This paper presents retrieval methods applied to HIS observations during FIRE II and doubling/adding model developed to simulate high-spectral resolution infrared radiances in a cloudy atmosphere. The capabilities of the retrieval methods and sensitivity studies of high-altitude aircraft based observations to cloud microphysical structure are conducted with the model

Ackerman, S. A.

Knuetson, R. O.

Ma, X. L.

Revercomb, H. E.

Smith, W. L.

English

NASA Technical Reports Server

N -22315
CIRRUS CLOUD RETRIEVALS FROM HIS OBSERVATIONS DURING FIRE II
S. A. Ackerman, W. L. Smith, X. L. Ma, R. O. Knuteson and H. E. Revercomb
ABSTRACT
This paper presents 1) retrieval methods applied to HIS observations dating FIRE II and 2)
doubling/adding model developed to simulate high-spectral resolution infrared radiances in a cloudy
atmosphere. The capabilities of the retrieval methods and sensitivity studies of high-altitude aircraft
based observations to cloud microphysical structure are conducted with the model.
OBSERVATIONS
Methods of detecting cirrus clouds and inferring their radiative properties have been developed at
the University of Wisconsin-Madison under the High-resolution Interferometner Sounder (HIS) program
(e.g., Smith and Frey, 1990; Ackerman et al, 1990; and Smith et al, 1993). The HIS instrument is a
calibrated nadir viewing Michelson interferometer which flies on board the NASA high altitude ER-2
aircraft. Figure l is an example of the retrieved cloud pressure altitude using the CO 2 slicing method
developed for high-spectral resolution observations (Smith and Frey 1990) for the period 17:50 to 18:04
UTC on December 5. The white line is the retrieved cloud altitude and the dark line the IR window
equivalent blackbody temperature. Dotted lines indicate the HIS in its calibration mode or no cloud
height retrieval. The HIS time series is overlain the CALS lidar image, provide oy Dr. J. Spinhirne.
While the CO 2 slicing method is detecting the presence of the cloud, its effective altitude varies more than
the lidar observed cloud. The cloud is very inhomogeneons, making it a difficult situation for any cloud
retrieval technique. We are employing model simulations to test the sensitivity of the CO 2 slicing
method, and other cloud retrieval techniques, to cloud microphysical properties (this will be presented at
the conference). Figure 2 depicts the tri-spectral approach to detecting cirrus cloud (Ackerman et al 1990)
for the same period as Figure I. Positive 8.3-11 _tm brightness temperature differences are indicative of
cloud. We are combing the tri-spectral technique with the CO 2 slicing technique and the cloud emissivity
techniques of Ackerman et al (1990) and Smith at al (1993) to improve the detection of cirrus cloud and
the retrieval of its radiative properties.
MODEL SIMULATIONS
Infrared observations at a spectral resolution of 1 cm" 1, or finer, have proven to be extremely
valuable in assessing line-by-line radiative transfer models and in retrieving atmospheric temperature and
moisture profiles (Smith et al 1989). Techniques have also been developed to infer, in cloudy sky
atmospheric conditions, cloud radiative properties in addition to temperature and moisture profiles from
the high spectral resolution observations. To develop, verify and test these cloud retrieval techniques
requires accurate simulations of observed radiances. These model based simulations have to accurately
account for multiple scattering by the cloud layer, as well as emission and absorption of the gases in the
atmosphere.
Assuming a plane-parallel horizontally homogeneous cloud, the IR radiative transfer equation is
- ' , ,2 a-, P(6,1J,,u )l(6,p ) dp'
where I(tS,/_) is the azimuthally average monochronmatic intensity, 8 is the optical thickness, coo is the
single scattering albedo, P(tS,/./, fl') is the azimuthally averaged phase function, B(T) represents the
Planck function at temperature 7",and fl = cos 0 where 0 is measured from the downward normal
direction. An accurate numerical technique to solve equation (1) is the doubling/adding method which
has been discussed in detail in previous atmosphere studies (Grant and Hunt, 1969; Wiscombe 1976;
Wiscombe and Grams 1976; Wiscombe and Evans 1977, Stephens 1980).
For the purposes of this paper, we have assumed the cloud is composed of spherical ice particles
distributed according to a modified gamma distribution. Scattering is neglected in the clear sky
atmosphere so that, for a single cloud layer, the atmosphere is divided into three layers: above, within and
below the cloud, layer. Radiances and transmittan¢es in the clear sky conditions ,are determined from
FASCOD3 calculations. The incident radiances at the cloud boundaries must be specified. Rather than
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run separate FASCOD3 calculations for each angle incident on the cloud, for the purposes of this paper,
FASCOD3 is used to the nadir and zenith angle radiance, the angular distribution is derived by weighting
the FASCOD3 radiance by the cosine of the incident angle. FASCOD3 is used to assign gaseous
transmittance within the cloud.
Examples of model simulations which correspond to conditions observed during the coincident
FIRE II Cirrus and SPECTRE field experiments, will be presented at the conference. Vertical profiles of
temperature, moisture and ozone were measured during 5 December 1991. Both ground based and ER-2
based observations were also available during this time period. Lidar observations indicated a cirrus cloud
between approximately 10 and 12.3 km. The model discussed above was used to simulate high-spectral
resolution observations for a variety of assumed cloud microphysical properties. Here we want to briefly
highlight the sensitivity of the observations to different microphysical properties.
An example of the sensitivity of the HIS observations to ice water path (IWP) is depicted in
Figure 3. The reference spectra is computed for a cloud with IWP=6.9 g/m 2, an effective radius of 30 p,m
and a variance of 0.25. In each subsequent calculation, only the IWP was changed (values are: 0.23, 0.69,
1.61, 2.3, 16.1, 23., 69., and 161 g/m2). Differences between the different IWP's are depicted in terms of
the difference in equivalent brightness temperature from the reference spectra (ABT). Negative values
indicate a larger IWP and positive values a smaller IWP. As expected the more opaque the band the less
sensitivity to IWP and the cloud in general. Maximum sensitivity occurs in the regions between
absorptance lines. For these high clouds, the aircraft based instrument has more sensitivity to the cloud
IWP in the spectral regions 500-600 cm "1 and 1300-1500 cm "1. Water vapor absorption is dominant in
these spectral regions, the majority of which lies between the surface and the cloud base obscuring the
view of the cloud from the ground-based instrument. In the region 1100-1300 cm" 1, the ER-2 view sees a
constant ABT, at least in-between absorption lines. For the spectral region 850-1000 cm'l,the ABT is
spectrally dependent. The spectral variation of ABT depends on the IWP though it is less than
approximately 2 °. This spectral variation in BT is driven by the spectral variation in the cloud optical
properties and therefore rooted to the cloud particle size distribution.
Sensitivity of the simulated spectre to changes in cloud particle size distribution are depicted in
figure 4. Again results are displayed in terms of the brightness temperature difference from a reference
spectra. The reference spectra has a particle size distribution with a=30 Ixm and a variance of b=0.25;
comparison are made for small effective radius (a=151_m) and larger effective radius (a=120_tm).
Comparisons were conducted for three equivalent IWP's, .23, 2.3 and 23 g/m 2. The positive ABT
represent differences between the 30 _m effective radius distribution and the 15 _tm distribution. As seen
in Figure 4, the smaller the IWP the less difference between spectra with different effective radii. The
scales were kept the same as the IWP comparison to demonstrate the dominating effect of the cloud ice
water path. The spectral region that appears to be most sensitive to particle size is the 950-1050 cm "l (>5
°). Variations in this spectral regime are larger than the IWP dependence. The magnitude depends on the
IWP, while the particle size controls the shape of the ABT curve.
Further sensitivity studies and data analysis will be presented at the meeting.
REFERENCES
Ackerman, S. A., W. L. Smith, J. D. Spinhirne, and H. E. Revercomb, 1990: The 27-28 October 1986
FIRE cirrus case study: spectral properties of cirrus clouds in the 8-12 I.tm window. Mort. Wea. Rev.,
118, 2377-2388.
Revercomb, H.E., H. Buijs, H.B. Howell, D.D. LaPorte, W.L. Smith, and L.A. Sromovsky, 1988:
Radiometric Calibration of IR Fourier Transform Spectrometers: Solution to a Problem with the i-Iigh
Resolution Interferometer Sounder, Applied Optics, 27, 3210-3218.
Smith W. L., H. E. Revercomb, D. D. LaPorte, H. M. Woolf, H. B. Howell, R. O. Knuteson, H. L. Huang,
L. A. Sromovsky and. S. Silverman, 1989: GHIS, the Geostationary High-resolution Interferometer
Sounder. CIMSS View, Vol. V, No. 1, available from the Space Science and Engineering Center,
Uuiverstiy of Wisconsin, Madison, Wisconsin, 53706.
Smith W.L., Xia Lin Ma, S.A. Ackerman, HE. Revercomb, and R.O. Knuteson, 1993: Remote Sensing
Cloud Properties from High Spectral ResolutiOn Infrared Observations. J. AppL Met, in press, 1993.
99
Smith, W. L. and R. Frey, 1990: On cloud altitude determinations from high resolution interferometer
sounder (HIS) observations. J. Appl. Meteor., 29, 658-662.
300
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REFERENCES
Ackerman, S. A.. W. L. Smith. J. D. Spinhirne, and H. E. Revercomb. 1990: The 27-28 October 1986
FIRE cirrus case study: spectral properues of cirrus clouds in the 8-12 _.m window. Ahm, tI'ea. Rev..
118, 2377-2388.
Revercomb, HE.. H. Buijs, HB. Howell, DD. LaPorte. W.L. Smith, and L.A. Sromovsk3', 1988:
Radiometric Calibration of IR Fourier Transform Spectrometers: Solution to a Problem with the High
Resolution lntefferometer Sounder. Applied Optics. 27. 3210-3218.
Smith W. L., H. E. Revercomb, H. B. Howell, and M -X. Lin: 1988: Multi-spectral window radiance
observations of cirrus from satellite and aircraft - November 2, 1986 "Project FIRE", FIRE Science
Results 1988, Proceeding of a conference held in Vail, CO July 11-15, pp 89-93.
Smith W. L., H. E. Revercomb, D. D. LaPorte, H. M. Woolf, H. B. Howell, R. O. Knuteson. H. L. Huang,
L. A. Sromovskv and S. Silverman, 1989: GI-I]S, the Geostationarv High-resolution lntefferometer
Sounder. CIMSS View, Vol. V, No. I, available from the Space Science and Engineering Center,
Umverstiy of Wisconsin, Madison, Wisconsin. 53706.
Smith W.L., XffLqh_a,=S.A. Ackerman, H.E. Revercomb, anflR.O. Knuteson, 1993: Remote Sensing
Cloud Properties from High Spectral Resolution Infrared Observations. ,I. Appl. Met, in press. 1993.
Smith, W. L. and R. Frey, 1990: On cloud altitude determinations from high res:_lution interferometer
sounder if[IS) observations, J. ,4ppl. Meteor., 29. 658-662.
Smith, W. L.. and R. Frey, 1991" Altitude specification of cloud motion winds. Workshop on Wind
Ex'traction from Operational Meteorological Satellite Data, Washington. D.C. September 17-19.
Table2. WISCONSIN AERI INSTRUMENT OPERATIONS
LOCATION: COFFEYVILLE, KANSAS
YEAR: 1991
REMARKS: (1) Observations are at 10 nunute intervals betweenstated STARt and END times. (2)
The letter H indicates that the ER-2 HIS was overhead. (3) OP # refers to an AERI operating period.
DATE TIME PERIOD CONDITIONS FROM VISUAL OBS
............................................
OP #
l II NOV 17:06 - 17:30
2 12 NOV 23:26 - 02:29
13 NOV 02:53 - 04:28
3 13 NOV 18:18 - 01:26
14NOV 02:13 - 03:41
4 17 NOV 17:58 - 21:12
18NOV 01:29 - 24:00
19 NOV 00:00 - 05:57
5 20 NOV 17:20 - 23:33
21 NOV 00:12 - 24:00
22NOV 00:00- 19:07
6 23 NOV 16:28 - 24:00
24 NOV 00:47 - 23:29
25 NOV 00:37 - 05:48
7 25NOV 16:19 - 23:52
26 NOV 00:29 - 24:00
8 27NOV 14:01 - 17:21
9 28 NOV 14:40 - 22:35
I0 29 NOV 15:00 - 24:00
30 NOV 00:00 - 17:34
1 ! 03 DEC 00:25 - 23:08
04DEC 23:55 - 06:41
12 04 DEC 17:16 - 24:36
05 DEC 01:23 - 24:00
06 DEC 00:00 - 05:31
13 06 DEC 14:52 - 20:33
07 DEC 00:54 - 05:52
14 07DEC 14:49 - 21:23
low overcast
cirrus
cirrus/clear
cirrus
thin cirrus
mixed cirrus to clear
clear
clear/cirrus/low thick cloud
clear
clear
cirrus/clear/rain
clear/mixediovercast
overcast/clear
overcast
alto-cumulus/scatter cirrus
clear/cirrus/mixed
low cloud
cirrus/overcast stratus
overcast/clear
clear/overcast
overcast/clear/cold
clear
clear/aerosol
clear/cirrus
thin cirrus
mixed cirrus/alto cu
clear/aerosol/low cloud
low overcast/broken low
............................................................... • - ....................................................
102


Cirrus cloud retrievals from HIS observations during FIRE 2

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