Preliminary Measurements with CODI: an Automated Compact Water Vapor DIAL

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Preliminary Measurements with CODIan Automated
Compact Water Vapor DIAL
Janet Machol, NOAA ETL / CIRES
Need for a New Lidar
Measurements
Continuous water vapor profiles would help
improve weather forecasts and climate studies.
Currently, routine water vapor profiles are taken
with twice-daily radiosondes. Lidars can
provide continuous profiles, but to date, most
water vapor lidars have been large, expensive and
complex instruments. We built an eye-safe
automated DIfferential Absorption Lidar (DIAL) to
profile water vapor in the lower troposphere.
CODI (COmpact DIAL) was designed to
be a prototype of a low-cost lidar which can
be duplicated for multiple deployments. The
lidars could be placed adjacent to wind
profilers to measure moisture transport.
Water vapor lidar deployed at CART site during
IHOP.
The first field test of the system was at the SGP
ARM CART site in Oklahoma during the IHOP
experiment. Failure of the air conditioners
precluded getting good data at IHOP.
Here we present nighttime profiles
taken in Boulder compared with both
ground-based in situ sensors and with a
radiosonde.
Design
CODI is a low power infrared DIAL based on diode
lasers and photon counting detection. A DIAL
measures water vapor by detecting the returns
from laser pulses emitted at two wavelengths, on
and off of a water vapor absorption line. The
instrument is in a weather-proof housing and can
run unattended for days. The lidar automatically
switches and locks the DIAL wavelengths and does
periodic adjustments of the optics. The seed
laser is a distributed feedback (DFB) laser
amplified by a flared diode amplifier. Typical
nighttime traces use 30-minute and 180-m averages
Comparison of vertical DIAL measurement with a
radiosonde.
On- and off-line atmospheric returns.
Results
These Boulder measurements validate the lidar,
but also demonstrate the need for several
improvements on the instrument the addition of a
near-field channel to obtain measurements below
500 m, a higher-power laser to permit better
resolution and daytime measurements, and improved
climate control in extreme conditions.
Horizontal water vapor profiles.
Specifications

wavelength 823 nm
seed power 16 mW
amplifier peak power 0.5 W
output pulse energy 0.15 mW
pulse duration 600 ns
pulse repetition freq. 8 10 kHz
telescope diameter 34 cm
field-of-view 180 mRad
Acknowledgements
Special thanks to Mike Hardesty, Tom Ayers, Karl
Schwenz, Keith Koenig, Mike Krainak, Jim Abshire,
Leo Hollberg, Hector Bravo, Dave White, Christoph
Senff, and Scott Sandberg. Funding was provided
by NOAA ETL, NOAA OGP, NCAR ATD, and NASA GSFC.
Comparison of horizontal DIAL measurements at
1000-m range gate with in situ sensors. Boulder
map at right shows sensor locations.