Analysis and Calibration of CRF Raman Lidar Cloud Liquid Water Measurements Page: 2 of 3
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filter used to select the water vapor Raman backscatter signal is only 0.4 nm wide. The
wide bandpass required to achieve reasonable signal-to-noise in the liquid water channel
essentially eliminates the ability to measure LWC profiles during the daytime in the
presence of large solar background, and thus all LWC observations are nighttime only.
Additionally, the wide bandpass increases the probability that other undesirable signals,
such as fluorescence from aerosols, may contaminate the observation. The liquid water
Raman cross-section has a small amount of overlap with the water vapor Raman cross-
section, and thus there will be a small amount of 'cross-talk' between the two signals,
with water vapor contributing a small amount of signal to the LWC observation. And
finally, there is significant uncertainty in the actual strength of the liquid water Raman
cross-section in the literature.
The calibrated LWC profiles, together with the coincident cloud backscatter observations
also made by the RL, can be used to derive profiles of cloud droplet effective radius. By
combining these profiles of effective radius in the lower portion of the cloud with the
aerosol extinction measurements made below the cloud by the RL, the first aerosol
indirect effect can be investigated using a single instrument, thereby reducing the
uncertainty associated with aligning the different sampling periods and fields of view of
multiple instruments.
We have applied a "first principles" calibration to the LWC profiles. This approach
requires that the relative differences in optical efficiency between the water vapor and
liquid water channels be known; this relative difference is easily computed using the
efficiency values of the beam splitters and interference filters in the lidar that were
provided by the vendors of these components. The first principles approach then
transfers the calibration from the water vapor mixing ratio to the LWC using the
difference in the optical efficiency and an interpolated value of the liquid water Raman
cross section from the literature, and the better established water vapor Raman cross
section.
After accounting for all known error sources, the vertical integral of LWC was compared
against a similar value retrieved from a co-located ground-based infrared radiometer.
The RL and infrared radiometer have significantly different fields of view; thus to
compare the two sensors the data were averaged to 5 min intervals where only cloudy
samples were included in the average of each. While there is fair scatter in the data
(r=0.47), there is also a clear indication of a positive correlation between the infrared and
the RL values. The value of the slope of the regression is 0.49, which indicates a
tendency of the RL measurements to underestimate the total liquid amount with respect
to the infrared retrieval. Research continues to investigate the source of the bias, but the
most likely candidate is the large uncertainty in the liquid water Raman cross-section as
there have been no direct measurements made of this parameter at the lidar's laser
wavelength of 355 nm.
The calibrated LWC profile was then used together with the cloud backscatter coefficient
profile from the RL to derive profiles of cloud droplet effective radius and cloud droplet
number density. These profiles of cloud droplet size together with the aerosol extinction
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Turner, D.D. Whiteman, D.N. Russo, F. Analysis and Calibration of CRF Raman Lidar Cloud Liquid Water Measurements, report, October 31, 2007; United States. (https://digital.library.unt.edu/ark:/67531/metadc886082/m1/2/: accessed March 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.