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Sky-Bound Lidar Offers Tool for Weather Projections

Photonics Spectra
May 1999
Margaret A. Whitney

Weather forecasting is getting a helping hand as scientists develop systems that take lidar to the sky. Projects such as the Multicenter Airborne Coherent Atmospheric Wind Sensor and the Space Readiness Coherent Lidar Experiment will use lidar for more effective wind and cloud measurements. Improved forecasting could help to prevent weather-related loss of life and property.

The Multicenter Airborne Coherent Atmospheric Wind Sensor was developed jointly by scientists at NASA's Global Hydrology and Climate Center and Marshall Space Flight Center in Huntsville, Ala.; the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory in Boulder; and the Jet Propulsion Laboratory in Pasadena, Calif. It uses airborne pulsed, scanning, coherent Doppler lidar to remotely sense wind velocity and aerosol backscatter in the troposphere and lower stratosphere.

The system incorporates a frequency-stable, pulsed, 9- to 11-µm transversely excited atmospheric CO2 laser transmitter that produces 0.6 to 1.0 J per pulse at a repetition rate of 1 to 30 Hz. During flight on a NASA DC-8 research aircraft, the system generates and transmits laser pulses at the atmosphere through a scanner mounted within the left side of the aircraft ahead of the wing.

The system's laser beam diameter for a single pulse is about 1 m at a 10-km slant range. Such fine scanning permits unambiguous velocity measurements near clouds and surface features without velocity bias that comes from ground clutter. It can measure the two-dimensional wind field over a broad area in real time, can scan three dimensions and does detailed vertical profiling.

The Space Readiness Coherent Lidar Experiment -- a collaboration of several groups led by Marshall, and scheduled for launch on the space shuttle in 2001 -- is expected to offer direct global wind measurements from space, which should have a considerable impact on numerical weather prediction models. This pulsed coherent Doppler lidar produces 100 mJ per pulse at a repetition frequency of 6 Hz, with a beam size of 0.25 m.

Other applications will include global profiling of cloud frequency; global change research; numerical weather prediction; wind profiling of solar system bodies; and pollution studies.

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