We describe preliminary results from a comprehensive computer model developed to guide optimization of a Raman lidar system for measuring daytime profiles of atmospheric water vapor, emphasizing an ultraviolet, solar-blind approach

Bisson, S. E.

Ferrare, R. A.

Goldsmith, J. E. M.

Lapp, M.

Melfi, S. H.

Whiteman, D. N.

English

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Performance: Modeling of Ultraviolet Raman Lidar Systems for
Daytime Profiling of Atmospheric Water Vapor
Richard A. Ferrare, David N. Whiteman, and S. H. Melfi
NASA Goddard Space Flight Center
Greenbelt, MD 20771
(301) 286-9089
and
J. E. M. Goldsmith, Scott E. Bisson, and M. Lapp
Sandia National Laboratories
Livermore, CA 94551-0969
(510) 294-2432
Abstract
We describe preliminary results from a comprehensive computer model developed to
guide optimization of a Raman lidar system for measuring daytime profiles of
atmospheric water vapor, emphasizing an ultraviolet, solar-blind approach.
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Performance Modeling of Ultraviolet Raman Lidar Sy::rems for
Daytime Profiling of Atmospheric Water Vapor
Richard A. Ferrare, * David N. Whimman, and S. H. Melfi
NASA Goddard Space Flight Center
Greenbelt, MD 20771
(301) 286-9089
and
J. E. M. Goldsmith, Scott E. Bisson, and M. Lapp
Sandia National Laboratories
Livermore, CA 94551-0969
(510) 294-2432
`Universities Space Research Association Research Associate
Summan;
We describe preliminary results from a comprehensive computer model under
development t... guide optimization of 7 , ultraviolet Raman lidar system for measuring
daytime profiles of atmospheric water vapor. The Raman lidar technique is a leading
candidate for providing the high-resolution, day/night profiling of water vapor that is
critical to research in global climate change. While Raman lidar is used currently to
perform meteorologically important, sustained, reliable nighttime p ► :,filing of water
vapor, daytime measurements present added challenges because of the difficulties
inherent in detecting Raman signals against solar backgrounds.
Raman lidar systems detect selected molecular species by monitoring their
corresponding wavelength-shifted backscattered Raman ren!rn signals. Solar-blind
0
Richard A. Ferrare et al., "Performance Modeling of Ultraviolet Raman Lidar .. "
operation is based on choosing an excitation wavelength such that the return :signal is
in the solar-blind region of the spectrum (x, -300 nm). Background daytime radiance
drops off exwmely rapidly below 300 nm (see Fig 1), because atmospheric ozone (as
well as other gases for the shorter wavelengths) absorb practically all of the incoming
solar radiation in this region of the spectrum, providing a "black backgrourd" for
detection of the Raman signal. However, tropospheric ozone (see Fig. 1 for
absorption cross-section) also absorbs the transmitted laser beam and the
wavelength-shifted, backscattered Raman signal, reducing the range to which signals
can be detected even in the absence of any background. Thus, the optimum excitation
wavelength must be short enough to result in only a small level of background
radiation, but long enough to result in sufficient atmospheric penetration.
We are developing a detai,ed Raman lidar instrument performance model, more
comprehensive than those reported previously, 1,2
 that will provide this optimum
excitation wavelength as a function of a variety of operational parameters. The model
simulates key characteristics of the lidar system, using realistic atmospheric profiles,
estimated background sky radiance, and experimentally determined values for the lidar
system parameters; model results demonstrate the tradeoffs among range,
measurement precision, and data acquisition time during daytime operation. Figure 2
displays the calculated range obtained versus wavelength in relatively clear air for P.
Raman lidar system, based on a high-average-power excimer laser, suc y . as the
instmment developed at the NASA Goddard Space Vlight Center and fielded by the
NASA/Sandia authors at the SPECTRE (Spectral Radiance Experiment) campaign in
Coffeyville, Kansas during late 1991. Measurements performed at SPECTRE using
248-nm excitation will be compared to our model calculations to guide further
development of the model, which will in turn be used to guide our development of
wavelength-shifting capabilities for providing the optimum wavelength.
2
Richard A. 5—rare et al., "Performance Modeling of Ultraviolet Raman Lidar .."
The preliminary results shown in Fig. 2 indicate that the optimum laser
excitation wavelength is in the range of 255-265 nrn. One factor that requires
significant further investigation is the influence of attenuation by the weak
("forbidden") Herzberg I molecular oxygen uv absorption bands, which could influence
signals over the several-kilometer atmospheric Faths probe: by the Daman lidar
system.. In addition, the calculation shown in Fig. 2 represents only a signal-to-noise
calculation, and does not take into account systematic effects, such as those
introduced by the strong wavelength dependence of the ozone absorption cross
section. These and other potential effects on optimum wavelength selection for solar-
blind operation are being addressed currently in model, laboratory, and field-based
lidar studies. We are also investigating the merits of a different approach for daytime
operation based on a narrow spectral bandpass, narrow field-of-view system.
This work is supported by the U. S. Department of Energy, Atmospheric
Radiation Measurement (ARM) Program, and by the NASA Radiation and Dynamics
Program.
3
Richard A. Ferrare et al., "Performance Modeling of Ultraviolet Ranson Lidar
References
1. K. Petri, A. Salik, and J. Cooney, "Variable-Wavelength Solar-Blind Raman Lidar
for Remote Measurement of A'mospheriG Water-Vapor Concentration and
Temperature," Appl. Opt. 21, 1212-1218 (1982).
2. D. Renaut and R. Capitini, "Boundary-Layer Water Vapor Probing with a Solar-
Blind Raman Lidar: Validations, Meteorological Observations and Prospects,"
J. Atmos. Oceanic Technol. 5, 585-601 k1988).
4
Rh-hard A. Ferrare et al., "Perfoi niance Modeling of Ultraviolet 	 i.war ..."
iFi 11 ure Ca tp inns
1. Ozone absorption cross-section and daytime sky radiance in the spectra', region
240-320 nm.
2. Observation range as a function of laser wavelength computed from the model.
This maximum range eorr^;sponds to a signal-to-noise ratio of 10 (based on
Poisson statistics) in the Raman water vapor channel. The laser output power
and lidar system parameters are assumed constant over t ;ie indicated spectral
region.
5
Daytime Sky Radiance (W/M2/sr/nm)
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Performance modeling of ultraviolet Raman lidar systems for daytime profiling of atmospheric water vapor

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