Better Weather: Water-vapor Profiling System Could Result in More Accurate, Less Costly Forecasting

A vehicle from the National Severe Storms Laboratory observes conditions around a tornado in Wyoming during June of 2009. Image credit: Mike Coniglio, NOAA/NSSL

Researchers at the Georgia Institute of Technology and Bennett Aerospace are leveraging light detection and ranging (lidar) technology to develop a ground-based system that will automatically measure water vapor in the atmosphere. Known as”WaV-ProTM,” for Water Vapor Profiler, the system aims to provide more frequent data collection that could dramatically improve weather forecasting.

Initial work on the project began in mid-2009, when Bennett Aerospace, in collaboration with the Georgia Tech Research Institute (GTRI), won a Phase I Small Business Innovation Research grant from the National Oceanic and Atmospheric Administration (NOAA) to develop design requirements for the system. Recently, the researchers won a Phase II grant to build and test critical components needed for a final operating prototype of WaV-Pro.

Lidar, also known as laser radar, is similar to radar, but transmits pulses of laserlight (instead of radio waves) and then measures properties of the scattered light to detect particles and measure atmospheric conditions. The Wav-Prosystem will enhance data collected by balloon-borne radiosondes that the National Weather Service (NWS) currently uses to measure air pressure, temperature, wind and relative humidity (water vapor).

Each day the NWS launches two weather balloons at about 100 sites around the country. Although the weather balloons provide valuable information on water vapor, a lot can happen in the 12 hours between launches. A small increase in water vapor can mean the difference between no precipitation and a major thunderstorm, researchers explain.

A spectacular thunderstorm and super cell clouds are observed in Enid, Oklahoma in May 2009. The new water vapor profiling system will help predict such dangerous weather.Image credit:Sean Waugh, NOAA/NSSL

“Our system would affordably profile water vapor on a continuous process, 24/7/365, which would translate into an orders-of-magnitude increase in data the NWS could feed into their computer models,” said Douglas Bennett, CEO of Bennett Aerospace, a research and development company based in Cary, N.C.

Such data could allow the NWS to predict thunderstorms earlier and pinpoint weather activity down to a city-block level.

“The commercial aviation industry loses $28 billion each year due to weather-relateddelays,” said Bennett. “With more accurate thunderstorm predictions, it would be possible to move into a realm of adaptive flight scheduling to prevent planes from sitting at airports, which would cut down on aviation costs and benefit passengers.”

In addition to improving weather predictions, WaV-Pro also has long-term applications for climate modeling, Bennett added. Water vapor absorbs terrestrial radiation more effectively than carbon dioxide, and more sophisticated data about water vapor concentrations would help scientists better understand climate change and global warming.

An important aspect of the WaV-Pro system is making it eye-safe to avoid harming humans who might inadvertently look into the beams.

“There are existing lidar systems that can profile water vapor, but they use ultraviolet light, which can cause severe eye damage if encountered by humans,”explained Gary Gimmestad, a principal researcher at GTRI and one of the project’s principal investigators.

Outdoor use of lasers is regulated by the FAA due to concern for airplane and helicopter pilots. “In addition, you never know who may come in contact with an unattended system,” Gimmestad added.

MakingWaV-Pro eye safe means leaving the ultraviolet range of the electromagnetic spectrum and using infrared wavelengths. “The human eye is sensitive to wavelengths from 400 nanometers (violet) to 700 nanometers (red),” said David Roberts, a senior researcher at GTRI who is also a principal investigator on the project.

But working with infrared wavelengths poses challenges for the researchers, particularly when it comes to detecting the laser light returning from the atmosphere.

Lidar systems transmit intense pulses of light into the atmosphere. When these photons encounter gas molecules or aerosol (dust) particles, they head in adifferent direction, which is known as scattering. A small fraction of these photons scatter back toward the lidar, where they are collected by a receiver telescope. A photodetector converts these photons into electronic signals,which are turned into digital data that a computer can process.

Two problems exist with infrared wavelengths, Roberts explained: “They are scattered much less to begin with, and photodetectors in the infrared region are very noisy compared to photodetectors that operate in the visible spectral region or the ultraviolet region.”

The signals from high altitudes are so weak that individual photons must be counted as they return from the atmosphere. Thus, the researchers are using a special type of detector known as a photon-counting infrared photomultiplier, and they are conducting rigorous testing to make sure the component can satisfy the performance requirements specified by computer simulations.

Phase II is expected to last about 18 months, which will enable the researchers to test the photon-counting system in different seasons and weather conditions.

“One of the most important things you can do with any lidar system is to put the photodetector through its paces,” Roberts said. “You can have a wonderful laser, great optics and sophisticated computers and software, but in this application if you can’t count photons, then it’s over. It’s important to reduce risk at this phase before moving on to a final prototype.”

*Source: Georgia Tech Research Institute (GTRI)

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