Subject: Re: Definition of "Dbz"
From: capskb@alliant.backbone.uoknor.edu (Keith Brewster)
Date: 14 Sep 1995 05:35:14 GMT
Message-ID: <438eui$ree@tiber.uoknor.edu>

In article <434uhr$j2@vixen.cso.uiuc.edu> AISS Staff <network@aiss.uiuc.edu>
writes:
>In another unrelated newsgroup in a discussion about weather radar the 
>unit "Dbz" was mentioned as a measure of precipitation intensity.  What 
>is the definition of a "Dbz"?  Thanks.
>

Actually, its dBZ.  dB is for decibels, and Z is the "reflectivity
factor", which describes the strength of the returned radar signal
(after normalizing the actual returned power for the strength of the
transmitted signal, distance from the radar, antenna gain, etc).

dBZ = 10 log (Z)

dBZ stands on its own as a radar measurement.  Trying to relate it
to precipitation intensity can only be an approximation, because
the reflectivity factor is a function of the size and shape
as well as the number of raindrops.  Rainfall is related to the
number and the cube of the drop diameters (water volume) while
Z is related to the number and sixth power of the drops' diameters.

Nevertheless, if a drop spectrum is assumed, a reasonable approximation
to rainfall can be obtained, and that is what is currently done
with the WSR-88D rainfall algorithm.  Using radar signals with
different polarizations (horizontal and vertical) you can improve that
estimate quite a bit, and also could help the hail detection
problem.  Proof of that came too late to get into the 88D specs.

-Keith

----------
Keith Brewster
CAPS/Univ of Oklahoma