Volume Scatter Special representation used for volume
scatterers (precipitation) in weather radar Resolution cell radians contains a large radians number N of R m scatterers with individual cross sections i N Total RCS is i … i 1 1 N i … so volume reflectivity is V i 1
RCS of a Water Droplet Water droplets modeled as conducting
spheres with small radius compared to wavelength (Rayleigh region) 2
K i 4 Di = drop diameter 5
6 Di
|K|2 0.93 for water, 0.193 for ice
So volume reflectivity of a resolution cell is
1 V
N
i 1
K 5
2
4
Di6
K 5
4
2
1 V
N
6 D i i 1
z Factor Define
1 z V
N
6 D i i 1
z is a measure of size of droplets related to precipitation intensity natural units are m6/m3, but often expressed as
mm6/m3 because droplet sizes are measured in mm Then
K 5
4
2
z
Use radar range equation to measure
, then
use above equation to convert from to z
NEXRAD Rainfall Scale Equivalence between z and rainfall rate similar scale used by commercial weather radars
Caution: these are 10log10(z) where z is in mm6/m3 z in m6/m3 is a factor of 10-18 lower Level
