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Doppler Lidar Maps High-Altitude Winds

Photonics Spectra
May 2002
Hank Hogan

According to Bob Dylan, the answer is blowing in the wind. That's certainly true for weather prediction, as winds throughout the atmosphere steer weather patterns and are an essential parameter for accurate forecasts. Unfortunately, the information on upper-level winds is spotty. Researchers at the University of Michigan and Michigan Aerospace Corp., both in Ann Arbor, have developed an instrument to map the winds aloft, a novel Doppler lidar system that employs a CCD detector instead of a photomultiplier tube and that boosts detection efficiency by two orders of magnitude.

A second-generation Doppler lidar system measures the winds above New Hampshire, using a CCD detector to measure shifts in the backscattered light.

The goal is to put the refined instrument on a satellite and to use it to measure winds from orbit to the ground, explained Todd D. Irgang of Visteon Technology Center in Redford, Mich., who helped build the system as a graduate student at the university. Polar orbiting satellites measure atmospheric temperature and humidity from the ground to altitudes of approximately 20 km, and they provide worldwide coverage twice a day.

Wind measurements, on the other hand, are limited to balloon instrumentation and, therefore, are lacking over the ocean and sparsely populated areas.

Doppler lidar systems can supply the missing information. They send a laser pulse to scatter from atmospheric molecules and suspended aerosols, and the frequency shift in the returning pulse indicates the air velocity. Measuring wind throughout an atmospheric column requires measurements of both molecular and aerosol backscatter. With standard Doppler lidar systems, this is challenging.

Two key changes to the design boost the Doppler lidar efficiency and measurement reach. The first is a CCD detector. With quantum efficiencies of up to 90 percent, the CCD wastes few of the incoming 355-nm photons.

The second is the use of some clever optical engineering, including a reflective cone that converts the circular fringes from a Fabry-Perot etalon into a line of spots that the CCD can read. The researchers also reflect the unused part of the aerosol backscatter into the molecular measurement section of the instrument, allowing more of the signal to be captured.

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