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*Topic:*
Update to implement cloud clearing information and processing converter
change
*Date/Time(UTC) Message Issued:*
06 January 2006, 1700 UTC
*Satellite(s) Involved:*
NOAA-15, 16, 17, and 18
*Instrument(s) Involved:*
AVHRR
*Product(s) Involved/Affected:*
All Products/no impact
*Date/Time(UTC) of Initial Implementation:*
25 January 2006, Time TBD UTC
*Details/Specifics of Change:*
On January 25, NOAA would like to update our AVHRR level 1b processing so that the
reserved CLAVR-x (_*Cl*_ouds from _*AV*_HR*_R_* E*_x_*tended cloud mask) bytes will
be utilized and the announced 1B converter changes will go into operation. A new
flag will be added to the header record (utilizing spares) that will indicate when
the CLAVR-x processing is actually turned on or off. A brief explanation of the
CLAVR-x is provided below along with a list of the affected level 1b bytes. Also,
you will find a brief explanation of the 1B converter change. This change was
announced for testing on December 13, 2005. Test data that includes both changes is
provided on the *ftp://metroweb.nesdis.noaa.gov/pub/test/* anonymous ftp site. This
data is updated orbitally and will be available until the implementation is
complete. Though we believe this to be a no impact change, we are providing all
instrument level 1b data for your review. Please take some time to try the test data
and provide your comments or concerns to us before January 23rd.
The CLAVR-x provides the cloud mask present in the AVHRR level 1b datasets. It was designed to operate under all conditions including all orbital configurations. The goal of the CLAVR-x cloud mask is to provide a useful first guess cloud mask for all AVHRR applications that require it. It is constrained to perform this task in a timely manner to avoid delays in subsequent AVHRR applications. The CLAVR-x cloud mask in level 1b data is stored as a 2-bit value for each pixel . The values of the cloud mask are 0= clear,1 =probably clear, 2= probably cloudy and 3 = cloudy. The cloud mask is a series of spectral and spatial tests based on a long heritage of AVHRR cloud detection within NESDIS. In addition to providing the 2-bit cloud mask in the level 1b data, the results of each CLAVR-x test are also available in HDF files to allow for application specific optimization of the cloud mask. A draft of the CLAVR-x cloud mask theoretical basis document can be found at http://cimss.ssec.wisc.edu/clavr/clavrx_docs/clavrx_mask_atbd.pdf .
The CLAVR specific bytes in the level 1B file are outlined below. This information can be found in the NOAA-KLM Users' Guide (http://www2.ncdc.noaa.gov/docs/klm/html/c8/s83-1.htm) under the NOAA-N format information.
*Header Record*
*Field Name*
*Start Octet*
*End Octet*
*Data Type*
*Word Size*
*Number of Words*
*Scale Factor*
*Units*
*/CLOUDS FROM AVHRR (CLAVR)/*
CLAVR Status Bit Field
989
990
u
2
1
0
bits 15-1: <zero fill>
bit 0: CLAVR processing (0=off; 1=on)
<Zero Fill>
991
992
i
2
1
0
*HRPT/LAC Data Record*
*Field Name*
*Start Octet*
*End Octet*
*Data Type*
*Word Size*
*Number of Words*
*Scale Factor*
*Units*
*/CLAVR/*
* *
* *
* *
* *
* *
* *
* *
<Reserved> [CLAVR Status Bit Field]
14977
14980
u
4
1
0
bits 31-1: <undefined>
bit 0: CLAVR status (0=disable, CCM codes zero-filled; 1=enable)
<Reserved> [CLAVR]
14981
14984
u
4
1
0
<Reserved> [CLAVR CCM (Clear/Cloudy/Mixed) Codes (0=clear; 1=mixed clear; 2=mixed
cloudy; 3=cloudy)]
14985
15496
u
2
256
0
Word 1
bits 15-14: CCM code, FOV 1
bits 13-12: CCM code, FOV 2
...
bits 1-0: CCM code, FOV 8
Word 2
bits 15-14: CCM code, FOV 9
...
bits 1-0: CCM code, FOV 16
...
(set of 8 CCM codes per word)
...
Word 256
bits 15-14: CCM code, FOV 2041
...
bits 1-0: CCM code, FOV 2048
*GAC data Record*
*Field Name*
*Start Octet*
*End Octet*
*Data Type*
*Word Size*
*Number of Words*
*Scale Factor*
*Units*
*/CLAVR/*
* *
* *
* *
* *
* *
* *
* *
<Reserved> [CLAVR Status Bit Field]
4049
4052
u
4
1
0
bits 31-1: <undefined>
bit 0: CLAVR status (0=disable, CCM codes zero-filled; 1=enable)
<Reserved> [CLAVR]
4053
4056
u
4
1
0
<Reserved> [CLAVR CCM (Clear/Cloudy/Mixed) Codes (0=clear; 1=mixed clear; 2=mixed
cloudy; 3=cloudy)]
4057
4160
u
2
52
0
Word 1
bits 15-14: CCM code, FOV 1
bits 13-12: CCM code, FOV 2
...
bits 1-0: CCM code, FOV 8
Word 2
bits 15-14: CCM code, FOV 9
...
bits 1-0: CCM code, FOV 16
...
(set of 8 CCM codes per word)
...
Word 52
bits 15-14: CCM code, FOV 409
bits 13-0: <zero fill>
The following gives a description of the changes to the converter:
A problem with certain data values in 1b data sets has been reported by users. This
problem involves 1-, 2-byte or 4-byte data fields where a valid data value has the
most significant bit set, with all other bits 0. Therefore, values affected are
1-byte fields with a valid signed integer value of -128, 2-byte fields with a valid
signed integer value of -32768, or 4-byte fields with a valid signed integer value
of -2147483648. These values may be erroneously stored as 0 in the 1b. The 1b
fields which have been reported as occasionally erroneous are AMSU-B and MHS
radiometry values and HIRS minor frame bit flags and minor frame header elements.
See December 13 notice for more details.
*Contact Person(s) Name/Email/Phone Number for Questions:*
John.Sapper@xxxxxxxx 301-457-9014 ext 148 - CLAVR questions
Emily.Harrod@xxxxxxxx 301-457-5247 ext. 117 - ops questions
*Web site(s) that contain other relevant information (where applicable):*
http://www.osdpd.noaa.gov/PSB/PPP/PPP.html - History of level 1B notices
http://www.ssd.noaa.gov/PS/SATS/messages.html - All user notices
http://www.oso.noaa.gov/poes/index.htm - SOCC polar satellite status
http://www.oso.noaa.gov/goes/index.htm - SOCC geostationary satellite status
http://www.oso.noaa.gov/daily-news/index.asp
<http://www.oso.noaa.gov/daily-news/index.htm> - SOCC morning reports
-- ******************************************* * * Emily D. Harrod * NOAA/NESDIS/OSDPD/IPD * Computer Operations Branch * Pre-Product Processing Group * E/SP11, Federal Building #4, room 0318 * 5200 Auth Road * Suitland, Maryland 20746-4304 * USA * voice 301-457-5247 ext 117 * fax 301-457-5199 * email Emily.Harrod@xxxxxxxx * *******************************************
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