Tropical thin cirrus clouds play an important role in the earth's climate system through their longw

1
RETRIEVING CIRRUS OPTICAL THICKNESS FROM MODIS
REFLECTANCE DATA
Kerry G. Meyer1, Ping Yang1, Bo-Cai Gao2, and
Bryan A. Baum3 1 Texas AM University, College
Station, TX 2 Naval Research Laboratory,
Washington, D.C. 3 NASA Langley Research Center,
Hampton, VA
Tropical thin cirrus clouds play an important
role in the earth's climate system through their
longwave radiative forcing and effect on water
vapor distribution. With a unique band centered
at 1.375-µm, the Moderate Resolution Imaging
Spectroradiometer (MODIS) instruments on both the
TERRA and AQUA satellite platforms provide an
unprecedented opportunity to study thin cirrus
clouds (Platnick et al., 2003). At Texas AM
University, we have focused on the retrieval of
the optical and microphysical properties of
cirrus clouds using publicly available MODIS
reflectance data in the hopes of gaining a
greater understanding of the forcing of these
clouds in the earth-atmosphere energy budget. We
have developed a new method for retrieving the
optical thickness of tropical cirrus clouds using
level-1b radiance data (Meyer et al., 2004),
which has subsequently been extended for
application to level-2 and -3 data. We have also
developed a new method to retrieve the optical
thickness and ice crystal effective size of
midlatitude cirrus clouds using 1.38- and 1.88-?m
reflectance data (Gao et al., 2004). Future work
includes creating a tropical cirrus climatology,
as well as collocating MODIS and Airborne
Visible/Infrared Imaging Spectrometer data for
cirrus retrieval.
I. Introduction
IV. Cirrus Optical Thickness and Ice Crystal
Effective Size from 1.38- and 1.88-?m Reflectance
Data
We have also developed a method to retrieve
midlatitude cirrus optical thickness and ice
crystal effective size simultaneously using
1.38-?m reflectance and 1.88-?m reflectance from
Airborne Visible/Infrared Imaging Spectrometer
(AVIRIS) data (Gao et al., 2004). The 1.88-?m
band, like the 1.38-?m band, is located in a
strong water vapor absorption band. We hope in
the future to collocate 1.88-?m AVIRIS
reflectance data with corresponding 1.38-?m MODIS
data for optical thickness and effective size
retrieval.
Left Visible MODIS image over Africa on January
30, 2003 (0.66-, 0.55-, and 0.47-?m channels).
Note the surface features evident in the upper
portions of the image. Right Corresponding
1.375-?m MODIS image. The white outline
indicates the region of retrieval.
Retrieved cirrus optical thickness corresponding
to the images at left. Note the sensitivity to
thin cirrus (blue and purple shades).
II. Tropical Cirrus Optical Thickness from MODIS
Level-1b Reflectance Data
Single-scattering phase functions for the 1.38-
and 1.88-µm channels for 21 midlatitude cirrus
size distributions. The assumed habit percentage
is shown at top left (Baum et al., 2000).
We have introduced a method to retrieve the
optical thickness of tropical cirrus clouds using
the isolated visible cirrus reflectance (Meyer et
al., 2004). The isolated cirrus reflectance is
inferred from level 1b calibrated 0.66- and
1.375-?m MODIS data. We created an optical
properties database and optical thickness look-up
library using previously calculated single
scattering data (Yang et al., 2000) with the
Discrete Ordinates Radiative Transfer (DISORT)
code (Stamnes et al., 1988). An algorithm was
constructed based on this look-up library to
infer the optical thickness for each pixel in a
MODIS image. This method is complimentary to the
operational cloud retrieval algorithm (King et
al., 1997) for the case of cirrus clouds, and has
been validated using Atmospheric Infrared Sounder
(AIRS) data.
Left Ice cloud optical thickness retrieved
using AIRS 1070-1135 cm1 spectral data (Wei et
al., 2004). Center Retrieved cirrus cloud
optical thickness inferred from MODIS 0.66- and
1.375-µm reflectance data. Right Comparison of
the cloud optical thickness values retrieved
using MODIS and those retrieved using AIRS
Brightness Temperature Difference (BTD) 900 -
1559.
III. Optical Thickness Retrieval from MODIS
Level-3 Data
Top Sample lookup-table with AVIRIS data
overlaid. Bottom Close-up of top panel. The
spatial resolution of the AVIRIS data in this
image is lowered to 76 ? 64 using the IDL rebin
function. This is done before the retrieval to
remove noise from the data.
The single-scattering phase functions for the
0.66- and 1.375-?m MODIS channels for nine CEPEX
size distributions used in this study. The
assumed habit percentage is also shown above
(McFarquhar, 2000).
Scatter plot of DISORT calculated 0.66- vs.
1.375-?m reflectance values, with linear curve
fitting. The slope is indicated at the top.
We have subsequently extended the method
described above (for level-1b data) for
application to level-3 daily averaged data.
Isolated cirrus reflectance from the MODIS Cloud
Product, as well as solar/sensor view geometry
information, is used as input for the retrieval.
This effort is focused on computing various
statistics (probability distribution functions,
etc.) with the goal of producing a tropical
cirrus climatology.
Top AVIRIS 0.66-?m image. Center
Corresponding 1.38-?m image. Bottom
Corresponding 1.88-?m image.
Left MODIS level-3 daily averaged derived
cirrus reflectance in the tropics from the Terra
satellite on July 27, 2002. Right Inferred
cirrus optical thickness corresponding to
reflectance image at left. Data is plotted
between ? 30º latitude.
Left 0.66-?m MODIS image over southeastern Asia.
Center Corresponding 1.375-?m MODIS image.
Right Corresponding derived visible cirrus
reflectance (Gao et al., 2002). Note the
similarity between the 1.375-?m image and the
derived cirrus reflectance.
Sample lookup-table plot of 0.66-?m reflectance
vs. optical thickness for varying effective
diameters (from nine CEPEX size distributions).
AVIRIS 1.38- vs. 1.24-?m scatter plot for the
above images. The slope of this plot is used to
remove atmospheric water vapor effects from the
data.
Top Retrieved cirrus cloud optical thickness
for the above images. Bottom Corresponding
retrieved ice crystal effective diameter (?m).
The total number of days between September, 2001,
and October, 2002, with cirrus optical thickness
greater than zero. Persistent cloud patterns due
to the ITCZ and orographic effects (Andes in
South America, etc.) are clearly visible in this
image.
B. A. Baum, D. P. Kratz, P. Yang, S. C. Ou, Y.
Hu, P. F. Soulen, S. C. Tsay, Remote sensing of
cloud properties using MODIS airborne simulator
imagery during SUCCESS 1. Data and models, J.
Geophy. Res., 105, 11767-11780, 2000. B. C. Gao,
P. Yang, W. Han, R.-R. Li, and W. J. Wiscombe,
An algorithm using visible and 1.375-?m channels
to retrieve cirrus cloud reflectances from
aircraft and satellite data, IEEE Trans. Geosci.
Remote Sensing, 40, 1659-1688, 2002. B. C. Gao,
K. Meyer, and P. Yang, A new concept on remote
sensing of cirrus optical depth and effective ice
particle size using strong water vapor absorption
channels near 1.38 and 1.88 ?m, IEEE Trans.
Geosci. Remote Sensing, 2004 (in press). M. D.
King, S. C. Tsay, S. E. Platnick, M. Wang, and
K. N. Liou, Cloud retrieval algorithms for
MODIS Optical thickness, effective particle
radius and thermodynamic phase, MODIS Algorithm
Theoretical Basis Document, NASA, 1997. G. M.
McFarquhar, Comments on Parameterization of
effective sizes of cirrus-cloud particles and its
verification against observations by Zhian
Sunand Lawrie Rikus (October B, 1999, 125,
3037-3055), Quart. J. Roy. Meteor. Soc., 126,
261-266, 2000. K. Meyer, P. Yang, and B.-C. Gao,
Optical thickness of tropical cirrus clouds
derived from the MODIS 0.66- and 1.375-?m
channels, IEEE Transa. Geosci. Remote Sens., 42,
833-841, 2004. S. Platnick, M. D. King, S. A.
Ackerman, W. P. Menzel, B. A. Baum, J. C. Riedi,
and R. A. Frey, The MODIS cloud products
algorithms and examples from Terra, IEEE Trans.
Geosci. Remote Sensing, 41, 459-473, 2003. K.
Stamnes, S.-C. Tsay, W. Wiscombe, and K.
Jayaweera, Numerically stable algorithm for
discrete-ordinate-method radiative transfer in
multiple scattering and emitting layered media,
Appl. Opt., 27, 2502-2509, 1988. H. Wei, P. Yang,
J. Li, B. A. Baum, H. -L. Huang, S. Platnick, Y.
X. Hu, and L. Strow, Retrieval of
semi-transparent ice cloud optical thickness from
Atmospheric Infrared Sounder (AIRS)
measurements, IEEE Trans. Geosci. Remote Sens.,
2004 (in press). P. Yang, K. N. Liou, K. Wyser,
and D. Mitchell, Parameterization of the
scattering and absorption properties of
individual ice crystals, J. Geophy. Res., 105,
4699-4718, 2000.
V. References
Total number of days with cirrus optical
thickness greater than zero during the winter
months of December, January, and February (left),
and the summer months of June, July, and August
(right). Note the northward movement of the ITCZ
(especially evident over land masses) from winter
to summer.
Left Visible MODIS image over the Indian Ocean
on July 13, 2002 (0.66-, 0.55-, and 0.47-?m
channels). Right Corresponding 1.375-?m MODIS
image. The white outline indicates the region of
optical thickness retrieval.
Retrieved cirrus optical thickness corresponding
to the images at left. Note the sensitivity to
thin cirrus (blue and purple shades).