NOAA-15 and 16 HIRS Visible Calibration Update

New HIRS visible coefficients for NOAA-15 and 16 are planned to go into
operations to correct the erroneous values currently used. Attached is a
copy of Dr. Changyong Cao's paper explaining the problem and correction.
Below is an excerpt from that paper giving you the highlights of the
changes. For most users this should be a no impact change, but if the
algorithm correction is not currently in your production software, you may
find that the new coefficients introduce a problem. We are currently
checking out the changes in the Product Systems Branch and will provide test
data to the user community as soon as this task is complete. In the
meantime, please evaluate your system to determine the impact of the change.
When the official notice and test data are provided to the user community, a
two week time period is planned for your check-out before the new
coefficients are included in operations. Please contact us if you will need
more than two weeks to test or effect changes to your system.

Excerpt from Dr. Cao's paper outlining the changes:
"In summary, the current operational calibration coefficients for the
visible channels of HIRS on NOAA-16 and –15 are incorrect. The new
coefficients derived using vicarious calibration should be used in the
operations, and the users of HIRS level 1b data should use the correct
formula (including the factor cos(solar zenith angle)) in retrieving the
albedo. The calibration uncertainty for the new coefficients is within
10%. This is poor compared to the 5% uncertainty achieved for the visible
channels of AVHRR. However, this accuracy is probably sufficient for cloud
detection and it represents a dramatic increase in accuracy from the current
operational calibration. Future updates to these coefficients may be needed
to increase the calibration accuracy and account for degradation over time
to meet the user’s needs." (This would be updates similar to the current
monthly visible calibration updates for NOAA-14.)

"It is found that the current operational calibration coefficients
overestimate the target albedo by ~127% for NOAA-15 HIRS, and underestimate
the target albedo by 14% for NOAA-16 HIRS."

Below are Calibration Coefficients for NOAA-15 and –16 HIRS taken from Table
1 of Dr. Cao's attached paper. Shown are the new vicarious values that
should replace the current operational values shown in column 2 and the
prelaunch values in column 3.

Vicarious Operation Prelaunch
Slope 0.03174 0.0674 0.02336
Intercept 47.1100 101.0635 36.0500

Vicarious Operation Prelaunch
Slope 0.02611 0.021354 0.021354
Intercept 62.3307 51.4852 51.4852

"To retrieve the albedo from the HIRS level 1b data, the following equation
should be used:
Albedo = (intercept + slope * C)/cos(z)
Where C is the Earth view count and z is the solar zenith angle for the
pixel. All items are available in the level 1b data. Leaving out the
cos(z) term in deriving the albedo, which is common among some users, is
simply wrong, and the error involved could be very large!
. . .

Another effect is that there would be a significant fall-off in albedo as
the solar zenith angle changes from one side of the scan to the other side,
which contradicts the very definition of surface albedo. This is especially
true for a terminator orbit satellite such as NOAA-15, where the solar
zenith angle varies greatly cross-scan."
* Emily D. Harrod
* Product Systems Branch
* Pre-Product Processing Group
* E/SP13, Federal Building #4, room 0318
* 5200 Auth Road
* Suitland, Maryland 20746-4304
* voice 301-457-5247
* fax 301-457-5199
* email

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