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, Rc is given by

 Rc = eB(Ts) tatm + óõ B dt
where Ts is the NCEP sea surface temperature, and tatm is the atmospheric transmittance computed with the forward model using the NCEP model fields for temperature and water vapor.

 RsC = eB(Ts)
is the radiance if the atmospheric transmittance was zero, and

 Robs = eB(Tst) tatmt + óõ Bt dtt,
is the observed radiance, where the superscripts denote ``true'' quantities. (Rc - Rsc) is our estimate of the ``atmospheric'' part of Robs, which we subtract from Robs in order to estimate just the sea surface emission part of Robs, or eB(Tts). Tts(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 éê ë Robs - (Rc - Rsc)e ùú û = Tst(est)
where BT[] turns radiance into brightness temperature. (Rc -Rsc) 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 Tts(est) for the 900 and 2616 cm-1channels. The clear flag is derived from FOVS that return true for the following three tests:

1. Is Mitch Goldberg's L2 flag (fov_clear_flag) set?
2. | Tts,2616 cm-1(est) - Ts | < 1K
3. | Tts,2616 cm-1(est) - Tts,900 cm-1(est) | < 0.5K

Since we know the true sea surface temperature Test-#2 above is quite good. However, in practice Ts 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 TEX by TTH, version 2.86.
On 18 Jun 2001, 09:23.