ISCCP KNOWN AND FIXED ERRORS IN DATA PRODUCTS
This page is currently under construction
- The BT for METEOSAT-7 from 06/1998 to 09/2001 for IR channel has a couple of systematic errors.(have been corrected)
- The clear sky composite for 1.6 micron channel is biased a little low.
- Missing DX Geostationary 1st additional channel [ARAD(1)]
- Spurious satellite zenith angle dependence (artifact in Indian sector)
- Change in land surface pressures from TOVS
- Incorrect snow cover (has been corrected)
- Incorrect precipitable water amounts and surface temperatures (has been corrected)
- Spurious sea ice reports
- Flipped array indices in North Polar DX (has been corrected)
- Spurious land pixels in METEOSAT DX
- D2 versions 0,1,2 contain un-corrected METEOSAT-3 reflectances (has been corrected)
- Cloud top temperature/pressure error
- Change to daily atmospheric temperature profiles
- Error in monthly-mean atmospheric temperature profiles
- Missing GOES-7 visible data
- Error in normalized calibration of GOES-8/9 water vapor channel
- Systematic decrease in surface temperatures due to changes in NOAA operational sounder analysis
The BT for METEOSAT-7 from 06/1998 to 09/2001 for IR channel has a couple of systematic errors(have been corrected)
Problem: There appears to be a sudden shift of about -3K in the nominal temperatures for radiance values around 12 and 15 Wm^-2.
Data type: BT, B3, DX, D1,D2
Data range: June 1998 to Sep 2001
Satellites: METEOSAT-7
Description: The problem was present due to a typing error in the nominal calibration table. This is used for the creation of look up tables for temperature and radiance. The consequence is a -2K shift in temperature values just bellow 295K and 310K. This potentially affects ISCCP analyses that use METEOSAT-7 B3 data.
Thanks to Dr. Gary J. Robinson, NERC Environmental Systems Science Centre for notifying us of the problem. - March, 2004
Corrected data is on ISCCP download site.
Data type: BT, B3, DX, D1,D2
Data range: June 1998 to Sep 2001
Satellites: METEOSAT-7
Description: The problem was present due to a typing error in the nominal calibration table. This is used for the creation of look up tables for temperature and radiance. The consequence is a -2K shift in temperature values just bellow 295K and 310K. This potentially affects ISCCP analyses that use METEOSAT-7 B3 data.
Thanks to Dr. Gary J. Robinson, NERC Environmental Systems Science Centre for notifying us of the problem. - March, 2004
Corrected data is on ISCCP download site.
The clear sky composite reflectance for 1.6 micron channel is biased a little low (DX ADD3 [NCSREF]).
Problem: Day time snow/ice covered surfaces clear sky composite reflectance for 1.6 micron channel for NOAA satellites is biased a little low.
Data type: DX only
Data range: October 2001 through June 2005
Satellites: NOAA-16 from Oct 2001 to May 2003 and NOAA-17 from July 2002 to June 2005
Description:
A code error with the effect of biasing the 1.6 micron clear sky composite reflectance a little low. However all the retrievals based on this clear sky composite are not affected because the threshold had been modified to match the performance of 3.7 microns channel.
Data type: DX only
Data range: October 2001 through June 2005
Satellites: NOAA-16 from Oct 2001 to May 2003 and NOAA-17 from July 2002 to June 2005
Description:
A code error with the effect of biasing the 1.6 micron clear sky composite reflectance a little low. However all the retrievals based on this clear sky composite are not affected because the threshold had been modified to match the performance of 3.7 microns channel.
Missing DX Geostationary 1st additional channel [ARAD(1)]
Problem: The first additional radiance channel was not written properly into the DX data set for Geostationary satellites.
Data type: DX only
Data range: October 2001 through December 2004
Satellites: all Geostationary
Description:
Starting in October 2001 the first additional channel of radiance for each of the geostationary satellites was not correctly written into the DX data. All values are coded as 254 for that channel. This does not affect the ISCCP cloud data sets in any way. These additional radiance channels are carried along for the users convenience but are not used in the cloud algorithm. This does not affect users of D1 and D2 datasets. This does not affect users of polar oribter DX datasets.
Thanks to Bruno Six, Université des Sciences et Technologies de Lille for notifying us of the problem. - Feb, 2006
Data type: DX only
Data range: October 2001 through December 2004
Satellites: all Geostationary
Description:
Starting in October 2001 the first additional channel of radiance for each of the geostationary satellites was not correctly written into the DX data. All values are coded as 254 for that channel. This does not affect the ISCCP cloud data sets in any way. These additional radiance channels are carried along for the users convenience but are not used in the cloud algorithm. This does not affect users of D1 and D2 datasets. This does not affect users of polar oribter DX datasets.
Thanks to Bruno Six, Université des Sciences et Technologies de Lille for notifying us of the problem. - Feb, 2006
Spurious satellite zenith angle dependence (artifact in Indian sector)
Global maps of cloud properties retrieved in the ISCCP analysis often show an
artifact between longitudes of 50E and 100E. This feature is the most
noticeable manifestation of a spurious, but weak dependence of the retrieved
cloud properties on satellite zenith angle: as cosine of the satellite
zenith angle (MUE) decreases from 1.0 to 0.3, total cloud amount increases by
about 15-20%, cloud top temperature decreases by about 4-6K, and cloud
optical thickness decreases by about 2-3. Most results are obtained for MUE
values > 0.4 which reduces the overall effect somewhat. The dependence of
total cloud amount is discussed in detail in Rossow and Garder (1993: J.
Climate, 6, 2370-2393). This effect is thought to be caused in part by
geometric effects on broken cloudiness and by variation in detection
sensitivity to optically thin clouds; however, the total effect is not
completely understood. The dependences of cloud top temperature and optical
thickness are thought to be caused in part by the use of an inaccurate model
of cloud microphysics in the retrieval model; the introduction of an ice
crystal model in the new D-series data in place of the liquid droplet model
for cold clouds in the C-series reduced but did not completely eliminate this
dependence. Research is on-going to understand and develop a correction for
this effect: D1 data report the value of cosine of satellite zenith angle
that could be used to make corrections. - March 22, 1996
Change in land surface pressures from TOVS
Between May and June 1986, NOAA increased the global mean surface pressure
used to calculate topographic surface pressures (functions of topographic
height and surface temperature only) over land from 1000 mb to 1013 mb. This
change increases surface pressures over land by about 1%, but the maximum
value is still 1000 mb. Surface pressure over ocean remains 1000 mb. Since
the TOVS atmospheric temperature and humidity profiles are used in the ISCCP
analysis, this change may also produce a very small change in the cloud top
pressures of lower level clouds over land. The change is partly mitigated by
retaining 1000 mb as the maximum surface pressure in processing the sounder
data. No correction of the data is planned. - March 22, 1996
Incorrect snow cover
In the first release of the new D-series datasets, there were two errors
involving snow cover data. In the D1 and D2 datasets for January - April
1990 (Version Number 0), the correct snow cover is reported, but the snow
cover was erroneously eliminated in the cloud analysis. This error will
produce small increases in cloud amount (because of false visible radiance
detections), decreases in surface reflectance and increases in cloud optical
thickness in areas where snow cover varies during the month. In the D1 and
D2 datasets for 1992 (Version Number 0), the correct snow cover was included
in the cloud analysis, but incorrect values are reported in the datasets.
The affected D1 and D2 datasets have been corrected and replaced (Jan90-Apr90 and Jan92-Dec92 Version 1).
- April 12, 1996
Incorrect precipitable water amounts and surface temperatures
A processing code error caused monthly mean values of the
atmospheric water vapor column amount (precipitable water) to be
excessively high at pressures < 560 mb. Since the monthly mean values
are sometimes used to fill in when daily observations are missing, a
small fraction of the daily precipitable water amounts are
erroneous. These excessive values also cause retrieved surface and
cloud top temperatures to be overestimated. The effect is largest for
surface temperatures. The DX/D1/D2 datasets for April, May, August and
October 1992 (Version 0 or 1) are affected, the first two months more
so than the other two. The TOVS dataset for 1992, and the DX/D1/D2 datasets
for April, May, August, and October 1992 have been corrected and replaced.
- September 9, 1996
Spurious sea ice reports
Beginning in 1992, a new sea ice dataset is employed in the ISCCP
cloud analysis that comes from an analysis of satellite microwave
observations. One problem with this sea ice product is the presence of
spurious sea ice along coastlines. A procedure was applied to attempt
removal of these spurious sea ice reports, which was not entirely
successful. The procedure does eliminate isolated sea ice at the scale
of the microwave map if it is surrounded by open water; however, it
does not eliminate such sea ice if it is adjacent to the coast or if
there is a cluster of sea ice values. Both of these conditions may
occur occasionally along complicated coastlines. Therefore, some
spurious values of snow-ice fraction in the D1/D2 datasets may occur
at coastlines -- the occurrence frequency appears to be very low. This
problem affects ONLY the snow-ice fraction since the cloud analysis is
essentially insensitive to the presence of sea ice in near-coastal
water: in other words, the cloud analysis is already so conservative
in such a mixed area that the presence of sea ice makes no
difference. Therefore, the cloud and surface properties reported are
unaffected by this error. The error occurs only in 1992 and 1993
data. The sea ice and snow product will be revised later to reduce
this problem and the information concerning the frequency and
locations of its occurrence will be documented. - December 31, 1996
Flipped array indices in North Polar DX
In the DX dataset the location of each pixel is given by
latitude/longitude and by array indices (i,j). The latter are used
primarily when displaying the DX data as pictures. The NOAA polar
orbiter dataset is divided into three lower latitude sectors and two
polar sectors. The array indices given in the North Polar DX datasets
were incorrectly reversed so that the data, when displayed, appeared to
be flipped from West to East (as if viewed from inside the Earth
instead of from outside the Earth). The associated latitudes/longitudes
for these data were correct, however. This error affected North Polar DX
data for 1986, 1990-1992, but did not affect data in other years.
The affected DX data has been corrected and replaced in the archives.
- June 24, 1997
Spurious land pixels in METEOSAT DX
In the METEOSAT DX data there are 4 pixels just south of the
African coast at 0 degrees longitude that are mislabeled as land
pixels. This error arises when the calculated longitude is exactly 360
degrees in floating point representation, causing an overflow in a
look-up table. Although the cloud analysis is somewhat different over
land and ocean, these differences are reduced for such isolated
pixels, so that the effects on the reported cloud and surface
properties should be minimal. This error affects METEOSAT DX data for
1986, 1990-1993, but does not affect data in other years. No
correction of this data is planned. - December 31, 1996
D2 versions 0,1,2 contain un-corrected METEOSAT-3 reflectances
In D2 1992 versions 0,1,2, the special METEOSAT day/land
reflectance correction (see cloud data document page 53) was not
applied to METEOSAT-3 while it was in the GOES-EAST position
(July-December). D2 1992 Version 3 is now
available which includes this correction. - March 20, 1997
Cloud top temperature/pressure error
In the ISCCP D-series dataset, there is a small error in the cloud
top temperature and pressure corrected for the effects of
transmitted radiation from below transparent clouds. The error does
not affect the values obtained assuming blackbody clouds. The
correction procedure is iterative but, for clouds with visible
optical thicknesses between about 2 and 6, the iteration was
terminated prematurely. This error produces a slight overestimate
of cloud top temperature and pressure, which decreases as cloud top
temperature and pressure increase. For the highest altitude clouds,
cloud top pressures are biased high by about 20 mb on average,
equivalent to a high bias of cloud top temperature of about 2 degrees.
The processing error has been corrected for all ISCCP D data for 1994
and later; D data for July 1983 through 1993 still contain this error.
- April 30, 2001
Change to daily atmospheric temperature profiles
The way ISCCP determines the surface temperature and tropopause
temperature in the daily TOVS files has changed. We no longer report
the values included in the NOAA TOVS product. Instead
Tropopause/Surface temperature values are extrapolated, linear in
pressure, to be more consistent with the mid-level temperature
values of the profile. This may cause only slight changes to the
reported surface/tropopause temperatures. This change does not imply
any error in the original values. Rather, changes in the original
NOAA processing over the whole record lead to some inconsistencies,
so we have made this change to provide a more consistent temperature
profile. As of this date, only the data products from 1994 onwards
have been changed, both the ISCCP TV data product and the atmospheric
information included in the Stage D1 and D2 datasets. - April 30, 2001
Error in monthly-mean atmospheric temperature profiles
The monthly mean atmospheric temperature profiles produced for ISCCP
contain erroneous temperatures near the surface and tropopause. This
error is not in the original NOAA TOVS product, but was produced by
the ISCCP calculation of monthly average profiles. Over oceans, the
error leads to errors in the lowest layer temperature and
near-tropopause temperature of about 7-10 degrees, a high bias. Over
land, where topography truncates the profile, the error may be slightly
larger. This error appears only in the Monthly-Mean datasets used to
fill in missing daily data, indicated by the Origin Code = 3 in the
ISCCP TV and D1 datasets. The presence of this error does not
affect the retrieval of cloud top or surface temperatures by very
much, but can affect the cloud top pressure for very low-level
clouds, causing an underestimate of about 50 mb. The processing error
has been corrected for all ISCCP TOVS and D data for 1994 and later;
however, as of this date, the TOVS and D data for July 1983 through
1993 still contain this error. - April 30, 2001
Missing GOES-7 visible data
During the period from January 1994 through December 1995, when GOES-7
was operating in the WEST position, rather than its original EAST position,
there was an inconsistency between the identification of the times of the
three nighttime images (local midnight plus and minus one 3 hr interval)
and the actual local time at the subsatellite point. Since the original
times for the EAST position were retained in processing these data, some
actually available visible radiance data were dropped. A new version of
these data has been produced to capture this dropped visible data. The
new B3 dataset has version number = 2 for January through June 1994 and
version number = 3 for July 1994 through December 1995. - May 10, 2001
Error in Normalized Calibration of GOES-8/9 Water Vapor Channel
Starting with GOES-8, all radiance values are given as 10-bit counts.
For all channels, except the Water Vapor channel (6.7 micron wavelength),
these counts are multiplied by 0.25 to reduce them to 8-bit counts. For the
Water Vapor channel, however, the counts are multiplied by 0.5 because of the
limited range covered by the original values. In determining the tables
that employ the nominal calibration to convert these counts to brightness
temperatures, this scale factor is taken into account. However, a change in
this scale factor from 0.5 to 0.4 was introduced by the ISCCP Sector
Processing Center in the processing of the radiance data but not incorporated
in the calibration normalization procedure. Thus, although the normalization
of these radiances to the comparable channel from the HIRS instrument on the
NOAA polar orbiting satellites produced the correct brightness temperatures,
the first version of the normalization coefficients were too large to
compensate for the systematic low bias of the count values. This error effects
the Water Vapor channel data in the GOES-8 B3 data for the period February
1996 through December 1997 and GOES-9 data for the period February 1996
through June 1998. Some of the GOES-9 B3 data were re-processed to correct
a navigation problem (August and October 1996, February, April, August,
September and October 1997, February and April 1998); for these B3 data
(version number = 2), the calibration error has been corrected. In any
case, this error is corrected in the BT data version number = 2.
- May 10, 2001
Systematic Decrease in Surface Temperatures Due to Changes in NOAA Operational Sounder Analysis
In mid-1998, the NOAA operational sounder analysis, which produces the TOVS
atmospheric temperature and humidity profile products used in the ISCCP
analysis, was changed to a new, more physical method. However, this change
introduced a sudden and systematic change in the water vapor amounts in the
upper troposphere, which were reduced by as much as 20-30% in the uppermost
levels, a reduction of about 1-2 mm in total precipitable water amount in the
tropics and correspondingly less at higher latitudes. Other than the direct
effect on the long-term statistics of precipitable water amounts, this change
does not affect the ISCCP cloud parameters (tropical cloud top temperatures are
decreased by only about 0.2 K), but it does cause a systematic decrease in
surface temperatures, ranging from about 0.2 K at higher latitudes (depending
on season) to about 1 degree in the tropics. - October 14, 2002
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