Simultaneous measurements of the liquid water content and particle size have assumed an important role in cloud physics as they help elucidate the mechanism of cloud particle formation and the mechanism of air mass-mixing in stratus clouds. Such measurements can reveal the modification of cloud air masses by anthropogenic aerosol particles (Coakley et al. 1987, Durkee 1989). Studies of the climatic impact of these modification processes on cloud microphysics seems to be urgent for understanding mechanisms of climate change. General Circulation Model (GCM) simulations can be improved by introducing a parameterization of cloud optical properties in terms of integrated liquid water content (liquid water path) and particle size (Slingo 1989). Motivated by the above mentioned circumstances, remote sensing techniques were developed for simultaneously retrieving the cloud optical thickness and effective particle radius, from which the liquid water path can be inferred. Statistical features of the cloud optical thickness (or liquid water path) and effective particle size for marine stratocumulus clouds are presented. These results were obtained during 4 days (7, 10, 13, and 16 July 1987) of observations with the Multispectral Cloud Radiometer (ER-2) and the Thematic Mapper (LANDSAT-5) during the First ISCCP Regional Experiment (FIRE)

King, Michael D.

Nakajima, Teruyuki

English

NASA Technical Reports Server

N90-28239
Cloud Optical Parameters as Derived from the Multispectral
Cloud Radiometer
TERUYUKI NAKAJIMAt AND MICHAEL D. KING
Laboratory for Atmospheres, Goddard Space Flight Center, NASA,
Greenbelt, MD 20771, USA
1. Introduction
Simultaneous measurements of the liquid water content and particle size
have assumed an important role in cloud physics as they help elucidate the
mechanism of cloud particle formation and the mechanism of air mass-mix-
ing in stratus clouds. Such measurements can reveal the modification of
cloud air masses by anthropogenic aerosol particles (Coakley et al. 1987,
Durkee 1989). Studies of the climatic impact of these modification processes
on cloud microphysics seems to be urgent for understanding mechanisms of
climate change. GCM simulations can be improved by introducing a parame-
terization of cloud optical properties in terms of integrated liquid water con-
tent (liquid water path) and particle size (Slingo 1989).
Motivated by the above mentioned circumstances, we have been develop-
ing remote sensing techniques for simultaneously retrieving the cloud optical
thickness and effective particle radius, from which the liquid water path can
be inferred. Nakajima and King (1989a) have shown a good agreement be-
tween the effective radius derived from in situ cloud microphysics observa-
tions and that derived from reflected solar radiation measurements, with a
slight overestimation occurring in the remote sensing method. Also Durkee
(1989) found a good correlation between the in situ value of the effective par-
ticle radius and the cloud reflectance at 3.7 _tm.
In this paper we will present statistical features of the cloud optical thick-
ness (or liquid water path) and effective particle size for marine stratocumu-
lus clouds. These results have been obtained during four days (7, 10, 13 and 16
July 1987) of observations with the Multispectral Cloud Radiometer (ER-2)
and Thematic Mapper (Landsat-5) during the First ISCCP Regional Experi-
ment (FIRE).
2. Results
The optical thickness ('¢c) and effective particle radius (re) for 7, 10, 13 and
16 July 1987 have been retrieved by analyzing reflected solar radiation mea-
t Permanent affiliation: Upper Atmosphere and Space Research Laboratory, Faculty of
Science, Tohoku University, Sendal 980, Japan.
85
https://ntrs.nasa.gov/search.jsp?R=19900018923 2020-03-19T21:11:29+00:00Z
surements at the 0.754, 1.65 and 2.16 pm channels of the MCR onboard the
NASA ER-2 aircraft. The algorithm used to derive these parameters has been
described by Nakajima and King (1989b). We have also applied the same
method to data obtained from band 2 (0.56_tm) and 7 (2.22 _tm) of the Landsat-
5 Thematic Mapper on July 7 and 16.
The areas used in the present analysis are approximately 30 km x 100 km
for the MCR data, and 150 km x 150 km for the Thematic Mapper. In situ
cloud liquid water content and effective radius were obtained from mea-
surements obtained with the Johnson-Williams (JW) liquid water content
meter and three different PMS cloud probes (FSSP, OAP-200X and OAP-200Y)
on board the University of Washington C-131A aircraft.
Figure 1 shows comparisons between the remote sensing-derived values
of the effective radius and the in situ values, where we have partitioned the
results into three different cloud optical thickness ranges, i. e., 5 < xc < 10, 10 <
% < 15 and 15 < % < 20. The remote sensing values have been adjusted to the
cloud center where the C-131A was flying (see Nakajima and King 1989b for
details), and the data with z < 5 have been omitted because of the uncertainty
in the retrieval for optically thin clouds. From the results presented in this
figure we see that for thin clouds (5 < zc < 10) the remote sensing values are in
excellent agreement with the in situ values. As the cloud optical thickness
increases, however, the remote sensing values become progressively larger
than the in situ values for re < 10 Bm, and vice versa for re > 10 Bm. The
former phenomenon is likely related to the so-called cloud absorption
anomaly in the near infrared region (NIR), i. e., the cloud looks darker than
the theoretical expectation from in situ microphysical parameters (e.g.,
Stephens and Platt, 1987). On the other hand, the latter phenomenon is
caused by the drizzle mode particles which can be measured by the PMS-OAP
cloud and precipitation probes. The remote sensing values have only a weak
sensitivity to these drizzle mode particles which tend to exist relatively low in
the clouds.
Figure 2 shows comparisons of the liquid water path (gm -2) estimated
from the remote sensing quantities as
Wremote = 2 rcenter _c / 3, (1)
and by the in situ measurements as
Wtn situ = torn Az , (2)
where rcenter is the effective radius derived from remote sensing after adjust-
ment for vertical inhomogeneity (Nakajima and King 1989b), xe the remote
sensing-derived cloud optical thickness, Wm the measured liquid water con-
tent from the JW liquid water content meter, and Az the cloud geometrical
86
0 5 10 15 0
.... i .... i .... , ....
o 5¢tau < t0
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uJ °o_,*.o• *
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5
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. . °° .
,<
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w
0 ......... " • • J
0 5, 10 15 0
EFFECTIVE RADIUS-PMS (l_m)
Fig. 1. Scatter diagrams comparing the in situ effective radius with the remote sensing
effective radius. Three different optical thickness range cases (i. e., 5 _<'_c < 10, 10 _< _c < 15 and
15 _<_c < 20) are shown in three separate panels.
thickness• The geometric thickness of the cloud has been estimated as an
equivalent thickness which gives the liquid water path obtained by integrat-
ing vertical profiles of the liquid water content.
Although there is large uncertainty in the estimation of Az, we observe a
tendency to underestimate the liquid water path retrieved from our remote
sensing method by 20% to 40%. From Eq. (1), together with Figs. 1 and 2, we
conclude that the remote sensing optical thickness is somewhat smaller than
87
200
JUL 7
JULIO
JUL13
° o •
0
0 50 100 150 200
LIQUID WATER PATH (IN SITU)
Fig. 2. Comparison of the liquid water paths derived from in situ measurements and
estimated from remote sensing methods.
the expected value from in situ measurements by -50% for thick clouds with
= 20 and re " 8 Izm, roughly corresponding to the case for 10 July 1987. For
such thick clouds the channel 1 (_. = 0.754 I.tm) intensity is mostly sensitive to
the optical thickness, with little sensitivity to particle radius, and thus the in-
tensity at k = 0.754 _tm is significantly smaller than the expected value from in
situ measurements.
Although there is some discrepancy between the in situ and remote
sensing values of W and re, it is worthwhile to examine the correlation
between these two quantites, especially given the recent interest in
parameterizing the shortwave radiative properties of clouds in terms of these
two parameters (Slingo 1989). Figure 3 shows two dimensional histograms of
W and re for the four days of our observations. The five contour lines for
each day show the 10, 30, 50, 70 and 90% occurrence levels. Other than 10 July,
the contour lines are confined in a rather small area having the different
peaks depending on days. The composite pattern of all contours with
different days shows a weak positive correlation between W and re. The
rough tendency may be expressed as
re = 0.333 W 1/4. (3)
A similar comparison for in situ quantities is shown in Fig. 4. Since we have
relatively large particles on 7 July and very large drizzle particles on 10 July,
88
E
:K
v
O3
a
rv
iii
>
t-
O
itl
ii
ii
iii
2O
15
10
5
REMOTE SENSING
JULY 1 3
JULY 10
)
/
t JULY 1
0
O 100 200
LIQUID WATER PATH (g/m 2)
Fig. 3. Joint probability density function for the liquid water path and the effective radius
derived from reflected solar radiation measurements. Five contour lines for each day shows 10,
30, 50, 70 and 90 % occurrence levels. The solid continuous line fits the rough tendency of the
contours.
the overall pattern is more complicated than that presented in Fig. 3. If we
disregard the drizzle particles, the effective radius is relatively insensitive to
the liquid water path. The difference in the sensitive altitude within clouds
between in situ and remote sensing is at least partly responsible for some of
the difference in the tendency shown in Figs. 3 and 4.
3. Concluding remarks
As a result of our investigation, we have observed some interesting rela-
tionships between the liquid water path and the effective particle radius. We
observed a systematic bias in the effective radius derived by our remote sens-
ing method, with the tendency to overestimate the effective radius increas-
ing as the cloud optical thickness increases. Since this can be a good guideline
for solving the NIR cloud absorption anomaly problem, we need to compile
more data in order to determine whether this is a general tendency for ma-
rine stratocumulus clouds. Although we also found some tendency of un-
derestimating the cloud optical thickness from reflection function measure-
ments, we need to be careful about drawing any conclusions because of the
large uncertainty in estimating the liquid water path using Eq. (2).
89
20
15
rr 10
0
0
,u I
IN SITUJULY 7
• JULY 10 ". ." .
• JULY 13 ,_." •• JULY 16 • • 8""_.
• _%.o •
• • • • • ee•oo •o_ • •
o4 e,e •
....t:..s -....
.,_ ". ; :" ..."
t
i l a a 1 L i _ i
100 200
LIQUID WATER PATH (g/m 2)
Fig. 4. Comparison of the liquid water path and the effective radius derived from in situ
measurements.
As for the dependency of the effective radius on liquid water path, we
conclude that the effective radius is relatively independent of the liquid water
path with some weak positive correlation for clouds lacking significant driz-
zle development. For reflected solar radiation the existence of drizzle particle
is not important, whereas the transmitted solar radiation is expected to have
some dependence on drizzle mode particles. Consequently we need to use a
vertically inhomogeneous cloud model with a two mode size distribution to
produce a consistent cloud radiative model valid for both reflected and
transmitted radiation.
Since the above results depend strongly on the calibration of the radiome-
ters and in situ instruments, more ground (aircraft) truth will be required to
further extend the results shown here.
Acknowledgements: Drs. L. F. Radke of the University of Washington
and J. D. Spinhirne of Goddard Space Flight Center are gratefully acknowl-
edged for valuable discussion and for providing their data.
9O
REFERENCES
Coakley, J. A., Jr., R. L. Bernstein and P. A. Durkee, 1987: Effect of ship-stack
effluents on cloud reflectivity. Science, 237, 1020-1022.
Durkee, P. A., 1989: Observations of aerosol-cloud interactions in satellite-
detected visible and near-infrared radiance. Proceeding of Symposium on
the role of clouds in atmospheric chemistry and global climate. Anaheim,
Calif., 157-160.
Nakajima, T., and M. D. King, 1989a: Cloud optical parameters as derived
from the multispectral cloud radiometer. IRS '88: Current Problems in
Atmospheric Radiation, J. Lenoble and J. F. Geleyn, Eds., A. Deepak
Publishing, 18-21.
__, and _____, 1989b: Simultaneous determination of the optical thick-
ness and effective particle radius of clouds from reflected solar radiation
measurements. Part I: Theory.. Submitted to J. Atmos. Sci.
Slingo, A., 1989: A GCM parameterization for the shortwave radiative
properties of water clouds. J. Atmos. Sci., 46, 1419-1427.
Stephens, G. L., and C. M. R. Platt, 1987: Aircraft observations of the radiative
and microphysical properties of stratocumulus and cumulus cloud fields.
J. Clim. Appl. Meteor., 26, 1243-1269.
91

MARINE STRATOCUMULUS
SESSION M03: Radiative Properties
CHAIRMAN: Howard P. Hanson
Monday, July 10, 1989
PAGE
M03.01 Satellite-Derived Cloud and Radiation Fields Over the Marine Stratocumulus IFO
Young, David F., Patrick Minnis, and Edwin F. Harrison
95
M03.02
M03.03
Analysis of ER-2 Radiometer and Lidar Observations from the 1987 Marine
Stratus Experiment
Spinhirne, J. D., R. Boers, T. Nakajima, and W. D. Hart
The Radiation Budget of Stratocumulus Clouds Measured by Tethered Balloon
Instrumentation: Variability of Flux Measurements
Duda, David P., Graeme L. Stephens, and Stephen K. Cox
101
103
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93



Cloud optical parameters as derived from the multispectral cloud radiometer

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