The First ISCCP Regional Experiment (FIRE) Cirrus Intensive Field Observations (IFO) provide an opportunity to examine the relationships between the satellite observed radiances and various parameters which describe the bulk properties of clouds, such as cloud amount and cloud top height. Lidar derived cloud altitude data, radiosonde data, and satellite observed radiances are used to examine the relationships between visible reflectance, infrared emittance, and cloud top temperatures for cirrus clouds

Alvarez, Joseph M.

Grund, Christian J.

Minnis, Patrick

Sassen, Kenneth

Young, David F.

English

NASA Technical Reports Server

28276
INTERPRETATION OF CIRRUS CLOUD PROPERTIES USING COINCIDENT
SATELLITE AND LIDAR DATA DUR/NG THE FIRE CIRRUS IFO
Patrick Minnls and Joseph M. Alvarez
Atmospheric Sciences Division, NASA Langley Research Center
Hampton, Virgina 23665-5225
David F. Young
Aerospace Technologies Division, Planning Research Corporation
Hampton, Virginia 23666
Kenneth Sassen
Department of Meterology, University of Utah
Salt Lake City, Utah 84112
Christian J. Grund
Department of Meteorlogy, University of Wisconsin
Madison, Wisconsin 53702
I. Introduction
The First ISCCP Regional Experiment (FIRE) Cirrus Intensive Field
Observations (IFO) provide a unique opportunity to examine the relationships
between the satellite-observed radiances and various parameters which
describe the bulk properties of clouds, such as cloud amount and cloud-top
height. In this paper, lidar-derived cloud altitude data, radiosonde data,
and satelllte-observed radiances are used to examine the relationships
between visible reflectance, infrared emittance, and cloud-top temperatures
for cirrus clouds.
2. Data
Lidar backscatter data taken over Ft. McCoy (FMC), Wausau, and Madison,
Wisconsin during the FIRE Cirrus IFO (October 19 November 2, 1986) were
used to define the cloud base, cloud-top altitude zt, cloud thickness h,
and effective cloud center altitude zc. The last parameter is based on the
distribution of backscatter intensity. Sassen et al. (1989) discuss the
instrumentation and measurements in more detail. Soundings from Green Bay,
Wisconsin, were used to determine the temperature-height relationships for
all of the data. Cloud-top temperature Tt corresponds to zt on the
soundings. Mean cloud-center temperature Tc is found from zc. Surface
temperatures were used to supplement the clear-sky temperatures T
CS
derived from the satellite data. Half-hourly VIS (0.65_m) and IR (ll.5_m),
4-km data from the Geostationary Operational Environmental Satellite (GOES)
were extracted along the wind vector at zc for areas 4-plxels wide and
- 16-pixels long. The VIS data were converted to reflectance p using the
calibration of C. H. Whltlock (1988, personal communication). Clear-sky
temperature is derived for each data strip whenever some clear pixels are
observed in the VIS data. Otherwise, alternate methods are used to compute
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Tcs. For all pixels (denoted as cloudy pixels) having an equivalent
blackbody temperature, T < T 3K, the effective cloud beam emittance is
CS
= {[B(T) - B(Tcs)] / [B(Tz) B(Tcs)]}, (i)
where T refers to the temperature at altitude z.
Z
calculated twice for each satellite data strip using
of T . It is also assumed that
Z
The emittance is
T c and T t in place
e = I - exp (-r e / _), (2)
where the cloud IR optical depth is r and # is cosine of the viewing
e
zenith angle. The cloud reflectance, Pc' is assumed to be related to the
observed reflectance in the following manner.
P = TaP c + PsTcTu + _sd(l - _d)(l T _ )
C C'
(3)
where T a is the transmittance of the atmosphere above the cloud" T and
' C
T are the transmittances of the cloud to downward and upward directu
radiation, respectively; Ps and _sd are the clear-sky bidirectional
reflectance and diffuse albedos, respectively; and _d and _ are the
C
albedos of the cloud to diffuse and total radiation, respectively. Thus,
_d = ad(rv)" _c = _c(rv' #o)', and Pc = Xc_ c. The cosine of the solar
zenith angle is #0. It is assumed that the anisotropic reflectance
correction factor Xc is independent of the VIS optical depth r . Values
V
for Xc are taken from the empirical model of Minnis and Harrison (1984).
The relationship bewtween r and cloud albedo is based on the results of
V
Takano and Liou (1989) for a cloud containing randomly oriented hexagonal
ice columns 125 _m long and 50 #m wide. Cloud transmittance also depends on
the VIS optical depth. Mean values of T were computed for each discrete
reflectance value.
In the ISCCP methodology, the scattering ratio, r, is used to relate the
VIS optical depth to r in order to correct for the semitransparency ofe
the cloud. This quantity is equivalent to r / r . It is calculated here
v e
as
N
r = E (r v / re) / N,
i
i=l
where N is the number of cloudy pixels for each data strip. Details
concerning the data and methodology are given by Minnis et al. (1989).
318
3. Results and Discussion
Figure I shows an example of the observed values of _ p, and a for
' C
cirrus clouds over FMC at 15 UTC, October 28, 1986. Averaging the
temperatures minimizes some of the noise introduced by cloud structure
variations• Mean values of r computed at each half hour for the cirrus
days during the IFO (October 22, 27, 28, and 30; November i and 2) are
summarized in Table i. It is expected that r is constant for a given
cirrus cloud. Thus, the systematic variation of r with scattering angle
e indicates that the values of Xc used here are inadequate. The mean
value of r = 2•1, however, is very close to the value of 2.0 expected for
large-particle clouds (e.g., Hansen and Travis, 1974). Assuming that the
computed mean value of r is correct, it is possible to compute more
realistic values of Xc by forcing the results to yield r = 2.1 for each
data strip• The average values of Xc resulting from this process are
shown as the dashed lines in Fig. 2. Obviously, the cirrus clouds scatter
radiation more anisotropically than the average cloud• Optically thin
clouds tend to mimic the single-scattering phase function• Optically thick
clouds tend to reduce anisotropy through multiple scattering. Scattering
ratios from Table I averaged according to T c are shown in Fig• 3. The
average values of r are 2 5 + 0 2 for T < 235 K and 1.9 + 0 2 for T >
• -- " C -- " C
235 K. The larger values of r are significantly different from 2.0
suggesting that the colder clouds contain a substantial number of small
particles. The temperature dependency of r is similar to that found by
Platt and Dilley (1981). The variation of cloud emittance (corrected for
_) with T given in Fig. 4 for the case study (October 27-28) is much like
c
that determined by Platt et al. (1987) from ground observations. The data
for the entire period, however, show greater increases of E with
temperature than found by Platt et al. (1987). Cloud thickness (Fig• 5)
during the case study is also similar to the Platt et al. (1987) results•
Ratios of E(Tt) / _(Tc) are plotted against Tc in Fig. 6. The nearly
linear dependency of these ratios on T c indicates that the actual cloud-
top temperature can be determined if T is found first. Other results
C
from these analyses (Minnis et al•, 1989) indicate that cloud shadows have a
significant effect on the retrieved cloud albedos. This shadowing can
result in "dark" pixels which are colder than T and darker than Ps"cs
Their effects must be considered in the retrieval of cirrus properties from
VIS and IR data•
4. Concluding Remarks
The analysis of IFO data shows consistency with earlier cirrus studies
suggesting that some simple parameterizations of cirrus cloud properties can
be effectively utilized. Other results indicate that additional parameters
may be derived from the satellite data. More cases taken over other areas
will be needed to confirm the results presented here.
319
REFERENCES
Hansen, J. E. and L. D. Travis, 1974: Light scattering in planetary
atmospheres. Space Scl, Rev., 16, 527-610.
Minnis, P., and E. F. Harrison, 1984: Diurnal variability of regional cloud
and clear-sky radiative parameters derived from GOES data, ParC III:
November 1978 radiation parameters. J. Cllm. ADD1 MeTeor, 23, 1032-1051. - ' ,
Minnis, P., D. F. Young, K. Sassen, J. M. Alvarez, and C. J. Grund, 1989:
The 27-28 October 1986 FIRE Cirrus IFO case study: Cirrus parameter
relationships derived from satellite and lldar data. Submicted co Mon.Wea. Rev.
Plait, C. M. R., and A. C. Dllley, 1981: Remote sounding of high clouds: IV.
Observed temperature variations In cirrus optical properties J. Atmos,
Sol., 38, 1069-1082.
Plait, C. M. R., J. C. Scott, and A. C. Dilley, 1987: Remote sounding of
high clouds, VI. Optical properties of midlatltude and tropical cirrus
J. Atmos. Sc_,, 44, 729-747.
Sassen, K., C. J. Grund, J. Spinhlrne, M. Hardescy, and J. M. Alvarez, 1989:
The 27-28 October 1986 FIRE IFO cirrus case study: A five lldar view of
cirrus cloud structure and evaluation. Submitted to We_/D__L___
Takano, y. and K. N. Liou, 1989a: Radiative transfer In cirrus clouds: II.
Theory and computation of multiple scattering in an anlsotroplc medium.
$. Atmos, $c_,0 46, 21-38.
UTC Cuea
lIFO)
1330 4
i 400 3
1430 2
1500 3
1600 5
1630 2
1100'" 6
i BOO 6
1830 I
1900 ?
1930 4
2O0O 8
Z030 5
2100 5
2130 4
2200 5
total • I
mea_a ! 71
[ 6
• All _ta IIFOJ
|' ) x v r
109 0,97 1.08
117 1.84 1.42
124 0.90 1.76
131 1.46 2.01
146 2.31 2.22
153 2.44 1.74
160 1.78 2.99
173 2.20 3.49
175 3.52 1.98
168 2.50 2.Ul
162 1.52 2.73
154 1.93 2.68
147 1.40 2.47
140 1.06 1.99
132 1.15 1.30
125 0.62 0.63
--- 1.69 2.08
Case St_iy
0.15 0.85
0.17 1.19
0.31 1.58
1.46 2.01
1.72 1.98
1.39 2.94
1,94 5.55
0,53 3.13
0.14 2.15
1.28 3,06
1.63 2.34
0.97 2,46
0.53 i.20
0.26 0.56
1.04 2.0?
Table I. Reflectance parameters computed
for all data.
|.O
O.O
04
0-q
(a)
0._
0.0
0.0
1.0 --
0-8
04
g
g 0.q
d
0.2
(b)
0.0
0.0
• # o
o
oo °
o# o
I.
#
:.'..
{ I I I I
0.2 O,q 0.6 O.O
VISIBLE REFLECTANCE
/
I I I I I
0.2 O.q O.& 0.8 1.0
CLOt.OF:IL.I_O0
I
1.0
Fig. I. Cloud emittances and observed (a)
reflectances and (b) cloud albedos
for cirrus clouds over FMC at 1500
UTC, October 28, 1986.
320
Fig.
Fig.
'®I_ '=
.._I I I I I I i i i :i .m
13 14 tS II 17 18 19 _ _1 U
UTC
2. Mean anisotropic reflectance
correction factors for the I FO.
The solid line is the nominal
model; the dashed line represents
the values derived for r = 2.1.
W
#.-
t--
tJ
| o0 --
O.B -
0.6 -
o.q -
0.2 -
0.0
210
I
220
I I i
23O 2_0 250
CLOU0TE_°ERflRJREI K)
4. Variation of cloud vertical beam
emittance with cloud-center
temperature.
tl m
>-
_s
W
(.J
b.
tL
t.J
Z
tJ
tO
I I I I
elO 22o 23o _o i_o
CLOUD TErlPEI_:ITURE I K}
Fig. 3. Variation of mean scattering ratio
with cloud-center temperature.
Fig.
w
tO
tO
9
d
o
I I I I I
210 220 23(} k"iO 280
CLOUDTEIIPERRTURE(K)
5. Dependence of cloud thickness on
cloud-center temperature.
l.O--
O.S
2i0
0.$
o
O.e
(_)
z
cz
_- 0.7
i-=
w
0.5
I I I I
CLOUD TEr'PERAfURE I K}
Fig. 6. Variation of emittance ratio with
cloud-center temperature.
321



Interpretation of cirrus cloud properties using coincident satellite and lidar data during the FIRE cirrus IFO

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