A description of the radiative properties of two cirrus clouds sampled on 10/28/88 in the FIRE cirrus IFO is presented. The clouds are characterized in terms of the broadband infrared effective emittance, cloud fractional absorptance, shortwave heating rate, cloud albedo and vertical velocity. The broadband fluxes used in these calculations were obtained from measurements made by pyranometers and pyrgeometers. The shortwave irradiances were corrected to a horizontal plane and normalized to the same time by taking into account Sabreliner flight information (i.e., pitch, roll, heading and angle of attack), as well as sun-earth geometry considerations. Since only one aircraft was used, broadband fluxes at different levels in the cloud were not measured simultaneously. As a result, sampling errors may occur due to the nonsteady state of the cloud field or due to the possibility that the flight legs were not flown directly above or below each other. To minimize these errors and to simplify the analysis, the necessary variables were averaged and the averages used in the calculations. The downwelling shortwave and longwave irradiances were used as selection criteria to remove cloud free data encountered along the data sampling leg

Cox, Stephen K.

Hein, Paul F.

Smith, William L., Jr.

English

NASA Technical Reports Server

RADIATIVE PROPERTIEB OF CIRRUS CLOUDS: FIRE IF0 CASE lO/ZS/SS 
PAUL F. HEIN, WILLSAM L. SMITH, JR., AND STEPHEN K. COX 
Department o/ Atmoiphrric SciSna, Cobdo sbtr Winradv, Ibr( Cdlinr, CO 805IS 
1. Data Analyeis Procedures 
A description of the radiative pro erties of 
two cirrus clouds sampled on lO/'28b8 in the 
FIRE cirrus I F 0  is presented. The clouds are 
characterized in terms of the broadband in- 
frared effective emittance, cloud fractional ab- 
sorptance, shortwave 1.3-2.8~ m) heating rate, 
cloud albedo and vertical velocity. The broad- 
band fluxes used in these calculations were ob- 
tained from measurements made by pyranome- 
ters and pyrgeometers manufactured by E p p  
ley Laboratories Inc. For a description of these 
radiometers and calibration procedures, see Al- 
brecht and Cox (1976), Albrecht and Cox (1977) 
and Smith Jr. et  aZ. (1988). In addition, the 
shortwave irradiances were corrected to a hori- 
zontal plane and normalized to the same time by 
taking in to  account Sabreliner flight information 
(i.e. pitch, roll, heading and angle of attack), as 
well as sun-earth geometry considerations (Ack- 
erman and Cox, 1981; hereafter AC). 
Since only one aircraft was used, broadband 
fluxes at  different levels in the cloud were not 
measured simultaneously. As a result, sampling 
errors may occur due to the nonsteady state of 
the cloud field or due to the possibility that the 
flight legs were not flown directly above or be- 
low each other. To minimize these errors and 
to simplify the analysis, the necessary variables 
were averaged and the averages used in the cal- 
culations. The averaging for each flight leg was 
done in the following ways: 
(1) Average the appropriate variables using 
every data point along a flight leg t o  determine 
a mean value for the cirrus cloud field. 
(2) Average some percentage of the data to  
stratify the cloud into separate regimes (Le. o p  
tically thicker or thinner than average). 
The downwelling shortwave and longwave ir- 
radiances were used as selection criteria to re- 
move cloud free data encountered along the data 
sampling leg. 
(a) Shortwave Stratification 
Find the lowest 30% of the downwelling short- 
wave irradiance values along each flight leg and 
average all irradiance variables corresponding to 
these points. 
(b) Longwave stratification 
Find the highest 30% of the downwelling long- 
wave irradiance values along each flight leg and 
average all irradiance variables corresponding to  
these points. 
The effective emittance ( E * )  is the ratio of the 
observed change in irradiance through a cloud 
layer to the change that would have occurred 
had the cloud been black. e*, as derived from 
the downward irradiance may defined as: 
where H( ) refers to the observed downwelling 
infrared irradiance, T is the mean temperature 
along the flight leg, the subscripts T and B re- 
fer to the top and bottom of the cloud layer, 
respectively, and u is the Stefan Boltzman con- 
stant. 
The cloud fractional absorptance (CFA) is de- 
scribed by AC and is defined as: 
where 
H 1 - H  f)T 
H 1 - H  f ) ~  
net irradiance at  cloud top, 
net irradiance at  cloud base, 
. Hsi EE net energy gain or loss through cloud 
sides (i.e., a ~ z / a z  + a ~ y / a y ) ,  
H J T E  incoming irradiance at  cloud top. 
In seeking ways to quantitatively express the 
spatial variability of the observed variables, two 
techniques have been applied: a histogram anal- 
ysis and a cumulative variance summary. 
The histograms were created by applying 
thresholds to detrended data and calculating 
the size in kilometers) of each section that ex- 
ceeded t 6 e threshold. These sizes were then 
combined to form the histograms. In Figure 1, 
i: 
159 
https://ntrs.nasa.gov/search.jsp?R=19910001166 2020-03-19T20:14:11+00:00Z
RADIATIVE P R O P E R T I E S  O F  CIRRUS CLOUDS: F I R E  I F 0  CASE 10/28/86 
Height 
(km) 
11.3 
10.0 
8 8  
an example graph, the +1 SD (standard devi- 
ation threshold is exceeded between points F 
the size used in creating the histogram with the 
threshold of +1 SD. The count of the appropri- 
ate bin size will then be incremented. Similarly, 
the distance between E and H is the size used to 
increment the count of the appropriate bin for 
the +O.O threshold. For the negative thresholds, 
-0.0 and -1 SD, the values must be less than the 
threshold (instead of being greater than for the 
positive thresholds) for the threshold to be ex- 
ceeded. Thus, the section between points B and 
E, and the section between H and K, exceed the 
-0.0 threshold. Similarly, the section between 
points C and D, and the section between points 
I and J,  exceed the -1 SD threshold. Sections 
of the graph at  the ends of the data set that 
exceed a threshold (i.e. section to the left of 
point B for the +O.O threshold are not used 
contains several different heights to increase the 
total number of sizes. Since the thresholds are 
based on standard deviations and not absolute 
values, the distribution of sizes a t  each height is 
expected to be similar. 
The cumulative variance graphs were created 
by first using Fast Forrier Transforms to cre- 
ate a variance spectra. The variances were 
then summed in a cumulative fashion from the 
largest scalelengths (wavelengths) to the small- 
est. No detrending was done to  the original data 
and no smoothing or averaging was done to the 
spectral points. Also, no points were removed 
due to the effect of the finite length of the file. 
This means that the spectral points at scale- 
lengths larger than about 4000 m (especially 
greater than 8000 m) may only be qualitatively 
correct. The cumulative variance at the smaller 
scalelengths should be correct because a more 
accurate representation of the variance at  the 
larger scalelengths should only redistribute the 
energy amongst the larger scalelengths. 
and A . The distance between F and G is then 
because the size is unknown. E! ach histogram 
S l u t  End 
Tim Lat Lon6 T i m  l a c  Long H e d i n g  
(CMT) (CMT) 
15:45:48 44.29 -87.89 1541)5.8 44.45 -87.41 I20 
1600.56 44.27 -87.89 16.M;ll 41.45 -87.36 119 
161647 4 4 s  -87.95 162041 44.44 -8736 115 
2. Flight Description 
The Sabreliner flight on the morning of 28 Oc- 
tober 1986 was flown over the western shore of 
Lake Michigan. There was a shortwave pres- 
sure) ridge over the region with a trougk ap- 
proaching from the west. 
The cirrus, the Sabreliner flew through, was 
thin and had a banded structure. Cloud base 
of the upper cloud deck varied from about 8.8 
km to 9.2 krn with broken undercast beneath. 
Cloud top was at  about 11.0 km. 
The Sabreliner flew a racetrack pattern with 
the fiight legs being oriented about 30 degrees 
3. Results 
Cloud fractional absorptance values were de- 
duced from pyranometer measurements in the 
.3-2.8 p rn spectral region with a correction 
made for sampling of finite cloud effects in the 
manner described by AC. 
Figure 2 shows a plot of the solar cloud frac- 
tional absorptance v s  the infrared emittance for 
the two, two layer cloud samples defined in Ta- 
ble l. The lower values represent a layer with 
a 1.3 krn geometric thickness while the higher 
values represent a 2.5 krn thick layer. Values 
derived from both the SW stratification and 
the LW stratification are shown. As we men- 
tioned above, the stratification of the data was 
necessary to eliminate the banded structure of 
cirrus cloud within each data leg. In essence, 
both clouds for both stratifications show very 
similar radiative properties with emittances be- 
tween -48 and .55 for the thicker layers and .28 
and .30 for the thinner layers; similarly the S W  
fractional absorptance varies from .08 to .09 for 
the thicker layers and .05 to .06 for the thinner 
layers. Within the errors of the observations, 
the slopes of the lines connecting these points 
160 
for the two clouds are essentially the same. 
Layer S W heating rates for these same cloud 
layers are given in tables 2 and 3. The heating 
rates for the two layers are encouragingly similar 
as would be expected from the previous figure; 
for the top 1.3 km layer they vary between .23 
and .29 O C/hr and for the lower layer between 
.15 and -17 O C/hr. 
Figure 3 represents the SW reflectance ob- 
served at  cloud top for Cloud 2 for three spectral 
bandpasses. This is an example of the structure 
of the observations in the raw data. 
Figures 4 shows the results of the histogram 
analysis of scale size variation on the measured 
vertical velocity. It is notable that most of the 
vertical velocity "cells" have horizontal dimen- 
sions less than  3 km. Figure 5 shows that the 
downward visible irradiance shows essentially 
the same structure as the vertical velocity. 
Figure 6 shows the cumulative variance anal- 
ysis for the albedo at the top of cloud 2. The 
continuous line is a fit to the plotted data points. 
This graphs may be interpreted to show the per 
cent variance explained by scale lengths larger 
than a given value. For example, this figure sug- 
gests that 85% of the variance arises from fea- 
tures whose horizontal scale is greater than 10 
km. Figure 7 illustrates the cumulative variance 
analysis for vertical velocity applied to each of 
the flight levels in the sample of cloud 2. The 
solid curve represents the 10.6 km height, the 
dashed curve - 9.4 km, and the dotted curve - 
8.5 km. Notable is the narrowing of the scale 
length with increasing height. Fifty percent of 
the variance is explained by features larger than 
13 km at the 10.6 km height, larger than 22 km 
at the 9.4 km level and larger than 50 km at  the 
8.5 km level. 
(.M) 
2 I 8.S Km 
4. Acknowledgements 
This research has been supported by the Na- 
tional Science Foundation under Grant ATM- 
8521214 and by the National Aeronautics and 
Space Administration under Grant NAG 1-554. 
(.181) (.158) 
REFERENCES 
Ackerman, S. A., and S. K. Cox, 1981: Aircraft 
observations of the shortwave fractional ab- 
sorptance of non-homogeneous clouds. J. A&. 
Albrecht, B., and S. K. Cox, 1977: Procedures 
for improving pyrgeometer performance. J. 
Albrecht, B., and S. K. Cox, 1976: Radiation 
data reduction procedures for Sabreliner, C- 
130 and DC-6 aircraft during the GARP At- 
lantic Tropical Experiment. Colorado State 
University Atmorphcn'e &+nec Paper NO. u, Fort 
Collins, CO., 100 pp. 
Smith Jr., W. L., S. K. Cox and V. Glover, 
19th: Temperature aensitivit of Eppley 
broadband radiometers. CorOrado State 
University ~ t ~ ~ p h c r i e  science popcr NO. IW, Fort 
Collins, CO., 12 pp. 
Md:or., 20, 0128-0133. 
Appl. M d c ~ r . ,  16 188-197. 
lbbh 2. Lyw hat+  rat.. (S2.8 rm) io  dqrca/hr. for Cloud I 
Z = 11.1 Km 
Z I 10.0 Km 
.I73 
Z I 8.8 Km 
*indicata b*acin( r a k  indrkrminak 
Tabla ¶. L y r  l u d n g  r a h  (S2.8 rm) io degrm/hr. for Cloud 2 
MEAN IR STRATIPICATION SW STRATIFICATION 
Z = 10.7 Km 
I * I  289 I .zm 
(.W) 
I .I28 I ,146 I .I48 
Z I 0.4 Km 
2.000 
I .so0 
I 
-1 .0004 J 
0 20 40 60 10 100 
Ol~lance  
Figure 1. Example plot illustrating procedure 
for generation of size histogram plots (figs. 
4 and 5). 
161 
RADIATIVE PROPERTIES OF CIRRUS CLOUDS: F I R E  I F 0  CASE 10/28/86 
100 
9 0 ' '  
Y 
c 
70.. 
e 6 0 -  
>" 50. .  
5 4 0 . .  
3 5 0 . .  E 5 20. .  
10 
O *  
FIRE CIRRUS IFO 28 OCT m FLi. 1 2  
- c 
HEIGHTS - 10.6 km - 9.4 km - 8.5 km 
' 
CLOUD 1 
0-OSW STRATIFICATION 
e - e LW SIRATIFICATION 
CLWO 2 
A-A SW STRATIFICATION 
A - A LW SIRATIFICATION 
(1.3 km) 
0.1 0.2 0.3 0.4 0.5 I 
EFFECTM EMIllANCE 
Figure 2. .3-2.8 c m CFA VB. effective emittance Figure 5. Same a.3 Figure 4 but for downwelling 
for Cloud I and Cloud 2. shortwave irradiance. 
0.400 
0.300 
0 0 w
f 
0.200 
TIRE CIRRUS If0 28 OCT 86 FLT. I 2  
ALBCW (10.7 km) 
- .3-2.8 pn ...... .I-2.8 pm 
- -  .3-.7 pm 
\ 
.. -..-...., ,.: ......... .... \.' 
0.IW 
0 50 100 150 200 
ZlYE (SECS) 
Figure 3. Albedo for Cirrus Cloud 2. 
FIRE Clrnm IF0 28 OET 86 FIt. I2 
Vedlsol VeIocIl)~ 
I 
15 I I 
0 J 6 ¶ I2 I 5  
Slre (km) 
Figure 4. Histogram illustrating observed verti- 
cal velocity scale sizes (SD is standard devi- 
ation from detrended mean). 
\ 
01 
I 0 0  1000 (E4 
Scoldength (m) 
5 
Figure 6. Cumulative variance of -3-2.8 fi  m 
albedo for Cloud 2. 
5 
Figure 7. Cumulative variance of vertical veloc- 
ity a t  different heights in Cloud 2. 
162 


Radiative properties of Cirrus clouds: FIRE IFO case October 28, 1986

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