Clear Flag Methodology
L1B Clear Flag Methodology
L. Strow
Updated: Jun 18, 2001
Document URL: http://asl.umbc.edu/pub/airs/exercises/clearmethod.html
Definitions
Our computed radiance, R_{c} is given by
R_{c} = eB(T_{s}) t_{atm} + 
ó õ

B dt 

where T_{s} is the NCEP sea surface temperature, and t_{atm} is the
atmospheric transmittance computed with the forward model using the
NCEP model fields for temperature and water vapor.
is the radiance if the atmospheric transmittance was zero,
and
R_{obs} = eB(T_{s}^{t}) t_{atm}^{t} + 
ó õ

B^{t} dt^{t}, 

is the observed radiance, where the superscripts denote ``true''
quantities.
(R_{c}  R_{s}^{c}) is our estimate of the ``atmospheric'' part of
R_{obs}, which we subtract from R_{obs} in order to estimate just
the sea surface emission part of R_{obs}, or eB(T^{t}_{s}).
T^{t}_{s}(est) denotes our estimate of the true sea surface temperature.
Therefore, our final estimate of the sea surface temperature from an
observed radiance is
BT 
é ê
ë


R_{obs}  (R_{c}  R_{s}^{c}) e

ù ú
û

= T_{s}^{t}(est) 

where BT[] turns radiance into brightness temperature. (R_{c} R_{s}^{c}) is very small for 2616 cm^{1}(few tenths of a K), but up to 4K
or more at 900 cm^{1}.
Clear Flag
I compute T^{t}_{s}(est) for the 900 and 2616 cm^{1}channels. The
clear flag is derived from FOVS that return true for the following
three tests:
 Is Mitch Goldberg's L2 flag (fov_clear_flag) set?
  T^{t}_{s,2616 cm1}(est)  T_{s}  < 1K
  T^{t}_{s,2616 cm1}(est)  T^{t}_{s,900 cm1}(est)  < 0.5K
Since we know the true sea surface temperature Test#2 above is quite
good. However, in practice T_{s} will have 0.5  1K errors, and
Test#3 might be quite important, although it is less sensitive.
Test#2 will help get rid of outliers that could make it through
Test#3.
File translated from
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by
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version 2.86.
On 18 Jun 2001, 09:23.