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Cloud
top
pressure/altezza
Temperatura Necessita il profilo, giorno/notte, emissivita’
Temperatura corretta Necessita il profilo, stima optical thickness, relazione
optical thickness emissivita’
A partire dal tipo (soggettiva e storica)
Ombra (illuminazione, cloud detection, shadow detection, calcolo). Solo di
giorno, sole allo zenith, solo bordi, no calibrazione, si risoluzione spaziale
(OK per VIS), complessita’ nel riconoscimento di forme, solo su superfici
riflettenti, nubi fine e senza contorni definiti (cirri)
Stereoscopia. >1 osservazione contemporanee (vento) (geostazionari:
perfettamente in fase, multiviewing: (A)ATSR (2), MISR (9), POLDER
(14)), geometria (no calibrazione), cloud detection, navigazione, cloud
recognition (difficile, limitato ai bordi), risoluzione spaziale, nubi fini e
senza contorni
Limb sounding + vede nubi fine, no calibrazione, problemi a scendere (<7
km), possibilie ambiguità con aerosols, numero di misure per orbita,
risoluzione spaziale (cirri sono estesi)
CO2 slicing
MLEV (minimum local emissivity radiance)
WV intercept method
Molecolar scattering (Raman scattering)
A-band assorbimento. Photon path length, solo giorno
Lidar
Cloud radar
OMBRA
http://www-research.ge.ucl.ac.uk/cloudmap/reports/firstreport.pdf
Stereoscopia
Stereoscopia
Hasler, BAMS 1981
Hasler, BAMS 1981
Hasler, BAMS 1981
IR-WV
Curva precalcolata
Misura clear sky
Misura broken cloudy
Stima Tb fully cloudy
MLEV
CO2Slicing
• Input
• Temperature and Water Vapor profiles
(representative of the FOV under
consideration)
• Observations for, at least, two channels
in the CO2 absorption band
CO2Slicing: Theory
Solving Equation:
Iob(n1)-Iclear(n1)
Iob(n2)-Iclear(n2)
=
Icloud(n1,pc)-Iclear(n1)
Icloud(n2,pc)-Iclear(n2)
The solution is given by the value of pc that minimizes the difference
between the right and left side
Pair Selection
Broad Band Spectrometer:
Interferometer:
Example
spectra
CO2Slicing: weighting function
space
MODIS
CO2 channels
Interferometer
CO2 channels
IR Retrieval Scheme for Clouds
Calibrated
data
Determination
of cloud
altitude,
thickness and
temperature
Cloud mask
Determination
of cloud
emissivity
Validation of
Products
Temperature and
water vapor
retrievals
in clear sky
FOVs
Retrieval of
microphysical
properties (optical
thickness, ice
water path,
particle size and
shape)
Cloud Emissivity
In
 In
N n  cloud
clear
In  In
observed



I
(I
clear
 I)

( I clear
clear
I  I cloud
I cloud
clear
I
 clear
cloud
clear
I
)( I
 I)
(I
 I cloud )



I
( I clear  I )

( I clear
I  I cloud
I cloud
clear
I
 clear
 I cloud )( I clear  I )
(I
 I cloud )
Minimum Local Emissivity
Variance (MLEV)
Observations between 750 and 900 cm-1
In
 In
n  cloud clear
In  In
observed
clear
 n  n 
n  min  

 n 
'
'
2
Retrieved
cloud at
9.5 km,
lidar
indicates
single layer
cloud
between
7.5 and 9.8
km.
lidar
The fact that the depth of solar Fraunhofer lines in scattered light is less than those observed in direct sunlight, was discovered by Shefov
[1959] [17] and Grainger and Ring [1962] [6] and is known as the ”Ring Effect” or ”Filling-in”. Several publications analysed this effect
and its origins, showing that rotational Raman scattering provides the dominant contribution to the Ring Effect [1, 10, 4, 5, 8, 3, 18]. The
majority of these studies however concentrated on cloud-free conditions.
Cloud radar
• INIZIO VECCHIE: VERIFICARE
Cloud top properties
(P. Menzel, R. Frey, K. Strabala, L. Gumley, et al. –
NOAA NESDIS, U. Wisc./CIMSS)



Cloud top pressure, temperature, effective emissivity
Retrieved for every 5x5 box of 1 km FOV’s, when at least 5 FOV’s
are cloudy, day & night
CO2 Slicing technique (5 bands, 12.0-14.2 µm)
– retrieve pc; Tc from temperature profile


– ratio of cloud forcing in 2 nearby bands
– most accurate for high and mid-level clouds
Previously applied to HIRS (NOAA POES, 20 km), GOES
sounder (~ 30 km)
Accuracy of technique ~ 50 mb
MODIS 1st satellite sensor capable of CO2 slicing
at high spatial resolution
S. Platnick, ISSAOS ‘02
CO2 slicing: theory
Solving Equation:
Iob(n1)-Iclear(n1)
Iob(n2)-Iclear(n2)
=
Icloud(n1,pc)-Iclear(n1)
Icloud(n2,pc)-Iclear(n2)
solution given by the value of pc that minimizes the difference
between the right and left side
CO2slicing:
weighting functions
Bands w/greater CO2 absorption
have weighting functions more
sensitive to high clouds
MODIS CO2 band weighting functions
Example spectra (~ 12.65-14 µm)
S. Platnick, ISSAOS ‘02
BT in and out of clouds for MODIS CO2 bands
- demonstrate weighting functions and cloud top algorithm
S. Platnick, ISSAOS ‘02