Weather research may benefit lidar market
WASHINGTON, DC: Light detection and ranging (lidar) stations should be deployed at some 400 sites around the U.S. to continually monitor conditions in the lower atmosphere, says a National Research Council panel commissioned to improve collection of weather data.
WASHINGTON, DC: Light detection and ranging (lidar) stations should be deployed at some 400 sites around the U.S. to continually monitor conditions in the lower atmosphere, says a National Research Council panel commissioned to improve collection of weather data. The lidar stations would supply data to a national “network of networks” that would collect and distribute data on atmospheric conditions needed to forecast weather and air quality, and could aid in monitoring climate. Together with the Global Atmosphere Watch project of the World Meteorological Organization, this could pump up the market for lidar systems.
U.S. agencies want to improve weather observation and forecasting on the ‘mesoscale,’ spanning distances of kilometers to hundreds of kilometers. That scale is critical because it includes important weather phenomena such as thunderstorms, squall lines, fronts, and precipitation bands. The panel says the country’s current network of weather stations does not measure key atmospheric data uniformly, and is not spaced closely enough or uniformly enough for mesoscale observations.
Current stations do not adequately measure the vertical variation of atmospheric properties such as humidity and wind velocity, writes the panel headed by Richard Carbone of the National Center for Atmospheric Research (Boulder, CO) in a report titled “Observing Weather and Climate from the Ground Up, A Nationwide Network of Networks.” Better data for air above 10 meters from the surface is needed for computer forecasting systems to improve their mesoscale weather predictions.
The panel says the top priority is building new stations that observe the lower troposphere with lidar as well as radio-frequency instruments. They propose a grid of some 400 observing sites separated by an average of 125 km.
Why lidar?
Balloon-launched instruments called radiosondes now collect the best data, but their expense limits launches to twice a day from sites several hundred kilometers apart. Ground-based radars can measure wind profiles, and humidity can be calculated from data generated by the global positioning system. But only lidar can measure accurately the height of a critical feature called the planetary boundary layer, says Ray Hoff, a panel member from the University of Maryland (Baltimore, MD).
The boundary layer is the part of the atmosphere that interacts directly with the surface. Air warmed by contact with the sunlit surface rises to the top of the boundary layer in eddies that then lose their energy and drop back toward the surface. Typically the boundary layer is 1 to 3 km thick in summer, and the top may be marked by patchy clouds where moisture condenses. The thickness of this layer is a critical quantity for weather models because it constrains atmospheric mixing.
Radar can detect the boundary layer, but its pulses are too long for precise measurement. Lidars, with pulses stretching only about a meter, can give the precision needed for weather forecasting, Hoff says. Lidars also can profile key air pollutants and their distribution in the air, important for predictions of air quality, but such lidars are more sophisticated and thus more expensive.
The panel deliberately avoided recommending a specific lidar designs for the network. Standard lidars measure backscattered light, but don’t identify specific molecules. Raman lidars monitor look for Raman scattering from molecules such as nitrogen and water vapor. Hoff said the ability to profile water vapor makes Raman lidars a priority because water vapor is critical in cloud formation and also affects the rate of temperature change with altitude.
Meanwhile, the World Meteorological Organization’s Global Atmosphere Watch (GAW) program is developing its own plans for a global lidar network, called GALION for the GAW Atmospheric LIdar Observation Network. Its main thrust is to study aerosols and their impact on climate, which requires regular measurements, initially twice a week. The specialized lidars needed for aerosol measurements are so expensive that GALION planners will rely on existing lidar stations and networks rather than trying to build their own network of uniform instruments. But with large gaps in coverage, particularly in the southern hemisphere, some new equipment is likely to be needed eventually.
The GALION plan is available on the web at http://www.wmo.int/pages/prog/arep/gaw/documents/gaw178-galion-27-Oct.pdf.
—Jeff Hecht
