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Satellite-delivered weather -
does it show clouds? 
The satellite-delivered radar image that you view
in the
cockpit on your portable or panel mounted display is derived from the
network of NWS WSR-88D NEXRAD Doppler radars. This is the source of the
image, but keep in mind that the product you see in the cockpit is not
the same as what comes out of these radars. But does it show
the
presence of clouds not producing precipitation?
Ask yourself this question. Do you want
a radar that
shows clouds that are not producing precipitation-sized drops or ice
crystals? Your answer should be no. The visible and
infrared satellite images do a fine job telling you the whereabouts of
clouds. Instead, we want the radar to tell us which clouds
are
actually producing precipitation in the form of rain, snow, ice
pellets, hail and virga. Clouds would only be a nuisance.
But from time to time the WSR-88D will indeed show
the
presence of non-precipitating clouds. As shown on the left (click here
to view a larger image), the radar image from the NWS radar site from
the Midland/Odessa weather forecast office (WFO) shows both
precipitation and non-precipitation returns.
This image clearly shows areas of convective
precipitation
in extreme western Texas and southeastern New Mexico.
However,
the returns shown in the center of the image (around the radar site)
include both ground clutter and returns from shallow boundary layer
clouds
called horizontal convective rolls.
Horizontal convective rolls such as this can produce a ripple-effect
you see in the NEXRAD image. They are produced by counter-rotating vortices
that commonly occur in the boundary layer. Highly capped
vertically-developed clouds are produced in the updraft portion between
the vortices and are frequently referred to as cloud streets.
So
will these convective roll returns be shown on your satellite-delivered
weather display? Not likely.
The WSR-88D will see anything that intersects the radar's
pencil
beam and backscatters the energy. The reflected energy
(called a
radar return or radar echo) is measured. Returns that pass a
certain backscattered energy threshold are recorded by the NWS radar.
However, as mentioned in the
beginning, the data that comes from the WSR-88D is not the same as what you
will see in the cockpit. It goes through a process that
endeavors to
remove all returns that are not associated with precipitation before it is
broadcast.
The XM-delivered satellite weather broadcast is
produced
by WxWorx located in Huntsville, Alabama (shown to the
right).
They run the data from the NWS through a process that includes some
filtering. It is this process that will likely filter out the
returns produced  by the
convective rolls.
Initially WxWorx applies a manual gross filter to
areas
that could not possibly produce precipitation. An area with
clear
skies (or nearly so) is not a likely candidate to produce
precipitation. This filter alone will reduce most of the
non-precipitation radar returns caused by ground clutter and anomalous
propagation (AP) that ordinarily occurs around each radar site when
skies are clear (especially at night and during the early morning).
For
the remaining returns, the process is a bit more selective and
proprietary.
WxWorx applies an algorithm that will use both the
reflectivity data as well as the velocity data (Doppler portion) from
the radar. At a minimum, WxWorx filters all low power returns that fall
below 10 dBZ. This will reduce a good portion of the
remaining
clutter and AP. The returns produced by the convective rolls
in
west-central Texas are all below 15 dBZ based on the reflectivity
legend on the left. In fact, most the returns around the
radar
site are indeed below 10 dBZ and will be filtered. The few
returns remaining that are 10 dBZ and higher around the radar site will
most likely get filtered as well, but in some circumstances this may
leak through the algorithm and produce an occasional area of light
precipitation that may show up on your XM-delivered satellite display.
It is also important to note that the
hardware/software vendor that receives the broadcast from the satellite
(i.e., Garmin, Avidyne, etc.) has the ability to take the filtered data
and display it in any fashion they choose. This means they
can
filter the data further or even reduce the resolution.
What about
echo tops? Echo tops are only generated by the NWS WSR-88D
radars
for returns that are greater than or equal to 18 dBZ. The
data is
also at a 5,000-ft resolution. Once again, these returns will
generally fall below the threshold produced by non-precipitating
clouds.
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