The 8 to 13 micron spectral region is an important atmospheric window for radiometric studies of the Earth's surface and clouds. Most of the Earth-atmosphere longwave radiative loss to space occurs in this spectral region. Selective gaseous absorption in this window occurs in the 9.6 micron ozone band with the remaining absorption dominated by the water vapor continuum. Cirrus clouds have a large impact on the transmittance of this atmospheric window region; it is therefore important to understand the interaction of cirrus cloud with the radiation field for climate studies and in the interpretation of satellite radiometric measurements. The focus was to employ observations of the High-resolution Interferometer Sounder (HIS) made during First ISCCP Regional Experiment (FIRE) to improve the understanding of the radiative properties of cirrus clouds within this window region. Studies were undertaken to investigate the coupling between the microphysical properties of cirrus clouds and their spectral variation within this window region. Extensions of the HIS studies to satellite measurements, with regards to remote sensing and interpretation, were also investigated

Ackerman, Steven A.

Smith, William L.

English

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ANALYSIS OF CIRRUS OPTICAL PROPERTIES WITH DATA FROM THE NASA
ER2 HIGH-RESOLUTION INTERFEROMETER SOUNDER (HIS)
Final Report on NASA Research Grant NAG-I-1015
For the Period of
i April 1989 to 31 March 1990
William L. Smith, Principal Investigator
Stcven A. Ackerman, Co-Investigator
Cooperative Institute for Meteorological Satellite Studies (CIMSS)
Space Science and Engineering Center
University of Wisconsin-Madison
1225 West Dayton Street
Madison, Wisconsin 53706
June 1990
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TABLE OF CONTENTS
I. INTRODUCTION ........................................................................................................................... 1
II. PRINCIPAL FINDINGS 9
A. Variability of Cirrus Emittance ...................................................................................... 2
B. Inference of Cirrus Cloud Microphysical Properties ............................................. 4
C. Satellite Remote Sensing of Cirrus ................................................................................. 6
III. CONCLUDING REMARKS ....................................................................................................... 9
IV. REFERENCES ............................................................................................................................... I0
V. APPENDIX A ................................................................................................................................. 1 1
I. INTRODUCTION
The 8-13 #m spectral region is an important atmospheric window for
radiometric studies of the earth's surface and clouds. Most of the earth-atmosphere
longwave radiative loss to space occurs in this spectral region. Selective gaseous
absorption in this window occurs in the 9.6 #m ozone band with the remaining
absorption dominated b__ the water vapor continuum. Cirrus clouds have a large
impact on the transmittance of this atmospheric window region (Platt, 1973; Liou,
1974; Stephens, 1980; Wu, 1984); it is therefore important to understand the
interaction of cirrus cloud with the radiation field for climate studies and in the
interpretation of satellite radiometric measurements.
The focus of this research was to employ observations of the High-resolution
lnterferometer Sounder _HIS) made during First ISCCP Regional Experiment
(FIRE) to improve our understanding of the radiative properties of cirrus clouds
within this window region. Studies were undertaken to investigate the coupling
between the microphysical properties of cirrus clouds and their spectral variation
within this window region. Extensions of the HIS studies to satellite
measurements, with regards to remote sensing and interpretation, were also
investigated. This final report summarizes the major findings of this grant.
Publications are included in Appendix A.
II. RESULTS
This grant supported the investigation of three research areas: the variability
of cirrus emittance in the infrared window, the inference of cirrus cloud
microphysical properties using HIS observations, and the simulation of satellite
observations using the HIS data for studying cirrus cloud detection algorithms.
The principle findings from these research areas are briefly discussed below.
A. Variability of Cirrus Emittance in the Infrared Window
The study by Ackerman et al. (1990) [Appendix A] indicated that the cirrus
emissivity in the 8-12 _nl spectral region decreases with decreasing wavelength. An
example of this spectral dependency is depicted in Fig. 1, in which a set of four
(clear, alto-cumulus, and two cirrus) HIS spectral measurements in the 8-13 _m
"window" region are shown for 2 November 1986 near Madison, WI. The individual
gaseous absorption lines, primarily due to water-vapor, are evident in the clear and
alto-cumulus cloud case depicted in Fig. 1. The effect of the water-vapor
continuum in the clear spectra is seen as a trend of decreasing equivalent
blackbody temperature with increasing wavelength in the 10-13 _zm window. The
high thick cirrus case also depicts a decreasing trend with a blackbody temperature
difference of 5°C between 10 and 12 _m, this change is due to cloud
Figure 1. Four HIS spectra(clear, altocumulusand two cirrus) over the spectral
range8-13 /_m observed during the 28 October 1986. The dotted and dashed lines
are theoretical calculations.
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radiative properties and is consistent with the absorption coefficient of ice. This
large brightness temperature difference is indicative of the spectral variability of
cirrus optical properties in the window region.
To further investigate this spectral variability, a method of inferring cirrus
cloud spectral emissivity using HIS and lidar data was developed under this grant
and applied to data collected during FIRE (Ackermanet al., 1990). The cirrus
cloud beam emissivity derived from data collected on October 28, 1986 were
primarily limited to between 0.4-0.9 with less than 30% of the cases studied having
beam emissivities less than 0.4. Fifty percent of the clouds on this day displayed a
difference in the beam emissivity between 10 and 12 _m of greater than +_0.2 The
temperature dependency of the effective beam emissivity on the integrated cloud
attenuated backscatter was similar to that observed by Platt and Dilley (1981): for
a given emissivity, colder clouds had a smaller attenuated backscatter.
B. Inference of Cirrus Cloud Microphysicai Properties
This project demonstrated a technique of detecting cirrus and inferring their
optical properties using an 8 om channel in conjunction with 11 and 12 _zm
channels. The method i:; based on the brightness temperature (BT) differences
between these channels. The technique is demonstrated in Fig. 2 as a scatter
diagram of the BT8-BTll versus BT I I-BTI2 observations made with the HIS
aboard the NASA ER2 during FIRE (November 2. 1986). Each symbol in the
figure represents a rang_._ in the BTII as noted in the legend. The difference in
the brightness temperatttres observed in these channels are very useful in detecting
the presence of cirrus clouds. The cloud-free regions, determined from lidar
observations (Spinhirne and Hart, 1990), have negative differences of BTs-BTI 1
Figure 2. HIS observed brightness temperature differences between 8 and 11 um
versus 11 and 12 #m.
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due to absorption by water vapor, while cirrus clouds have positive differences
owing to the optical properties of ice. The cirrus BTs-BT 11 are greater than the
BT II-BTI2 as expected from the theoretical models. For liquid water clouds we
expect the BTs-BTII to be less than the BTII-BTI2, and would therefore lie to the
right of the dashed line in Fig. 2.
An envelope of results from model calculations, assuming a distribution of
spherical ice particles, is depicted in Fig. 2 by the solid line. From theoretical
calculations it was found that the large BT differences observed by the HIS are
associated with cirrus particle size distributions which have a small effective
radius. The magnitude of the BT differences is also related to the ice water path
(e.g., very thin clouds are similar to clear sky values). A method of determining
the cloud effective radii from the HIS measurements was demonstrated by
Ackerman et al. (1990) (Appendix A).
Differences between theoretical calculations and observations are seen in Fig.
2 at the larger BT8-BTII differences. This difference may be attributed to
particle shape. The efft',ct of particle shape is demonstrated in the figure by the
dashed line which is thc envelop for a cirrus cloud consisting of small ice crystals
of cylindrical shape. W.,• plan to continue investigating the relationship between
particle shape and the s0cctral observations in the 8-12 #m window.
C. Satellite Remote Sensing of Cirrus
One of the objec:ives of the FIRE was to provide verification and improve
satellite cloud retrieval tcchniques (Cox et al., 1987). The 10-12 #m band is a
standard spectral region for meteorological satellite observations and has been uscd
extensively for applications in the remote sensing of clouds and of the earth's
surface. Bi-spectral measurements in the 10 to 12 um region have been developed
for remotely sensing th(: presence of cirrus clouds and their optical propertics (e.g.,
6
Inoue, 1985;Prabhakaraet al., 1988). A drawback of a bi-spectral technique at
these wavelengths is that the brightness temperature generally decreases between 10
and 12 #m, whether viewing a cloud or a clear region.
The 8, 11 and 12 #m three channel technique was demonstrated above with
the HIS data. The HIRS/2 on board the NOAA-9 satellite has an 8.2 and an 11.1
_m channel, while the AVHRR channels 4 and 5 provide 11 and 12 _m
observations. The HIRS/2 and AVHRR channels have a much broader spectral
bandwidth than that of the HIS three channel technique demonstrated in Fig. 2.
An advantage of the HIS observations is that they can be integrated over the
bandpass and spectral response function of narrow band instruments to simulate
measurements made from satellites. This allows one to study the capabilities of the
HIRS/2 and AVHRR combined observations for cirrus cloud studies. Figure 3 is
the brightness temperature difference between these simulated HIRS/2 and
AVHRR channels from HIS observations made on November 2, 1986. The pattern
depicted in Fig. 3 is similar to that of Fig. 2 in which optimum wavelengths for
cirrus cloud detection were selected. The broader spectral channels of the HIRS/2
and AVHRR primarily result in a change in the threshold for cirrus cloud
detection. For example, clear regions in Fig. 3 have BT8-BT 11 of less than -2°C as
opposed to the 0°C threxhold of Fig. 2. Collocated observations from the HIRS/2
and AVHRR on the NOAA-9 therefore provide a potential data set to study the
radiative properties of cirrus clouds using the three channel technique developed
under this grant. We arc presently expanding the software to apply the three
channel technique to collocated HIRS/2 and AVHRR observations.
Figure 3. HIRS/2 simulated brightness temperature differences between 8.2 and
11.1 pm versus the HIS simulated AVHRR difference between 11 and 12 #m.
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IIl. CONCLUDING REMARKS
The major finding_ of the research grant can be summarized as follows:
, The HIS observations depict spectral variations in equivalent blackbody
temperature in the window region of greater than 5"C for a given cirrus cloud.
¢ A method of inferring cirrus cloud spectral emissivity using HIS and lidar
data was developed and applied to data collected during FIRE.
¢ The brightness temperature differences between 8 and 11 _m are useful
parameters for detecting the presence of cirrus clouds. For narrow spectral
observations of the HIS this difference is negative for clear regions and positive
for cirrus clouds. For the broader spectral bandpass of the HIRS/2, brightness
temperature differences greater than -2°C are indicative of cirrus.
¢ Theoretical calculations indicate that the magnitude of the spectral variation
in brightness temperature is related to the particle size. The smaller particles are
associated with the larger brightness temperature differences.
* The magnitude of the brightness temperature differences are also related to
the particle shape. Calculations assuming spherical particles are in better
agreement with the majority of HIS observations than similar calculations
assuming cylindrical particles.
# Collocated HIRS/2 and AVHRR NOAA-9 observations provide a potentially
useful data set for developing and applying a cirrus cloud retrieval method from
observations at 8,11 and 12 ore.
IV. REFERENCES
Ackerman, S. A., W. L. Smith, J. Spinhirne and H. E. Revercomb, 1990: The 27-28
October 1986 FIRE IFO Cirrus case study: spectral properties of cirrus
clouds in the 8-12 om window. Accepted for publication in the Mon. Wea.
Rev.
Cox, S. K., D. S. McDougal, D. A. Randall, and R. A. Schiffer, 1987: FIRE - the
First ISCCP Regional Experiment. Bull. Amer. Meteor. Soc., 68, 114-118.
Heymsfield, A. J., and C. M. R. Platt, 1984: A parameterization of the particle size
spectrum of ice clouds in terms of the ambient temperature and the ice
water content. J. Atmos. Sci., 41, 846-855.
Liou, K. N., 1974: On the radiative properties of cirrus in the window region and
their influence on remote sensing of the atmosphere. J. Atmos. Sci., 31, 522-
532.
Platt, C.M.R., 1973: Lidar and radiometric observations of cirrus clouds. J. Atmos.
Sci., 30, 1191-1204.
Platt, C. M. R., and A. C. Dilley, 1981: Remote sounding of high clouds. IV:
Observed temperature variations in cirrus optical properties. J. Atmos. Sci.,
38, 1069-1082.
Spinhirne, J., 1988: Lidar and Radiometer Results from the ER-2 for the FIRE Field
Experiments. Presented at the FIRE Science Team Workshop, July 11-15,
Vail, CO.
Stephens, G. L., 1980: Radiative properties of cirrus clouds in the infrared region.
J. Atmos. Sci., 37, 435-445.
Wu, M. L. C., 1984: Radiative properties and emissivity parameterization of high
level thin clouds. J. Clim. Appl. Meteor., 23. 1138-1147.
10


Analysis of cirrus optical properties with data from NASA ER2 High-resolution Interferometer Sounder (HIS)

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