Subject: Re: CASILIST : Why Velocity has breaks on NEXRAD?
Date: Sun, 21 Mar 1999 17:24:48 EST
From: Atari2601{at}aol.com
To: j{at}weatherwatchers.org
If the "breaks" in data referred to are areas of Range Folded, or "RF"
data,
it's a limitation of the radars in the way they detect velocity.
Contrary to popular belief, it is actually currently impossible to
construct a
true DOPPLER radar. Any radar which can detect velocity (including the
NWS
WSR-88Ds) actually are measuring phase shift, not doppler frequency
shift.
This is because even with high wind speeds, the actual doppler frequency
shift
is less than .001% of the original frequency -- too small to be
accurately
detected and displayed.
When the radar sends out a pulse into a storm, it remembers the phase of
the
transmitted signal rather than the frequency. Phase refers to whether
the
signal started on a crest (amplitude +1) or a trough (amplitude -1).
When the
wave is reflected back to the radar, the phase of the returned signal is
compared to the phase of the wave that left.
The maximum velocity that can be determined is actually related to the
phase
shift of the wave. In low to moderate wind speeds, simple equations can
relate the wind speed to the amount of phase shifting detected in the
returned
signal. With high wind velocities, the wave will actually be
phase-shifted
more than one entire wavelength -- which means the radar can't
immediately
identify whether the wave was shifted just a partial wavelength, or a
higher
multiple of a wavelength! In this case, a "Velocity Dealiasing
Algorithm"
tries to extract, or unfold, the true velocity in that region using a
variety
of methods. If even this algorithm fails to determine a true wind, the
velocity information at that point is marked as Range Folded --
displayed as
purple on the NWS base velocity display.
The number of pulses per second transmitted out from the radar antenna
AND the
length of each pulse determine a value called the Maximum Unambiguous
Velocity...often called Vmax. This value is adjustable from +/- 22 kts.
in
clear air mode (VCP 32) up to around 80 kts in preciptation mode (VCPs
21 or
11.) Suppose we're operating the radar with a Vmax of 50 kts. The
following
chart will explain what happens:
Phase Shift Actual Radial Wind Speed
0 deg 0 kts
45 deg 6.25 kts
90 deg 12.5 kts
135 deg 18.75 kts
.. ..
315 deg 43.75 kts
360 OR 0 deg 50.00 OR 0 kts...they're distinguishable!!!
405 OR 45 deg 56.25 OR 6.25 kts...they're indistinguishable!!!
More problems occur due to inbound versus outbound velocities, since
they
phase shift the wave in different directions...but we'll just ignore
that for
now! Also, problems arise when using higher and higher Vmax values
because
that cuts down the maximum range that you can detect the position of
echoes
accurately, the so-called Rmax value...another thing we'll ignore for
now.
Some internet radar providers delete the range folded data altogether,
which
will create an apparent "break" in the data continuity.
Hope this helps!
Tim Armstrong