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NEXRAD Doppler radar - What's
a
negative dBZ? 
Of all of the weather products available to pilots
prior to a flight, the images produced by the NWS
WSR-88D NEXRAD Doppler radars are likely the most widely
used. These images have an extremely high glance value and
are packed with guidance assuming that a pilot knows how to interpret
all the pretty colors.
The colors you see on the NEXRAD image
displayed by your favorite website depends on many factors.
Unfortunately, accepted standards do not exist per se.
The private organization, subscriber or government agency is
free to map the data (reflectivity) to colors of their choosing.
Depending on their operational requirements, they may use
three colors representing light, moderate or heavy precipitation or
they are free to use 30 different colors. So it is
important to always reference the scale (such as the one shown to the
left) that is normally located somewhere on the image or around the
image border.
More importantly, there are many kinds
of images and products that you may encounter. Therefore,
knowing the kind of radar image you are viewing is also paramount.
The image may display base reflectivity from a single NWS
NEXRAD radar that is in clear air mode. Or it may be one of
the volume products such as composite reflectivity or echo tops.
Another likely possibility is that the image may represent a
radar mosaic that has combined the base reflectivity data from two or
more sites into a regional or national image.
While
forecasters at the various NWS
local Weather Forecast
Offices (WFOs) do have real-time access to all of the data,
the "raw" data that is generated by the WSR-88D NEXRAD Doppler radars
is not distributed directly to other government and private
organizations in real time. Therefore, a NEXRAD
Information Dissemination Service (NIDS) has been established that
includes only a subset of the entire WSR-88D base and derived
products for use by external users.
One of the most ubiquitous base
products is called reflectivity (note that the term base does not
mean lowest
as most pilots are taught). How you interpret the base
reflectivity image will depend on the mode of the radar.
The WSR-88D radar operates in one of
two modes: precipitation and clear air. The main
difference between the two is that clear air mode offers the advantage
of greater sensitivity due to a slower antenna rotation rate, which
allows more energy to be returned back to the radar. However,
clear air mode takes twice as long to generate a product so it suffers
from temporal resolution, but is able to detect smaller objects in the
atmosphere such as light snow or drizzle.
The radar sends out a known pulse of energy in the
microwave band (10 to 11 centimeter wavelength). Some of this
energy strikes airborne objects referred to as hydrometeors.
This includes rain, snow, hail, dust, birds, insects, etc.
and the power returned is referred to as the reflectivity parameter or Z. In
basic terms, Z
is the density of water drops (measured in millimeters raised to the
sixth power) per cubic meter of air. As you might expect,
there is a very wide range of possible Z values.
Converting Z
to dBZ
(decibels of Z)
makes that range smaller and easier to use.
When the radar is in precipitation
mode, the range of dBZ
values displayed can be as low as 5 to a maximum of 75, whereas clear
air mode offers a range from -28 to +28. The reason negative dBZ values can occur
in clear air mode is because the dBZ
is a logarithmic
function. So an increase of 3 dBZ actually
represents a doubling of power returned! Anytime Z is less than 1 mm6/m3,
dBZ
becomes negative. Negative dBZs
are only found when the radar is in clear air mode such as shown to the
right (click here to view a larger
image). This radar image shows light snow falling around the
Bismarck, North Dakota area. Given that light snow is
falling, the radar operator needs the radar in its most sensitive mode,
namely, clear air mode. Notice the negative dBZ values in the dark
taupe color. If the radar were in precipitation mode, the
amount of coverage would be limited to the blue areas.
A negative dBZ means that the
radar is detecting very small hydrometeors. As mentioned
above, this is great way for forecasters to detect very dry light snow
or drizzle which have lower reflectivities. It may also be
useful to detect outflow boundaries and drylines.
One of the disadvantages of clear air mode is that any dBZ value under 10
gets filtered by the satellite-delivered XM weather. The
NEXRAD clear air mode image from Bismarck shows a rather wide area of
precipitation. However, the XM weather image on the left (click here to view a larger image)
only includes the returns that are greater than 10 dBZ. The
areas shown in the taupe color have been filtered out due to their
lower dBZ
values.
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