Hurricane study could save lives and property


To improve understanding and prediction of hurricanes such as the storm now threatening the eastern seaboard, two specially equipped NASA aircraft are collecting high-altitude information about Atlantic hurricanes and tropical storms. One of those airplane's wingpods carries MIT's contribution to the mission: a sophisticated microwave radiometer built in the Research Laboratory of Electronics.

The Convection and Moisture Experiment (CAMEX) mission is taking place in August and September, which are prime Atlantic hurricane months. Its results may increase warning time -- saving lives and property -- and decrease the size of evacuation areas while giving scientists a better understanding of these dramatic weather phenomena. CAMEX will yield high-resolution information on hurricane structure, dynamics and motion.

Led by the Atmospheric Dynamics and Remote Sensing program at NASA headquarters in Washington, DC, the experiment unites eight NASA centers, other government weather researchers and the university community.

"We only know what goes on in the bottom half of a hurricane -- from sea level to 27,000 feet," said atmospheric expert Robbie Hood of the Global Hydrology and Climate Center at NASA's Marshall Space Flight Center in Huntsville, AL. "With all of the agencies and the university community working together, we now can learn about these storms from top to bottom -- and hopefully improve hurricane prediction."

When a hurricane or tropical storm forms in the Atlantic, a NASA Dryden Flight Research Center DC-8 -- equipped with instruments to measure the storm's structure, environment and changes in intensity and tracking -- will fly into the storm at 35,000-40,000 feet.

At the same time, a specially equipped Dryden ER-2 -- a high-altitude research plane -- will soar above the storm at 65,000 feet. The modified U-2 will measure the storm's structure and the surrounding atmosphere that steers the storm's movement.

On the ground, the storm research team will launch weather balloons and monitor land-based sensors to validate the high-altitude measurements taken by instruments aboard the planes.

Ms. Hood and her team plan to fly the NASA planes in conjunction with scheduled storm flights of the National Oceanic and Atmospheric Administration (NOAA) that will take off from MacDill Air Force Base in Tampa, FL, and the "Hurricane Hunters" -- the Air Force's 53rd Weather Reconnaissance Squadron from Keesler Air Force Base in Mississippi.

The Hurricane Hunters and NOAA routinely fly into tropical storms and hurricanes to determine their location, motion, strength and size. Information gathered by the two organizations is used to predict the potential strength and size of the storms as well as landfall.

In addition to providing Doppler radars on each research plane, NASA for the first time will bring state-of-the-art airborne instruments to measure moisture and wind fields around the hurricanes under observation.

A BETTER RADIOMETER

The Microwave Temperature Sounder (MTS) component of the Aircraft Sounder Testbed (NAST) is the latest and most capable in a series of microwave radiometers built in MIT's Research Laboratory of Electronics (RLE).

This device will allow scientists to observe more closely the internal structure of intense storms such as hurricanes, to measure the microwave-transmission characteristics of the Earth's atmosphere with unprecedented accuracy, and to develop new methods of gathering and interpreting future data.

About the size of a foot locker, the MTS is mounted in one of the aircraft wingpods with the NAST-I imaging infrared spectrometer. MTS measures the heat in the Earth's atmosphere radiated by electromagnetic waves with radio frequencies near 54 and 118 gigahertz and wavelengths near 5mm and 2.5mm, respectively.

MTS covers 16 microwave frequencies by scanning from side to side beneath the aircraft to a distance of approximately 40km. The resulting images are approximately 80km wide at the surface, running along the full trajectory of the aircraft flight path.

These images represent the thermal radiation emitted by the environment at microwave frequencies; this radiation is the microwave equivalent of the heat one feels in front of a fire.

Each microwave frequency responds differently to temperatures at different altitudes, as well as to different constituents such as water vapor and precipitation or ice. By combining this information with observations made at infrared and visible wavelengths, much can be learned about the structure of weather systems and the surface.

The data are digitized and recorded by an on-board computer, which is also capable of transmitting these data in real time to experimenters on the ground who can be in radio contact with the pilots.

Three important scientific results are sought from the CAMEX-3 observations with NAST-MTS:

1. Observations of the internal structure of intense storms. Microwave frequencies penetrate clouds much more readily than infrared or visible sensors, thus revealing much structure that is otherwise unseen. Most useful is the ability of NAST-MTS to detect hail and other ice thrust aloft in intense convective systems, and to measure more precisely their location, altitude and intensity. Precipitation beneath clouds can also often be observed.

The temperature structure can be particularly revealing in hurricanes, where the eye can be perhaps 10���������C warmer than the surrounding air. This warming is directly related to the wind speed in the hurricane. Hurricane winds can also be sensed from microwave images as they respond to the wind-roughened ocean surface. Finally, the water vapor distribution horizontally and vertically can be determined by combining this information with observations at other microwave frequencies, revealing where hurricanes and other intense storms might draw additional energy as the storm migrates and evolves.

2. Development and validation of new methods for interpreting data. By comparing observations in two microwave bands that respond the same to temperature profiles except when clouds are present, scientists can detect and characterize cloud water and ice distribution by altitude. Similar comparisons with data from the NAST-1 infrared spectrometer can characterize clouds that impact infrared and visible sensors.

Through CAMEX-3, researchers can learn a great deal about how to combine these new unprecedented images of microwave and infrared data. Most important will be new methods to detect and correct the effects of clouds on both microwave and infrared data, and similarly to compensate for the effects of surface variations.

The NAST-MTS also views space directly above the aircraft every few seconds as the aircraft ascends and descends, permitting the microwave transmission chracateristics of the atmosphere to be measured with unprecedented accuracy. These results also will be used to improve the interpretation of future satellite data.

3. Improved future systems. Future microwave satellites for meteorological purposes are now being designed. The results of CAMEX-3 will be critical in refining and validating the design and performance expectations for these systems. Of greatest interest is the choice of microwave frequencies to be used and their required accuracies.

The NAST-MTS investigator team for CAMEX-3 comprises principal investigator Dr. Philip W. Rosenkranz, principal research scientist in RLE's Remote Sensing and Estimation group; co-principal investigator David H. Staelin, professor of electrical engineeringand computer science (EECS) and assistant director of Lincoln Laboratory; and co-investigators Dr. Michael J. Schwartz, research scientist; John W. Barrett, sponsored research technical staff; and EECS graduate students William J. Blackwell, Carlos Cabrera Mercador and Fred W. Chen.

The overall NAST development effort was led by Lincoln Laboratory. The hurricane study is part of NASA's Earth Science enterprise to better understand the total Earth system and the effects of natural and human-induced changes on the global environment.

A version of this article appeared in MIT Tech Talk on September 26, 1998.


Topics: Earth and atmospheric sciences

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