation rate
R (mm/h) is proportional to D3.7. Also, we must assume something about drop sizes. For
64 drops of diameter 1 mm (0.04 in) or one drop, diameter 2 mm (0.08 in), Z is same, R is
not. A good first guess is the classical Marshall Palmer equation:
Z = 200R
1.6
which equals to R = Z/200
0.625
A.3
Geometry
The Earth is spherical. Microwaves do not follow the surface. Because of the atmospheric
optics, they usually bend down a bit, at a radius of 4/3 of the Earth.
The following figure shows the height of some radar beams as function of distance from
radar at dierent elevations.
Some energy is spread to directions outside the nominal beamwidth. These are called
sidelobes.
IRIS Product and Display User Guide M211319EN-H
298 RESTRICTED