is working on a groundbreaking system of forecast models,
lidars, and other tools to track airborne toxins as part
of the nation’s antiterrorism efforts.
Ever since the September 11 terrorist attacks, U.S. defense
officials have been eyeing new technologies to defend potential
terrorism targets. Now they have tapped NCAR to help develop
a groundbreaking system to protect the Pentagon and its occupants,
who often number more than 25,000, from airborne toxins.
Watching the winds.
Instruments used in the Pentagon Shield
project measure the relative strength of surface
winds around the Pentagon, as shown in this
graphic. The information is added to a plume
model to help determine the path of a toxic
The project, known as “Pentagon
combining data from four models continually in real
time, thereby providing officials with remarkably
detailed information about the local atmosphere and
any signs of a toxic release.
“Knowing how to properly
respond to an attack or a toxic industrial incident
requires the best modeling tools and sensors available today,
and these must all work in a coordinated fashion
in real time,” says RAP’s Scott Swerdlin,
the project leader. “This is by far the most
challenging project we’ve ever faced.”
Under NCAR’s guidance, researchers from
several organizations performed a field test at
the Pentagon from April 15 to May 15 that involved
an array of lidars and other sophisticated instruments,
as well as gas releases to simulate how chemical
or biological agents would flow in and around the
building. Results from the test should help in
developing an operational system.
Pentagon Shield is the latest in a series of
antiterrorism projects at RAP. All have a common
theme: to provide information about winds, temperature,
humidity, and other weather conditions so security
officials can predict the path of a toxic plume
and quickly evacuate people. Two years ago, for
example, RAP used its four-dimensional forecasting
system (4DWX) to provide security support at the
Winter Olympics in Salt Lake City, and it’s
currently designing systems to provide information
to emergency crews in urban areas in the event
of the release of an airborne toxin. (See the November
2003 issue of Staff Notes Monthly.)
To predict weather features that could affect
toxic plumes, RAP draws on years of experience
in developing systems that detect wind shear and
turbulence at airports, as well as on NCAR’s
sophisticated modeling capabilities.
The Pentagon system, sponsored by the Defense
Advanced Research Projects Agency (DARPA), is especially
complex. It involves mapping atmospheric conditions
on scales ranging from an entire region (the mid-Atlantic)
to a single building (the Pentagon). The system
combines computer weather forecasting models and
high-tech sensors, including a lidar developed
Understanding air circulation around the Pentagon
is a unique challenge, Scott says. The air circulations
are very complex because of the building’s
size and unusual geometry. Temperature inversions,
especially at night, could allow an airborne hazard
to spread below rooftop height, which adds to the
complexity of a monitoring system.
To tackle the problem, NCAR and its partners
in the private sector and academia built a nest
of concentric computer models—each with a
different strength—that predict weather conditions
from the entire region inward to the Pentagon itself.
Information is routed among them every 15 minutes.
“The weather modeling system tested here
is one of the most complex ever constructed,” says
RAP’s Tom Warner, lead scientist on the project.
The project involves a number of scientists and
engineers in RAP, including Dan Breed, Jeff Copeland,
Rod Frehlich, David Hahn, Jason Knievel, Yubao
Liu, Bob Sharman, Rong Sheu, and Al Yates. CU scientist
and MMM visitor Jeff Weil, who specializes in transport
and dispersion modeling, is also playing a major
role in the program. In ATD, Shane Mayor and Scott
Spuler designed an experimental aerosol lidar that
is being evaluated for use in the system.
Other organizations involved in Pentagon Shield
include Coherent Technologies, CU–Boulder,
the Naval Surface Weapons Center, NOAA’s
Air Resources Laboratory, the U.S. Army’s
Dugway Proving Ground, and several other private
firms and government labs.
Most modern weather forecasts target areas the
size of a county, not a single building. To develop
a unique, fine-scale weather monitoring and forecasting
system needed to protect the Pentagon, NCAR and
its partners relied on a breakthrough blend of
high-tech instruments and weather forecasting models.
A multiscale weather forecast
model. Every 15 minutes, this software
pulls information from a high-resolution regional
weather analysis and generates a set of wind
forecasts with increasingly finer detail at smaller
scales. The forecasts draw on data from Doppler
radars and lidars, and numerous surface and upper-air
meteorological observations. At its finest scale,
the system charts air flow every seven feet (two
meters) immediately around the Pentagon. (See
Lidars (laser-based radars). With
a beam much shorter than that of a conventional
radar, a lidar is ideal for mapping tiny particles
at relatively short distances in clear air. Coherent
Technologies is providing a Doppler lidar for monitoring
winds, while ATD is testing a new lidar with fine
spatial and temporal resolution that is designed
to quickly detect even small-scale suspicious plumes.
Unlike many other lidars, ATD’s Raman-shifted
Eye-safe Aerosol Lidar (REAL) is safe for use in
urban areas because it doesn’t pose any hazards
to the vision of people in the area.
Other sensors. Local
weather sensors and stations have been designed
by other organizations to spot airborne toxins
as they pass a single point. Such sensors can relay
To test the system, researchers used a 30-foot-long
instrumented balloon tethered above the Pentagon.
Deployed by CU, the setup included sensors studded
along the balloon’s tethering wire. As the
balloon rose and fell, the sensors sampled air
flow, temperature, and turbulence.
The test also involved periodic releases of sulfur
hexafluoride (SF6). This inert, invisible, nontoxic
gas helped scientists verify the accuracy of the
computer models and sensors that track dispersal
of airborne material. One component of the test
consisted of measuring the amount of gas that entered
the building under various heating and air conditioning
NCAR and CU provided the overall experimental
design and wrote a comprehensive test plan. NOAA
coordinated the gas releases with forecasting assistance
from the Dugway Proving Ground.
“It was a very challenging exercise,” Scott
says. “We called on a lot of experienced
players and advanced weather forecasting systems
in order to precisely time the releases. None of
us had ever worked in an environment with such
a high level of physical security, which presented
Scott says the development of Pentagon Shield
may help NCAR create systems to protect other areas
that could be targeted by terrorists. “Our
intent is to protect high-value targets all over
the world,” he explains.
Defense efforts aside, such systems may generate
scientific side benefits by expanding our understanding
of extremely fine-scale processes in the atmosphere.
Typical weather models have a resolution of several
kilometers, which is about 1,000 times coarser
than the finest-scale model in Pentagon Shield.
“The experience gained from the use of
atmospheric models that show weather processes
on scales of metropolitan areas, neighborhoods,
and street canyons or individual buildings will
significantly contribute to our scientific understanding
of urban weather and how to predict it,” Tom
As they implement Pentagon
Shield, Scott and his colleagues are gaining
a deeper appreciation of microclimates in urban
settings and the way winds move along buildings
and down streets. As Scott says, “In the event of a toxic release
in a dense urban setting, one side of the street
versus the other—20 feet—can make a
large difference between what parts of the city
end up getting contaminated and who gets exposed
to potentially lethal
The model used for Pentagon Shield comprises
a number of systems. They include:
- A high-resolution data assimilation
system, known as RT-FDDA, that was
developed by RAP’s Yubao Liu.
It runs with the Penn State/NCAR MM5
Mesoscale Model and provides regional
weather forecasts. It also has an interior
grid centered over the Washington metropolitan
area that runs at very high resolution.
- The Variational Doppler Radar Assimiliation
System, or VDRAS. Developed by Jenny
Sun and Andrew Crook (both MMM/RAP),
it provides detailed and frequently
updated information on wind, rain,
and other real-time weather developments.
- A Doppler lidar variant of VDRAS
called the Variational Lidar Assimilation
System, or VLAS. Also developed by
Jenny and Andrew, this provides similar
information as VDRAS, but at very high
resolution, suitable for estimating
winds at the neighborhood scale.
- A building-scale,
computational fluid dynamics model
to track the winds and movement of
particles and chemicals around the
Pentagon. Although this system is
being developed by another organization,
NCAR is testing its own version,
developed by MMM’s Piotr
Smolarkiewicz. It’s called EuLag.
about RAP's homeland security projects
about ATD's REAL lidar
Also in this issue...
the NCAR Science Store
Wilmot “Bill” Hess