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MULTIPLE REFLECTION: In the center of the North Water (Carey Islands),
the amount of low clouds in winter and spring is markedly higher than at other
stations around the Polynya (Ohmura,1978).Therefore, for some of the flights, a
temperature correction had to be introduced to compensate the effect of multiple
reflections of radiation between cloud bottoms and ground surface. The error in
infrared thermometry caused by a homogenous cloud layer is given by the follow-
ing equation:
=]
= E(T,) = (E(T) + E(T)r)(1-r_r)
where am is the reflectivity of the ground surface, Ye the reflectivity
of the cloud, E(T.) and E(T_) the engergies of black body radiation at surface
C
temperature T and cloud temperature Te: The transmissivity for a homogeneous
cloud was assumed to.be zero. The refléctivity for different cloud types in the
spectral band of 9.5 to 11.5 um was measured by the author in a separate study
and will be reported elsewhere. The resulting temperature correction for con-
ditions with overlying Stratus clouds is given for water and ice surfaces in
Fig. 4a,b. The temperature correction to be applied in the case of Altostratus
clouds are 20-40% smaller than the ones shown in Fig. 4a,b. For Cirrus clouds
the error can be neglected.
Temperature corrections AT in the spectral band of 9.5 to 11.5um
Lo compensate {or an overling homogeneous Stratus cloud (o^
a water surface (a) and an ice surface (b)
AE
Figures 4a,b:
a ) -10 o 10 20
> T T T T T T
2 E
_ =
10 [7.7 mio
tL 4
°C |.g -4
o ee 1
id
-5
- — 30
20 [- A»
+3
-$ AT*T, Tear
L [ STRATUS 7
ICE- SURFACE
AT*T, T, -5 ET! l3
1 | L 1 1 1
+120 ur STRATUS --30 240,0 -30 -20 10 40
TrarCC)
„3 WATER -SURFACE
[ T
1 x ; -40
10 i Tusc 20
40
With the application of all these corrections, it is possible to
obtain an accuracy of +/.15°C when using a well calibrated PRT-5 instrument
from an altitude of 300 m.
771
