This paper describes modifications to the University of Wisconsin High Spectral Resolution Lidar (HSRL) which permit very precise depolarization measurements in addition to optical depth, backscatter cross section, and extinction cross section measurements. Because HSRL separates the lidar return into aerosol and molecular contributions, they can be measured separately

Eloranta, E. W.

Piironen, P.

English

NASA Technical Reports Server

Eloranta,E. W., P.Piironen,"Depolarizationmeasurementswith thehighspectralresolutionlidar",
SeventeenthInternationalLaserRadarConference,Sendai,Japan,1994.
DEPOLARIZATION MEASUREMENTS WITH THE
HIGH SPECTRAL RESOLUTION LIDAR
E. W. Eloranta and P. Piironen
University of Wisconsin
1225 W. Dayton Street, Madison, Wisconsin 53706, USA
Phone: 608-262-7327 Fax: 608-262-5974
Email: elorantaQlidar.ssec.wisc.edu, paivi_hsrl.ssec.wisc.edu
/ /
Many investigators have used depolarization of lidar
returns to characterize atmospheric particles 1. This pa-
per describes modifications to the University of Wiscon-
sin High Spectral Resolution Lidar which permit very
precise depolarization measurements in addition to opti-
cal depth, backscatter cross section, and extinction cross
section measurements made by the instrument _,_.
The HSRL rotates the polarization of the transmit-
ted laser pulse by 900 on every other pulse (see figure
1). This allows measurement of the parallel and cross
polarized lidar returns without the use of separate de-
tectors for the two polarizations. The parallel return
is measured on the first pulse and the perpendicular re-
turn on the second pulse. The HSRL operates at a pulse
repetition rate of 4 kHz. The parallel and perpendicu-
lar returns are summed in separate buffers. Each profile
consists of the sum of 4000 pulses of each polarization.
Since only 250 t_s separates individual laser pulses, it
is possible to compute depolarizations from successive
pulses without having the wind change the particles in
the sample volume. Because the optical path and de-
tector are the same for both polarizations, the depo-
larization ratios can be measured without the need to
calibrate the relative sensitivity of the two channels.
,,
_g.,__ruu-_J-__ _ _ II
;: l --; ',',
:E 1 ,__ .,/d_ qlk
Figure 3. The transmitter section of the HSRL.
The transmitted polarization is rotated by 90 ° between
successive pulses by a Pockels cell.
Accurate measurements depend only on the linear-
ity of the detector, the polarization purity of the trans-
mitter, and the cross polarized leakage of the receiver
polarization filter. The photon counting system of the
HSRL is very linear. Pile-up and after-pulsing correc-
tions are applied to all data; however, low after-pulsing
photomultipliers and high bandwidth counting electron-
ics make the corrections very small for all returns execpt
those from dense low altitude water clouds. Measure-
meats of the polarization of the laser output show resid-
ual cross-polarizations of ,_ 0.1% when measured with a
polarization cube which is specified to have a 0.1% cross
polarization rejection. Model calculations suggest that
the cross polarization response of the receiver is also less
than 0.1% with the major portion of the error resulting
from the ellipticity of light reflected from the primary
and secondary aluminized mirrors of the 0.5 m diam-
eter Dall-Kirkham receiving telescope. Thus, although
we have not devised a rigorous experimental test of the
total cross polarization leakage in the system, we expect
that it is .-, 0.1%.
Figure 2. Range-squared corrected aerosol and
molecular lidar returns from aerosols (0 - 4 km), ice crys-
tal virga (4-5.2 km), a water cloud (5.4-5.7 km) and a
cirrus cloud (5.7-5.8 km) are shown on the left. Partic-
ulate depolarizations are shown in the center and molec-
ular depolarizations on the right.
Because the HSRL separates the lidar return into
aerosol and molecular contributions, the aerosol and
molecular depolarizations can be measured separately.
Figure 2 shows separated aerosol and molecular profiles
observed with the HSRL on November I1, 1993 between
1:55 and 2:01 UT. Ice crystal virga (at altitudes between
4.0 and 5.2 kin) is falling out of a thin water cloud lo-
cated at altitudes between 5.4 and 5.7 km. Below 4
kin, the backscatter from aerosols is weaker than from
molecules. The depolarization of the ice is nearly con-
stant between 34% and 42%. At the base of the wa-
https://ntrs.nasa.gov/search.jsp?R=19960051011 2020-06-16T03:12:17+00:00Z
ter cloud the depolarization drops to 2% as result of
scattering from spherical droplets. Higher in the cloud
the depolarization increases, apparently as the result of
multiple scattering and the presence of ice. Because the
HSRL receiver field of view is only 160 pradians, multi-
ple scattering depolarization appears to be under 10% in
nearly all clouds we have observed. The 30% depolariza-
tion at the top of the water cloud is due to the presence
of ice at the cloud top. Figure 3 summarizes the depo-
larizations measured in clouds on 14 days between Aug.
2 and Nov 11, 1993. High depolarization ratios easily
distinguish ice clouds from water clouds which produce
much smaller depolarization.
50 .... ,-.-.,--.-,..-.,.-..,.-..,---.,....,..--,--.-,....,.-..,....,..--,-..-, ....
,o (2 _
40
35 '_/
• -- n - q _A
_" 30
,_ 25
15
10
+ o "
0 V _ 0
-60 -50 40 -30 -20 -10 0 I 0 20
Atmospheric Temperature (C)
Figure 3. Depolarization as a function of tempera-
ture for particulate scattering in clouds.
Notice that all clouds colder than the -40°C homoge-
neous nucleation temperature of water are ice. Above
0°C all returns show low depolarizations. Although
spherical water droplets are expected to yield no depo-
larization from single scattering, this plot shows depo-
larizations up to 10% in clouds which appear to be water
clouds. Since many of these clouds are observed to have
ice crystal virga falling from their bases, it is apparent
that at least part of the increased depolarization is due
to ice in mixed phase clouds. For mixed phase clouds
the scattering from water droplets dominates the scat-
tering cross section; thus, the depolarization is closer to
that of pure water clouds than of cirrus. Inside water
clouds we see a characteristic increase of depolarization
with penetration depth caused by the presence of mul-
tiple scattering. For cases studied thus far, it appears
that the 160 pradian angular field of view of the HSRL
limits multiple scattering induced depolarization to less
than 10% in water clouds. We are surprised to find no
water cloud depolarizations less than 0.8%, although it
is clear that the HSRL can detect smaller values.
The molecular depolarization measured by the HSRL
is constant at a value of 0.7% to 0.8% when viewed with
a 1 nm filter bandpass. This is larger than expected
for Cabanne line depolarization since the filter admits a
small fraction of the closest rotational Raman lines and
the I2 filter of HSRL blocks the center of the Cabanne
line 3. When the filter is removed and the laser is tuned
off the Is filter, rotational Raman scattering increases
the molecular depolarization increases to 1.5%.
With traditional lidars, aerosol depolarization mea-
surements are complicated by the presence of molecu-
lar scattering which often dominates the received sig-
nal in clear air. Since the BSRL separately mea-
sures the aerosol depolarization, accurate measure-
ments can be obtained. Figure 4 presents a his-
togram of aerosol depolarizations observed below the
clouds from the data set used to compile figure 3.
025 , - . - - , .... , ..... - - - - • ....
 °51ill0000
o.oo ma=ees|l_l|e------m_l_l-- - -
0.00 0.05 0.10 0.15 0.20 025
Figure 4. The probability of occurrence of depolar-
izations for aerosol scattering between altitudes of 500
m and cloud base. Data points consist of 5 rain averages
of data in 15 m range bins. Statistics include only points
with backscatter ratios greater than 0.2.
ACKNOWLEDGMENTS
This work was supported by grants from the Office of
Naval Research (N00014-91-J-1558) and the National
Aeronautics and Space Administration (NAG-l-882).
REFERENCES
1. Sassen, K., 1991: The Polarization Lidar Technique
for Cloud Research: A Review and Current Assess-
ment,Bulletin of Am. Meteor. Soc., 72, 1848.
2. Grund, C.J., and E.W. Eloranta, 1991: University
of Wisconsin High Spectral Resolution Lidar, Opti-
cal Engineering, 30, 6-12.
3. Piironen, P. and E. W. Eloranta, 1994: Demon-
stration of an iodine absorption filter based high
spectral resolution lidar, Optics Letters, 19,234.
4. Eloranta, E. W. and P. Piironen, 1994: Proceedings
of the 17'h ILRC, July 25-29, 1994, Sendei Japan.


Depolarization Measurements with the High Spectral Resolution Lidar

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