ISCCP KNOWN AND FIXED ERRORS IN DATA PRODUCTS

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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.

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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.

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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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