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May 2004

Shielding the Pentagon

NCAR 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 release.

The project, known as “Pentagon Shield,” involves 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.

Scott Swerdlin

Scott Swerdlin

“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 at ATD.

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.

Key components

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. These include:

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 below.)

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 an alert.

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 situations.

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 considerable challenges.”

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 says.

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
airborne agents.”  •David Hosansky

The systems

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.

On the Web:

More about RAP's homeland security projects

More about ATD's REAL lidar

Also in this issue...

Streamlining the NCAR Science Store

Wilmot “Bill” Hess

Cooling us off

Short Takes

Spring Fling

Mentoring Latina students

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