AIRS

Frequently Asked Questions


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Q.  How were the 100 support pressure levels determined.  

A.  The support pressure levels were not chosen to match an altitude grid.
Instead, they were chosen to match the AIRS spatial resolution and the
AIRS rapid transmittance algorithm (RTA) used to forward calculate the
radiances from the physical state of the atmosphere (profiles of
temperature, water vapor, ozone and other trace gases).

Well before AIRS was launched in May of 2002, a pressure grid was
determined using a relation developed by our Science Team members from
NOAA. We started with 30 levels based on HIRS. Our RTA covers the
spectral band from 4.3 microns to 15.4 microns, and to ensure that the
calculated radiances were accurate to better than 0.1 K we ended up
with 100 levels. Line-by-line code was used to ensure accuracy in our
choice. 50 levels would have been OK over much of our spectral band,
but the 15 micron Q-branch required 100 levels for accurate
calculation of radiances.

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