blowing from west to east at the height of the display.
At point 2, the zero line is oriented east to west. In this
instance, inbound velocities are to the south so the
wind direction must be 180° near the surface.
Figure (D) is an example of the WSR-88D velocity
display. As you can see, areas of red and orange
indicate flow away from the radar, while areas of green
and blue indicate flow toward the radar. The colors can
then be compared to the color scale above to determine
Again, successful coverage from this product
depends on the size and amount of scatterers available.
When ground clutter or anomalous propagation
contaminate the radar beam, all velocities will be
biased toward zero. When range folding occurs, purple
shading might obscure valuable data, causing you to
miss significant features. Keep in mind that all
velocities are relative to the RDA, not the PUP.
Derived products are enhanced renditions of base
products that provide observers and forecasters with a
unique perspective of radar information. In the
following text, we will discuss some of the more
commonly used derived products.
Composite Reflectivity (CR) Product
Recall that base reflectivity provides a birds eye
view of the radar coverage area. While this is very
useful, base products provide data from only a single
elevation angle. Thus, only a slice of the overall
atmosphere is presented, and valuable information
above or below the radar beam may be overlooked. To
sample the entire volume scan, radar operators must
view each slice individually. This time-consuming
process is impractical for the operational user. The
WSR-88D offers Composite Reflectivity (CR) as a
The CR product contains information found in
base reflectivity. However, one very important
composite reflectivity operates on a summation
principle. That is, the algorithm first compiles data
from all elevations (volumetric), and then produces a
product which displays only the strongest returns for
all regions of the radar coverage area. In building the
CR product, the algorithm considers only intensity as
its criteria. Size, shape, characteristic, and altitude are
The CR offers a sneak-peek advantage over
base reflectivity, but should never replace the use of
other reflectivity products. When using CR, operators
are less likely to miss significant targets since only
reflectivity maximas are displayed. The major
downside of this product is its loss of target heights.
This limitation poses serious problems since echo
heights relate closely to storm development. Without
height data, targets become deceiving. This product is
normally accompanied by an attribute table that ranks
storms according to severity and includes forecast
movement and the likelihood of each storm to produce
a variety of conditions (hail, mesocyclones, tornadoes,
Keep in mind that values displayed for a given
location could have come from any altitude or
elevation angle. In fact, extensive ground clutter may
severely contaminate this product, creating the illusion
of intense storms where nothing exists. This
occurrence is common when superrefractive
conditions are present. Figure 2-37 is an example of the
Composite Reflectivity product.
Vertically Integrated Liquid (VIL) Product
Most WSR-88D products emphasize a targets
horizontal details. The Vertically Integrated Liquid
(VIL) product provides an estimate of atmospheric
liquid-water content in the vertical. It serves a
multitude of purposes, but is primarily designed to
evaluate storm severity.
The VIL product is compiled from extensive
reanalysis of base reflectivity data. It totals reflectivity
within a given column of the atmosphere and then
displays a product of tallied values. The function of the
VIL algorithm is to estimate the amount of liquid water
contained in a storm, and then display that value
(kilograms/meter squared) in a graphical form. In its
initial stages, the VIL algorithm holds many
similarities to composite reflectivity. It builds a
volumetric product by compiling reflectivity data from
all elevation angles. The difference is that VIL
displays tallied values for the entire column (fig. 2-38).
CR displays only the reflectivity maxima regardless of
altitude. Users can quickly evaluate storms by
comparing VIL columns.
VIL is useful when monitoring general echo pat-
terns for signs of development. In convective situa-
tions, VIL is directly related to updraft strength, which
translates into storm severity. The VIL, product was
designed to distinguish severe from nonsevere storms,
but it is also used as a hail indicator (very high dBZs).