The CO2 slicing method is generally recognized as the most accurate means of inferring cloud altitude from passive infrared radiance observations. The method is applicable to semi-transparent and broken clouds. During the cirrus FIRE and COHMEX field experiments, CO2 channel radiance data suitable for cloud altitude specification were achieved from moderate spectral resolution satellite sounders (NOAA-TOVS and GOES-VAS) and from a High spectral resolution Interferometer Spectrometer (HIS) flown on the NASA U2/ER2 aircraft. Also aboard the ER2 was a down-looking active lidar unit capable of providing cloud top pressure verifications with high accuracy. A third instrument, the Multispectral Atmospheric Mapping Sensor (MAMS) provided 50 meter resolution infrared window data which is used wtih radiosonde data to verify the heights of middle and low level clouds. Comparisons of lidar and MAMS/radiosonde ground truth cloud heights are made with those determined from: high resolution (0.5/cm) HIS spectra, HIS spectra degraded to the moderate resolution (15/cm) of the VAS/TOVS instruments, and spectrally averaged HIS radiances for individual pairs of VAS spectral channels. The results show that the best results are achieved from high resolution spectra; the RMS difference with the ground truth is 23 mb. The RMS differences between the infrared radiance determination and ground truth increase by 35 percent when the spectral resolution is degraded to the moderate spectral resolution of the VAS/TOVS instruments and by 52 to 183 percent, depending upon channel combinations, when only two spectral channels at VAS/TOVS spectral resolution are used

Frey, Richard

Smith, William L.

English

NASA Technical Reports Server

Cloud A l t i t u d e  Determinations from I n f r a r e d  S p e c t r a l  Radiances 
William L. Smith and Richard k e y  
Cooperative I n s t i t u t e  f o r  Heteorological S a t e l l i t e  S tudies  
1225 &st Dayton S t r e e t  
Hedison, Wisconsin 53706 
Abstract  
c loud a l t i t u a e  from pass ive  i n f r a r e d  rad iance  observat ions.  
t ransparent  and broken cloud. 
rad iance  d a t a  s u i t a b l e  f o r  cloud a l t i t u d e  s p e c i f i c a t i o n  were achieved from moderate s p z c t r a l  resolu- 
t i o n  sa te l l i t e  sounders (NOM-TOVS'@ and GOES-VAS5) and from a High s p e c t r a l  r e s o l u t i o n  I n t e r f e r o -  
meter Spectrometer (HIS) flown on t h e  NASA U2fER2 aircraf t .  
a c t i v e  l i d a r  u n i t e s 7  capable  of  providing cloud t o p  p r e s s u r e  v e r i f i c a t i o n s  w i t h  high accuracy ( ~ 5  
mb). 
r e s o l u t i o n  i n f r a r e d  %indou" d a t a  which i s  used w i t h  radiosonde d a t a  t o  v e r i f y  t h e  h e i g h t s  o f  middle 
and low l e v e l  clouds. 
h e i g h t s  are made with those determined from: HIS s p e c t r a ,  (b) HIS 
s p e c t r a  degraded to  t h e  moderate r e s o l u t i o n  (15 cm f of  t h e  VASfrOVS ins t rwnents ,  and (c )  s p e c t r a l -  
l y  averaged HIS rad iances  f o r  ind iv idua l  p a i r s  of VAS s p e c t r a l  channels. The r e s u l t s  show t h a t  b e s t  
r e s u l t s  are achieved from high r e s o l u t i o n  s p e c t r a ;  t h e  RnS d i f f e r e n c e  with t h e  "ground t ru th"  is 23 
mb. The RHS d i f f e r e n c e s  between t h e  i n f r a r e d  rad iance  determinat ion and ground t r u t h  i n c r e a s e  by 
35% when t h e  s p e c t r a l  reso lu t ion  is degraded to  t h e  moderate s p e c t r a l  r e s o l u t i o n  of t h e  VAS/TOVS 
instruments  and by 52% to  18B, depending upon channel combination, when only  two s p e c t r a l  channels  
a t  VAS/TOVS s p e c t r a l  reso lu t ion  a r e  used. 
i a s  
Ihe * T O  s l i c i n g "  method ' ' is g e n e r a l l y  recognized as t h e  most a c c u r a t e  means o f  i n f e r r i n g  
The method is a p p l i c a b l e  to  semi- 
During t h e  cirrus-FIRE and C O W X  f i e l d  experiments, CO channel  
Also aboard t h e  ER2 was a down-looking 
A t h i r d  instrumente, t h e  M u l t i s p e c t r a l  Atmospheric Mapping Sensor (HAMS) provided 50 meter 
In t h i s  paper, comparisons of l i d a r  and MAMSfradiogpnde "ground t ru th"  c loud 
(a)  ht  h r e s o l u t i o n  (0.5 cm 
1. Int roduct ion  
The working equat ion of the  C02 s l i c i n g  method2 is 
P a B  (v, T(p)) 
f 'T(v,P) d l n p  
a Inp - C(PC) I1 (VI - I2 (VI I 
I1 (v,) - I* (v2) 
pC 
P a B  (vo, T(p)) 
f (Vo, p) d lnp  
pC a lnp  
(1) 
where I is an observed s p e c t r a l  rad iance  a t  wavenumber (or s p e c t r a l  channel)  V, s u b s c r i p t s  1 and 2 
r e f e r  t o  geographica l ly  independent f i e l d s  of  view, and t h e  s u b s c r i p t  o r e f e r s  t o  a r e f e r e n c e  
wavenumber (or s p e c t r a l  channel). 
cloud e m i s s i v i t y  is t h e  same for wavenumbers V and V 1, 
ponding t o  t h e  temperature T, p is pressure ,  P is ear face  pressure ,  and T(V,p) is t h e  atmospheric  
t ransmi t tance  of  t h e  atmosphere between t h e  inetrument  and t h e  pressure  p. The cloud t o p  p r e s s u r e  
(p ) which y i e l d s  t h e  minimum d i f f e r e n c e  between the le f t -hand  s i d e  and t h e  r ight-hand s i d e  o f  Eq. (15 is t h e  cloud pressure  estimate. 
emiss iv i ty .  
The CO 
d i f f e r e n t  c?oud amounts andfor  c loud e m i s s i v i t y  w i t h  t h e  cloud e m i s s i v i t y  assumed t o  be independent 
of  s p e c t r a l  wavenumber. In p r a c t i c e ,  it is at tempted t o  u t i l i z e  a *lclear" a i r  rad iance  which is 
r e p r e s e n t a t i v e  of  t h e  cloudy a r e a  of i n t e r e s t  toge ther  with a cloudy rad iance  to d e f i n e  t h e  l e f t -  
hand s ide  of  Eq. (1). m e n  using a c l e a r  sky re ference ,  t h e  s igna l - to-noise  ra t io  is maximized and 
t h e  cloud he ight  need only be cons tan t  over  a s i n g l e  f i e l d  of  view. 
2. The HIS Instrument 
It is assumed t h a t  t h e  cloud r a d i a t e s  a s  a "greybody" (f.e., t h e  
B(V,T(p)) is t h e  Planck rad iance  corres- 
Note t h a t  t h e  s o l u t i o n  is independent of cloud amount and cloud 
s l i c i n g  method assumes t h a t  one can f i n d  t w o  s p a t i a l l y  d i f f e r e n t  rad iances  due to  
9,lO 
The HIS is a Michelson i n t e r f e r o m e t e r  which measures upwelling r a d i a t i o n  (3.5-17.Oum) a t  
high s p e c t r a l  r e s o l u t i o n  ( A l A A  > 1000/1). The spectral ran e of t h e  instrument  is p a r f i t i o n e d  i n t o  
t h r e e  bands. Band 1 (9.1-17.0um or 600-1100 cm 1, band 2 t5.0-9.lUm or 4QO-2000 cm 1, and band 3 
(3.5-5.O~m or 2000-2800 cm 1. The maximum s p e c t r a l  r e s o l u t i o n  is 0.28 cm . One c a l i b r a t i o n  c y c l e  
c o n s i s t s  of  t w o  cold blackbody views, two h o t  blackbody views and s i x  e a r t h  views followed by two 
more c o l d  and h o t  blackbody views. During 1986, t h e  HIS was flown aboard NASA U2fER2 a i r c r a f t  a t  a 
65,000 f o o t  a l t i t u d e  during t h e  F i r s t  ISCCP Regional Experiment (FIRE) and t h e  Cooperative Hunts- 
v i l l e  Meteorological  Experiment TCOHMEX). 
COIIMEX-aircr a f t f 1 i g h G .  
t i o n  of  t h e  Go2 s l i c i n g  method. 
f i e  i n s t r & n t  was n a d i r  viewing durT?jg t h e  FIRE-and 
The h igh  s p e c t r a l  r e s o l u t i o n  o f  t h e  HIS-In t h e  700-900 cm-' region makes i t  i d e a l  f o r  a p p l i c a -  
r e s o l u t i o n  t h e r e  is a v a i l a b l e  a very l a r g e  number o f  A t  a 0.5 cm 
1 35 
https://ntrs.nasa.gov/search.jsp?R=19910001161 2020-03-19T20:13:58+00:00Z
spectral "channels'*. 
F igure  la rhows a t y p i c a l  rad iance  spectrum me8surement from the HIS i n  the 600-1100 cm 
w i t h  t h e  spectral bands o f  the VAS superimposed. 
funct ions f o r  the HIS compared with those  f o r  the VAS. 
HIS is r e a d i l y  apparent  and enables  more a c c u r a t e  c loud a l t i t u d e  determinat ions w i n g  the C02 
As w i l l  be  shown, the h igh  r e s o l u t i o n  enables  a c c u r a t e  c loud h e i g h t  estimates. 
Figure l b  shows two CO channel weight ing 
The s u p e r i o r  v e r h c a l  r e s o l v i n g  power o f  t h e  
region 
_ _  
s l i c i n g  method. 
n 
Y 
w 
U 
< n w a 
I w 
I- 
v) 
U 
2 
3 
iE 200' ' 
E 
c3 
a n 1 1 1  I 
600 700 800 900 1000 1100 
320 
296 
272 
248 
224 LAT: 35.56 LON: 84.04 
Fig. la: HIS long wavelength band s p e c t r m  
with VAS s p e c t a l  i n t e r v a l s  shown. 
Fig.  lb:  Planck rad iance  weight ing func t ions  
a t  HIS and VAS r e s o l u t i o n s .  
- - - V A S  
HIS 
I ,  I I 
25 50 75 100 
PLANCK RADIANCE WEIGHTING FUNCTION 
!5 
3. Methodology 
The f i r s t  s t e p  i n  the a p p l i c a t i o n  of  t h e  C02 s l i c i n g  method is determining a r e p r e s e n t a t i v e  
%lear*l radiance.  
corresponding t o  a geographical  sample of  rad iance  s p e c t r a .  During P r o j e c t  FIRE, a down-looking 
l i d a r  uni t6 '7  was mounted on t h e  NASA U2/ER2 a i r c r a f t .  
cloudy regions as w e l l  as def in ing  t h e  cloud t o p  pressure  "ground t r u t h "  wi th  high accuracy (%S mb). 
An average of t h e  largest radiances from among those near  t h e  time of i n t e r e s t  was chosen as t h e  
%learq8 reference  radiance.  
lagpd f o r  each poin t  on t h e  spectrum between 675 and 920 cm 
cm . 
the  va lue  of  t h e  s p a t i a l  d i f f e r e n c e  a t  a re ference  wavenumber, chosen to  be 899.7 cm 
was compared to  t h e  cloud pressure  func t ion  (the r ight-hand side of  Eq. (1) evalua ted  from t h e  
sur face  pressure  t o  50 mb f o r  every p o i n t  on t h e  spectrum. 
la ted us ing  t h e  l ine-by- l ine  algori thm v1FASCOIT*13 (Clough et al . ,  1986) while  temperature  and water 
vapor p r o f i l e s  were obtained from rawinsonde soundings. 
vapor p r o f i l e s  could be  obtained by sounding r e t r i e v a l  from t h e  HIS spectra.12 
cloud pressure  func t ion  (p ) f o r  which a minimum d i f f e r e n c e  e x i s t e d  between t h e  le f t -hand  and 
r ight-hand s i d e s  of  Q. (15 was adopted as t h e  cloud p r e s s u r e  estimate. 
e s t i m a t e  was obtained f o r  every  poin t  on the  spectrum. 
@*Clear1@ is i n  quotes  s i n c e  i t  r e f e r s  t o  t h e  clearest f i e l d  o f  view of  those  
This provided a means of l o c a t i n g  c l e a r  and 
N e x t ,  d i f f e r e n c e s  between t h e  'Iplearll and cloudy rad iances  were calcu-  . The spacing between p o i n t s  was .275 
The r e s u l t  was normalized by d i v i d i n g  t h e  s p a t i a l  d i f f e r e n c e  a t  each poin t  on-she spectrum by . T h i s  r e s u l t  
Atmospheric t ransmi t tances  were ca lcu-  
(Al te rna t ive ly ,  t h e  temperature and water 
The va lue  of t h e  
I n  t h i s  way, a cloud h e i g h t  
A reasonable  procedure to  achieve a s i n g l e  c loud t o p  pressure  estimate from a range of frequen- 
cies is to form a weighted average over  t h e  frequency range. Thus, 
is used where t h e  weight %" r e p r e s e n t s  t h e  s e n s i t i v i t y  t o  cloud h e i g h t  and is given by t h e  der ivs-  
t i v e  of t h e  cloud p r e s s u r e  func t ion ,  C(Pc), with r e s p e c t  to the n a t u r a l  log of p r e s s u r e  (proport ion-  
a l  t o  he ight ) .  Mathematically, 
136 
.64 
.L(3 
700 720 7'w 760 780 890 
I I I I 
- 300 HB - 
- - 
SENS. 
500 HB 
703 720 7% 760 78.3 
I I 1  I I n 
700 720 7W 761) 700 830 
I.IAVEIJUI1BER ( 1/CI11 
Fig. 2: Sens i t i v i t i e s  of spec t r a l  radiance t o  var ia t ions  of cloud top pressure. 
7 37 
Infrared cloud pressures to be higher (low heights) than the  ground t ru th .  
seen t o  be spec t r a l  resolution dependent is due to  the reai-transparency of the cloud.' 
to Table I, the most accurate r e s u l t s  are achieved when the  determinations are based upon high 
resolution HIS spectra. The discrepancies increased by 358 when the  HIS spec t r a l  resolution is 
degraded t o  tha t  of the  VAS/rOVS sounding instrurwnts. Men the  determinations are made using p a i r s  
of Infrared spec t ra l  channels per ta in ing  to the VAS, as opposed to  using the  continuous spectrum, 
the discrepancies increase by as much as 183k, depending upon the channel combination used. I n  
general, bes t  r e s u l t s  f o r  VAS channel combinations are achieved when a *%wind&' channel is w e d  as a 
reference; the discrepancy i n  t h i s  case is less than a fac tor  of two poorer than tha t  achieved with 
high resolution HIS spectra.  
Table 1. 
dats with l i d a r  and HAHSfradiosonde ground truth.  
This tendency which is 
Referring 
RMS di f fe rences  (mb) between cloud top pressure estimates from HIS and 'simulated VAS 
RMS&ror (mb) 
H i  h Cloud Middle Cloud Low Cloud A l l  Clouds' 
- + 3 i V - n w - - 7 R f i 5 T - 7 = 5 T  
Method -
HIS 
HIS 
VAS 
VAS 
a t  high reso lu t ion  26 13 26 
a t  VAS/TOVS reso lu t ion  31 14 42 
1 
Channels (simulated from HIS, window channel reference) 
3/8 46 18 NA 
4/8 49 20 NA 
5 /82  34 31 44 
3/4 45 54 NA 
3/ 5 2  48 17 NA 
4/5 54 74 NA 
1 
Channels (simulated from HIS, near channel reference) 
23 
31 
35 
37 
37 
50 
36 
65 
m e  half bandwidtfi spept re l  limits of VAS channels-*re: 
%onsidered t o  be the channel combination. 
'Computed from the  average of the e r r o r  variance for  each height category. 
(1) Channel 3, 695-711 ~m-~ ;1 (2 )  
Channel 4, 706-724 cm ; (3) Channel 5, 742-758 cm ; and, (Ir) Channel 8, 822-960 cm . 
C 
1w 
400 
c -  
< 
f 
eo0 
VAS W l K C O l  R E F  VAS CCZ R E F  
Fig. 3: Sca t t e r  diagrams of ground t r u t h  ( l idar)  versus CO s l i c i n g  inf ra red  cloud pressure heights 
(mb) for  (a) HIS high resolution spectra,  (b) moderate (VAS? reso lu t ion  spectra,  (c) VAS channels 
us ing  a window channel ( 8 )  as a reference, and (d) VAS channels using a COP channel ( 5 )  as a 
reference. 
138 
kknawledgnenta  
We wish to  thank  Henry Revtrcmb,  Rarold M. W o l f ,  H. B. H o w e l l ,  and R.-L. Hrung for t h e i r  
a s e i s t a n c e  wi th  t h e  HIS data processing requi red  f o r  thirr restarch. We g r a t e f u l l y  acknowledge Jim 
S p i n h i m e  f o r  provid ing  t h e  l i d a r  data used to  v e r i f y  t h t  rad iometr ic  c loud b i g h t  d e t e m i n a t i o n s ,  
Our thanks to Laura Beckett and Steve Ackerman for t h e i r  h e l p  i n  the prepara t ion  of t h i s  manuscript. 
Zhis research  was supported by NASA under contract NAS1-18272. 
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14 
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139 


Cloud altitude determination from infrared spectral radiances

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