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DLOC Principles of Meteorological Doppler Radar:
WSR-88D Fundamentals

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Previous Page: Reflectivity

Why We Use dBZ

Reflectivity vs. Decibels of Reflectivity (Z vs. dBZ)

Range-normalized values of reflectivity, Z, can range over many orders of magnitude. To compress this large range of values for operational use, Z is displayed in decibels of Z, that is, dBZ. Converting Z to dBZ is simply done by using

 
Equation 5.  Conversion of Reflectivity (Z) to Decibels of Reflectivity (dBZ).  Click for long description.
 

For example, if Z = 4000 mm6m-3, then dBZ = 10(log10 4000) ~ 10 x 3.6 = 36 dBZ.

Due to the WSR-88D's sensitivity, reflectivities as low as -32 dBZ can be detected in clear air mode near the RDA. How can there be such a thing as a negative dBZ? If 0 < Z < 1, log10Z < 0 and thus dBZ < 0. Very low reflectivity values indicate the presence of extremely small sized particles (e.g., ash, dust, etc.) or Bragg scattering from differences in refractive index.

The WSR-88D can also detect reflectivity values as high as 95 dBZ. As an example, a one cubic meter volume containing just one 38.3 mm (~1.50 inch) diameter water-coated hailstone would yield a reflectivity value of approximately 95 dBZ. However, severe-sized hail frequently occurs with reflectivities less than 70 dBZ. This is a good indication that such large targets do not meet the Rayleigh approximation (i.e., they are Mie scatterers).

Table 4 illustrates why dBZ is used instead of Z to portray reflectivity.

Table 4: dBZ and Z values

dBZ
Z(mm6m-3)
-32
0.000631
-28
0.001585
-10
0.1
0
1
5
3.162
18
63.1
30
1,000
41
12,589
46
39,810
50
100,000
57
501.187
95
3,162,277,660

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