High resolution infrared radiance spectra achieved from the NASA ER2 airborne HIS experiment are used to analyze the spectral emissivity properties of cirrus clouds within the 8 to 12 micron atmospheric window region. Observations show that the cirrus emissivity generally decreases with increasing wavenumber (i.e., decreasing wavelength) within this band. A very abrupt decrease in emissivity (increase in brightness temperature) exists between 930/cm (10.8 microns) and 1000/cm (10.0 microns), the magnitude of the change being associated with the cirrus optical thickness as observed by lidar. The HIS observations are consistent with theoretical calculations of the spectral absorption coefficient for ice. The HIS observations imply that cirrus clouds can be detected unambiguously from the difference in brightness temperatures observed within the 8.2 and 11.0 micron window regions of the HIRS sounding radiometer flying on the operational NOAA satellites. This ability is demonstrated using simultaneous 25 km resolution HIRS observations and 1 km resolution AVHRR imagery achieved from the NOAA-9 satellite. Finally, the cirrus cloud location estimates combined with the 6.7 micron channel moisture imagery portray the boundaries of the ice/vapor phase of the upper troposphere moisture. This phase distinction is crucial for infrared radiative transfer considerations for weather and climate models, since upper tropospheric water vapor has little effect on the Earth's outgoing radiation whereas cirrus clouds have a very large attenuating effect

Howell, H. B.

Lin, M.-X.

Revercomb, H. E.

Smith, William L.

English

NASA Technical Reports Server

M u l t i - s p e c t r a l  Window Radiance Obse rva t ions  o f  Cirrus 
from S a t e l l i t e  and A i r c r a f t  - November 2, 1986 " P r o j e c t  FIRE" 
W. L. Smith,  H. E. Revercomb, H. 8. Hwe11, and H.-X. t in1  
c o o p e r a t i v e  I n s t i t u t e  f o r  Meteo ro log ica l  S a t e l l i t e  S t u d i e s  
U n i v e r s i t y  of Wisconsin-Madison 
Madison, Wisconsin 
1 S t a t e  He t e o r o l o g i c a l  Admin i s t r a t  i o n  
N a t i o n a l  Me teo ro log ica l  Bureau 
B e i j f n g ,  China 
A b s t r a c t  
High r e s o l u t i o n  i n f r a r e d  r a d i a n c e  s p e c t r a  ach ieved  from the NASA ER2 a i r b o r n e  HIS exper iment  are 
used  t o  a n a l y z e  the  s p e c t r a l  e m i s s i v i t y  p r o p e r t i e s  o f  CIRRUS c l o u d  w i t h i n  t h e  8-l?um a tmospher i c  
%indow" r eg ion .  
wavenumber ( i .e. ,  d e c r e a s i n g  wave leng th )  w i t h i n  t h i s  band._ A v e r y  a b r u p t  d e c r e a s e  i n  e m i s s i v i t y  
( i n c r e a s e  i n  b r i g h t n e s s  t empera tu re )  exis ts  between 930 cm 1 ( 1 0 . 8 U m )  and 1000 cm 1 ( l O . O U m ) ,  t h e  
magni tude  of t h e  change b e i n g  a s s o c i a t e d  w i t h  t h e  c i r r u s  o p t i c a l  t h i c k n e s s  as observed  by l i d a r .  
HIS o b s e r v a t i o n s  a r e  c o n s i s t e n t  w i t h  t h e o r e t i c a l  c a l c u l a t i o n s  of t h e  s p e c t r a l  a b s o r p t i o n  c o e f f i c i e n t  
€or ice. 
Obse rva t ions  show t h a t  t h e  c i r r u s  e m i s s i v i t y  g e n e r a l l y  d e c r e a s e s  w i t h  i n c r e a s i n g  
The 
The HIS observa t iorm imply t h a t  c i r r u s  c loud  can  be detected unambiguously from t h e  d i f f e r e n c e  i n  
Th i s  a b i l i t y  is demonst ra ted  u s i n g  s imul t aneous  
b r i g h t n e s s  t empera tu res  observed  w i t h i n  t h e  8.2um and 1 l . O u m  window r e g i o n s  o f  t h e  HIRS sounding  
r ad iomete r  f l y i n g  on t h e  o p e r a t i o n a l  NOAA s a t e l l i t e s .  
25 km r e s o l u t i o n  HIRS o b s e r v a t i o n s  and 1 km r e s o l u t i o n  AVHRR imagery ach ieved  from t h e  NOAA-9 
s a t e l l i t e .  F i n a l l v ,  t h e  cirrus c loud  l o c a t i o n  e s t i m a t e s  combined w i t h  6.7um channel  moi s tu re  imagery 
p o r t r a y  t h e  boundar ies  of t h e  i c e / v a p o r  phase  of t h e  upper  t r o p o s p h e r e  moi s tu re .  
d i s t i n c t i o n  is c r u c i a l  f o r  i n f r a r e d  r a d i a t i v e  t r a n s f e r  c o n s i d e r a t i o n s  for wea the r  and c l i m a t e  models, 
s i n c e  upper  t r o p o s p h e r i c  wa te r  vapor  h a s  l i t t l e  e f f e c t  on t h e  e a r t h ' s  ou tgo ing  r a d i a t i o n  whereas 
c i r r u s  c loud  has  a v e r y  l a r g e  a t t e n u a t i n g  e f f e c t .  
Th i s  phase  
1. Importance of C i r r u s  De tec t ion  
The d e t e c t i c n  o f  c i r r u s  c l o u d s  from s a t e l l i t e s  is impor t an t  f o r  a t  l e a s t  t w o  reasons :  (1) t h e  
mon i to r ipg  o f  long-term changes of c l o u d  c o v e r ,  s i n c e  c i r r u s  v a r i a b i l i t y  g r e a t l y  Impact; t h e  
greenhouse  e f f e c r ,  and ( 2 )  t h e  e l i m i n a t i o n  of i n f r a r e d  r a d i a n c e s  observed  by sounding  r ad iomete r s  
e f f e c t e d  by c loud  prior t o  t h e  t e n p e r a t u r e  p r o f i l e  r e t r i e v a l  p r o c e s s  s i n c e  unde tec t ed  c i r r u s  c loud  
con tamina t ion  l e a d s  t o  a s y s t e m a t i c  c o l d  b i a s  i n  t h e  r e s u l t .  
2 .  P r i o r  Work 
S e v e r a l  a u t h o r s  have addres sed  t h e  o p t i c a l l y  t h i n  cirrus d e t e c t i o n  problems (Prabahkara ,  1987; 
I n o u e ,  1985, Wu, 1987). 
c l o u d  can  be  d e t e c t e d  by t h e  d i f f e r e n c e  i n  b r i g h t n e s s  t empera tu re  obse rved  i n  t h e  3.8l.m and L l l m  
window r e g i o n s  because  of t h e  d i f f e r e n t  P lanck  r a d i a n t  energy  dependence upon t empera tu re  a t  t h e s e  t w o  
wavelengths .  
c l o u d  and w a r m  s u r f a c e  t empera tu res .  Because o f  t h e  h i g h e r  o r d e r e d  dependence o f  r ad iance  upon 
t empera tu re  a t  3.8um, t h e  l l u n  b r i g h t n e s s  t e m p e r a t u r e  will be  observed  t o  be lower than  t h e  3 . 0 ~  
b r i g h t n e s s  tempera ture  in  t h e  p re sence  o f  t h i n  c i r r u s .  However, t h i s  r e l a t i o n s h i p  is no t  un iqce  i n  
tha t  s i m i l a r  d i f f e r e n c e s  occur  d u r i n g  t h e  dayt ime due  t o  d i f f e r e n t i a l  r e f l e c t e d  s u n l i g h t  c o n t r i b u t i o n s  
to t h e  two chance l s  and f o r  broken midd le  or low c l o u d s  due to  t h e  d i f f e r e n t i a l  P lanck  e f f e c t .  More 
r e c e n t l y ,  Inoue (1985) and Prahahkara  (1987) p o i n t e d  oCt t h a t  t h e  a b s o r p t i o n  f o r  i c e  is l a r g e r  a t  l 2 u m  
than  a t  l l u m  so t h a t  t h e  " s p l i t  window" d i f f e r e n c e  ( i . e . ,  t h e  d i f f e r e n c e  i n  b r i g h t n e s s  t empera tu res  
obse rved  i n  t h e  l l - m  and 12% c h a n n e l s  o f  t h e  Advanced Very High Reso lu t ion  Radiometer (ASrtfRP.) f l y i n g  
aboa rd  t h e  o p e r a t i o n a l  N O U  s a t e l l i t e s )  can  be used  t o  d e t e c t  t h i n  c i r r u s  c loud .  
d i f f e r e n t i a l  r e l a t i o n  is rict unique  e i t h e r ,  s i n c e  w a t e r  vapor  a b s o r p t i o n  can produce a s i m i l a r  e f f e c t .  
S n i t h  e t  a l .  (19691, Arking (1?85) ,  and Mi (1987) p o i n t  ou t  t h a t  thLn c i r r u s  
When v iewing  through t h i n  c i r r u s ,  t h e  measured r a d i a n c e  is a f u n c t i o n  of bo th  t h e  c o l d  
However, t h i s  
3. HIS C b s e r r a r i o n s  
Most r e c e n t l y ,  s p e c t r a l  r a d i o n e t r i c  o b s e r v a t i o n s  of c i r r u s  c l o u d s  were achieved  d u r i n g  t h e  First 
ISCCP Regional Experiment (FIRE) conducted  o v e r  s o u t h c e n c r a l  Wisconsin d u r i n g  October-Sovember 1986. 
The o b s e r v a t i o n s  were cor.ducted w i t h  t h e  High r e s o l u t i o n  I n t e r f e r o m e t e r  Sounder (HIS) f l y i n g  aboard 
t h e  NASA ERZ a i r c r a f t .  Here w e  d i s c u s s  t h e  c i r r u s  observed  o v e r  Wisconsin on November 2 ,  19E6. As 
shown i n  t h e  Fig. 1 exanp ie ,  t h e  HIS s p e c t r a l  r a d i a n c e s  r e v e a l  an i n c r e a s e  i n  c i r r u s  c l cud  a b s o r p t i o n  
and e m i s s i v i t y  (dec rease  i n  r a d i a t i n g  b r i g h t n e s s  t empera tu re )  a c r o s s  the l l0-12um window r e g i o n  v i t h  a 
v e r y  s h a r p  i n c r e a s e  t a k i n g  p l a c e  between 10 and l l u m  (i.e., 1000-900 c m  
and l O U m  ( n o t  shown) r e v e a l  no s i g n i f i c a n t  change i n  c i r r u s  e m i s s i v i t y  between 8 and 10m. 
o b s e r v a t i o n s  are i n  good agreement w i t h  t h e o r e t f c a l  c a l c u l a t i o n s  o f  i ce  a b s o r p t i o n  by I m i n e  and 
P o l l a c k  (1968) as shown i n  Fig. 2. 
t h e  ER2.) 
. HIS o b s e r v a t i o n s  between 8 
The HIS 
( F i g u r e s  4 and 5 shew sa t e l l i t e  images o f  t h e  c i r r u s  observed from 
89 
https://ntrs.nasa.gov/search.jsp?R=19910001152 2020-03-19T20:15:21+00:00Z
WAVENUMBER (CM-11 
730 810 870 ' 930 880 1030 
280 I s I 1 1 I I I . I 
266 
252 
TtHP 
(kl 
238 
HIS SPECTRA 
20: 34-20: 37 UTC 2 NOVEMBER 1986 
Fig. 1. 
the NASA ER2 on November 2, 1986 near Madison, Wisconsin. 
backscatter between 8 and 12 km (no significant return outside this altitude range) of light pulses 
t rom a lidar (Spinherne, 1982) also aboard the ER2. 
each spectrum i s  shown above the lidar. 
A set of IO-12um lvwindowsl region cirrus cloud brightness temperature spectral observed from 
Also shown is an image of the cirrus cloud 
The field of view of the HIS corresponding to 
4. ADptication to Satellite Observations 
Because water vapor absorption is generally larger in the 8-9um window region than within the 
11-12pt region, a positive brightness temperature difference between these two window regions is a 
definite indicator of the presence of cirrus cloud. 
1l.lum channel observations of the High Resolution Infrared Radiation Sounder (HIRS) radiometer flying 
aboard the polar orbiting N O M  satellites. 
Figure 2 shows the spectral response of the HIRS 1l.lum (Ha) and 8.2um (H10) channels relative to 
This principal is demonstrated using the 8.2 and 
the volume absorption coefficient for ice. As can be seen, there should be a significant difference 
1500 
1400 - - VOLUUE ABSORPTION COEFFICIENI FOR ICE 0 
z 
t- 
- 10  g 
HIRS SPECTRAL RESPONSE - 0 9  5 
- (AFTER IRVINE A N 0  POCLACK. 1968) 
,. : '*, 
I :; - 0 6  
I ::: - 0 5  b 
- 0 7  * : --. 
;cn i o  :, 
I 
., 
I 
1 
I  
.' ' - 0  
B 
In 
0 -  
100 - 
eo0 800 moo I too tzoo 1300 
WAVENVYBER (em"] 
I E m 
1000 - 
900 - 
400 - 
100 - 
600 - 
500 - 
400  - 
300 
zoo - B 
Fig. 2. Volume absorption coefficient for ice and the spectral response of HIRS channels 8 and 10. 
Fig. 3. 
and 1 l . l u m  (Ha) channe l s  a s  a f u n c t i o n  o f  1 1 . 1 U m  (Ha) b r i g h t n e s s  t empera tu re .  
i n d i c a t e  t h e  presence  o f  s e m i - t r a n s p a r e n t  c i r r u s  c loud .  
S c a t t e r  diagram of the d i f f e r e n c e  in b r i g h t n e s s  t empera tu re  observed  i n  t h e  HIRS 8.2cm (H10) 
P o s i t i v e  d i f f e r e n c e s  
Fig .  4. An itnag? of c loud  c o v e r  a t  20 GNT on Kovembcr 2 ,  1986 a s  obse rued  by t h e  A V I R ?  l l u n  channel 
( C h l ) .  
11.L~n window charenel d i f f e r e n c e s .  The c r o s s e s  deno te  t h e  gap i n  e a r t h  coverage  due t o  i n s t r c r e n t a l  
c a l i b r a t i o n .  
Supertmpcsed a r e  circles d e l i n e a t i n g  a r e a s  o f  c i r r u s  c loud  d e t e c t e d  from t h e  HIRS 8 .2  2nd 
in ehe  b r i g h t n e s s  t empera tu re  observed  by t h e s e  two channe l s  when v iewing  semi - t r anspa ren t  c i r r u s  
c loud .  
HIRS 8.2~m channel  (H10) and t h e  HIRS 1l . lum channel  (Ha) as a f u n c t i o n  o f  ehe 1 l . l U m  b r i g h t n e s s  
tempera ture  o b s e r v a t i o n  ove r  t h e  upper  midwes t  on November 2, 1986 ( s e e  Fig. 4). 
t h e  o b s e r v a t i o n s  a r e  €ree of c i r r u s  c l o u d s  a s  i n d i c a t e d  by t h e  n e g a t i v e  d i f f e r e n c e s .  
c loud  c o n d i t i o n s ,  however, do c o v e r  a wide  range  o f  o p t i c a l  t h f c k n e s s e s  as i n d i c a t e d  by t h e  wide r ange  
F igu re  3 shows a s c a t t e r  diagram of t h e  d i f f e r e n c e  i n  b r i g h t n e s s  t empera tu re  observed in t h e  
As shown, most o f  
Apparent c i r r u s  
Fig. 5 .  
observed by t h e  GOES-VAS 6.7um H20 absorp t ion  channel. 
cirrus as detec ted  from the  NOAA-HIRS 8.2um and 11.1um b r igh tness  temperature d i f f e rence .  
An image of upper t ropospher ic  water vapor and c i r r u s  cloud a t  20 GMT on November 2 ,  1986 a s  
A s  i n  Fig. 4, t h e  circles show regions  o f  
of 1 l . l u m  b r igh tness  temperature obsprva t ions  (220-268 'K)  assoc ia t ed  wi th  pos i t i ve  ind ica t ions  of 
c i r r u s  c louds  ( i . e . ,  plus  1110-IIR br igh tness  temperature d i f f e rences ) .  Assuming a mean sur face  skin 
temperature of  28O0K and a probable c i r r u s  cloud temperature o f  220°K,  t h e  temperature a t  the  10 kin 
c i r r u s  he ight  depicted by l i d a r  (Fig. l ) ,  t h e  c i r r u s  o p t i c a l  th ickness  for the  1 l . l u m  channel is a s  
low a s  0 . 2 ,  i nd ica t ing  t h a t  cLrrus  with t r a n s m i s s i v i t i e s  a s  high a s  80% were de tec ted  by the  HIRS in  
t h i s  case. 
Figure 4 shows t h e  loca t ion  (denote by c i r c l e s )  of  H I R S  detec ted  cirrus superimposed upon an 
AWRR in f ra red  image. Because the  instrumental  no ise  of the  HTRS is about 0.25-K for t y p i c a l  cloud 
temperatures,  a threshold  of -0.5OK for  the  d i f f e rence  was used t o  d i sc r imina te  c i r r u s  contaminated 
HIRS fields-of-view. The c i r c l e s  depic t  t he  geographical dimension of  the  2 5  km HIRS field-of-view. 
Based on comparison wi th  the cloud morphology in  the At't'RR image, t he  ob jec t ive  c i r r u s  derec t ion  
technique seems t o  work very we l l ,  except when the  c i r r u s  occupies only  a small por t ion  of the  IlIRS 
f ield-of-view. 
5 .  h'ater Phase Detection 
The comhination o f  6 .7um,  8 .2um,  and 11.1um b r igh tness  temperature da t a  can be used to 
d i f f e r e n t i a t e  between upper l eve l  moisture in t he  vapor phase from t h a t  which has sublimated into t he  
i c e  phase, T h i s  d i s t i n c t i o n  is vey: important wi th  regard t o  the  a n a l y s i s  of the  "greenhouse e f f ec t "  
on the  e a r t h ' s  c l lmate  s ince  upper leve l  water vapor i s  l a rge ly  t r anspa ren t  t o  the  outgoing r ad ia t ion  
to  space whereas c i r r u s  cloud produces a very l a rge  decrease  i n  t he  r a d i a t i o n  t o  space. 
Upper t ropospher ic  moisture imagery, ob ta iced  from 6.7um g e o s t a t i o n a r j  s a t e l l i t e  measurevents is 
rou t ine ly  used fo r  t h e  depic t ion  o f  l a r g e  scale weather pa t t e rns .  
have an 8.2Um channel a s  pa r t  of the  geos ta t ionary  imagery system so t h a t  the phase of  the  upper l eve l  
moisture depic ted  by the  6 . W n  chaanel could be diagnosed. An example of the  r e s u l t s  which could be 
achieved is generated by superimposing HTRS c i r r u s  cloud dep ic t ions  ove r  a GOES-VAS 6.7pm moisture 
image obtained a t  nea r ly  the same t k e .  Figure 5 shows the  r e s u l t .  I f  one had a time sequence of  
such i a fe rences ,  t he  phase change process could be o b s e n e d ,  thus  providing inpor tan t  measurerents of 
upper tropospheric cloud dynamics a s  w e l l  a s  the  upper t ropospher ic  r a d i a t i v e  p rope r t i e s  important for 
weather and c l i o a t e  ana lyses .  
It would be extremely use fu l  t o  
6. Additional Considerations 
It is worth not ing  t h z t  c i r r u s  cloud microphysical p r o p e r t i e s  (e .g . ,  p a r t i c a l  size d i s t r i b u t i o n )  
might be deduced iron t he  high r e so lu t ion  HIS spec t r a  and the  simultaneous l i d a r  observa t ions  
conducted from t he  ER2 a i r c r a f t  (an example of which i s  shown in FiR. 1). The o p t i c a l  th ickness ,  1, 
of the  cloud fo r  "window" wavelengths i n  between gaseous absorp t ion  l i n e s  can be s h o w  t o  be given by 
92 
T!i 
where 8 is the a b s o r p t i o n  covf C l c l e n t ,  AT t h e  geometr ica l  t h i c k n e s s ,  I 
and T "are t h e  s u r f a c e  and c loud  temprrn ture ,  r e n p c c t l v e l y ,  and R 1s t&: Planck r a d l n n c i ~ .  
surfaEg s k i n  tempernture  T 
t aken  from a c l e n r  sky tem$rra turc  p r o f i l e  6 t  t h e  c loud  a l t i t u d e  d e p i c t e d  by t h e  Ildar. 
t h i c k n e s s  o f  t h e  cloud Ai! 1 s  also provided by t h e  l l d a r  f n r  o p t i c a l l y  t l i tn  c l o u d  (art. FIR. 1). 
Consequent ly ,  t h e  cnmblnation o f  HIS nnd l i d n r  obSI?rvattons p r o v i d e s  a measure of t h e  infrarc?rl 
a b s o r p t i o n  c o e f f i c i e n t  spectrum f o r  t h e  c loud .  
p a r t t c l e  p r o p e r t i e s  through t h e  r e l a t i o n  
is t h v  ol~na*rvc!cl raillanct., 
Thc 
can hc* observcd  from netghbor lng  c l o u d - f r r e  f l e l d s - o f - v l w  aiicl T 1 %  
33wC#comr-trlc 
The a b s o r p t i o n  c o e f f t c i e n t  1s re1ntc.d t o  t h e  clotid 
where K ( Q  , 0,) 1s a func t ton  of t h e  tmagtnary and real  p a r t s  o f  t h e  index r e f r a c t i o n  f o r  i c e ,  r is 
t h e  r a d i u s  of t h e  ice p a r t t c l e s  (assumed to  b e  e f f e c t i v e  s p h e r e s )  and N(r) is t h e  s i z e  d i s t r t b u t t o n .  
I f  it is assumed t h a t  the  s l z e  d i s t r i b u t i o n  can be  expressed  as  a l i n e a r  combinat ion of Caussian 
d i s t r i b u t i o n s ,  1.e.. 
N(r) = E nlrl(o)l exp { 
i= 1 2Oi2 
theii e s t l m a t e s  of t h e  size d i s t r i b u t i o n  might be o b t a i n e d  by s o l v i n g  f o r  t h o s e  
whlch b r s t  s a t i s f y  t h c  ohservetl a b s o r p t i o n  covf f l c l e n t  s p e c t r a .  Thls r e s e a r c h  
f u t u r e  paper .  
Acknowledfiments 
r (01 and u values  
14 t h e  s u b j h c  o f  a 
Ke thank  11. Wnolf of  CIMSS for h t s  a s s i s t a n c e  w i t h  t h e  HIS d a t a  reduct ion .  J. Splnl i i rne 
(NASAKSFC) g r a t i c i i s l y  provided t h e  ER2 l lt lar d a t a .  S p e c i a l  thanks  go t o  t h e  f l l g t i t  crew of the  NASA 
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Multi-spectral window radiance observations of Cirrus from satellite and aircraft, November 2, 1986 Project FIRE

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