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RADAR
OPERATIONS:
The most
important responsibility of any NWS field
office is the timely issuance of severe weather warnings. The WSR-88D Doppler
radar has dramatically helped in this task. Our radar tower is located
next to our parking lot outside our front door. Since 2000, AWIPS has been configured
so that
radar monitoring and warning issuance are now performed at AWIPS workstations.
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How we issue a severe storm warning:
Radar
surveillance is always part of the job; it is a basic part of conducting
a continuous weather watch. However,
when there are storms on the radar, this is a much more time-consuming
matter. Therefore, a
meteorologist
is
solely
assigned this task. In active weather regimes, multiple meteorologists
might be monitoring radar data as it becomes available. The workload may
be divided geographically (one person handles the eastern part of the Warning
Area, while another takes the northwest, and a third takes the southwest),
or by hazard (a forecaster assesses the wind/hail threat, while the other
looks
out for
flooding). It is preferable to have two people working a threat/assignment
as a team, as the interaction typically assists, especially when feature
are subtle.
After
interrogating a storm, if a radar meteorologist determines that a warning
is required, he/she clicks on an icon that's in
the upper right corner of the AWIPS graphic screen. This activates the
warning program. A cursor pops up on the screen, along with a dialog
box (seen to
the
left).
The cursor is moved to the storm of concern on the current radar image,
and a radar image from 10 to 15 minutes prior. This is so the computer
can calculate
storm movement. The meteorologist then makes sure the proper items are
clicked in the box, including warning type, valid time, and any appropriate
instructions,
and then clicks an icon at the bottom of the screen to compose the warning.
AWIPS
then composes the warning, and includes the names of communities in the
path of the storm, and the time they will be first affected. The meteorologist
has a chance to make any necessary additions or adjustments on a text
workstation before
issuing the warning.
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The advantage of Doppler radar is its ability
to not just detect where the precipitation is falling, but also determine
movement of particles within the storm. In the image below left, the areas
colored green depict movement toward the radar (which is off the image
to the right), and the areas colored red depict movement away from the
radar. From this, one can infer rotation in this spot since fairly strong
winds are depicted in opposite directions side by side. However, this accounts
for the wind field at just one level. We have the ability to view radar
data at a number of levels. Thus, if a similar scenario is found at the
same location at different, adjacent levels, this is a rotating column
of air, which is a precursor of tornado development.
This was the case in this image, when there was
an F4 tornado on the ground near Frostburg. This tornado occurred during
the tornado outbreak in western Maryland and eastern West Virginia on June
2, 1998. A reflectivity image from this storm at the same time can be seen
to the right.
We also have the ability to dial to other radars
to get a different view of storms. That feature was useful in this instance,
as the storm was closer to the WSR-88D radar in Pittsburgh, PA. It also
serves as good backup coverage. In other words, if the radar were to develop
mechanical difficulties, we can always obtain radar data from surrounding
sites, and maintain surveillance. AWIPS makes this task much easier since
the data gathering is done automatically.
As indicated above, the radar can view storms
at multiple levels. Instead of viewing the data in each "slice" along the
Earth's surface, it is also possible to view the data with respect to height.
The image below is a cross section of a hailstorm on April
23, 1999 that moved from the eastern West Virginia panhandle across northern
Virginia and into southern Maryland. The scale along the bottom is distance
along the ground, and the scale on the side is height above ground. With
AWIPS, the starting and ending points of the cross section can be placed
anywhere and in any orientation. The tilted white core is indicative of
the large hail in this storm. We can determine it is hail since its return
is a lot stronger than that of water particles.
The radar's
computer can also estimate rainfall
based on the intensity and duration of the returns, and is able to display
maps with 1 hour totals, 3 hour totals, and
storm total rainfall. There are also equations built in to the software to assist
us with hail detection, and determining the size of potential hail stones. To
provide further assistance, AWIPS has a software tool called Flash Flood Monitoring
and Prediction (FFMP), which compares flood guidance to rainfall rates. The display
can be seen in the upper left section of the image below. The colors correspond
to inches of rain per time interval (adjustable between 30 minutes and 6 hours).
The "hotter
colors" (such as red and pink) indicate the higher rates, and highlight a greater
risk for flooding. The software allows for zooming in on a county, and can pinpoint
the exact location of concern. In the image below, FFMP is a part of a 4-panel,
along with topography/geography (upper right), instantaneous precipitation (lower
left), and one hour precipitation estimates (lower right).
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