FD On-Line Data and Images
Features of the ISCCP-FD Radiative Fluxes, Both Real and Distorted
These figures show the deviations of monthly mean values of various components of the radiative fluxes reported in the ISCCP-FD analysis (Zhang et al. 2004): (first row) upwelling LW at the top-of-atmosphere (TOA) for the tropics and globe and global upwelling and downwelling LW at the surface (SRF), (second row) upwelling SW at TOA for the tropics and globe and global upwelling and downwelling SW at SRF, and (third row) total net flux at TOA for tropics and globe and global total net flux at SRF and in the atmosphere (ATM). The left-most column of panels shows the TOA flux components for the tropics (±20 latitude) compared with the record from the ERBS instrument. All the other figures are global averages. The last figure shows the upwelling SW at TOA in terms of planetary albedo deviations.
|LAT: 20S - 20N||GLOBAL||GLOBAL||GLOBAL|
|UPWELLING TOA||UPWELLING TOA||UPWELLING SRF||DOWNWELLING SRF|
|NET TOA||NET TOA||NET SRF||NET Atmosphere|
In the first row, the slow increase of global upwelling LW flux at TOA from the 1980's to the 1990's, which is found mostly in lower latitudes, is confirmed by the ERBE-CERES records. However, the sudden increase in upwelling LW flux in late 2001 may be exaggerated because it is associated with a spurious change of the atmospheric temperatures in the NOAA operational TOVS products that are used in the calculations. The effects of spurious changes in the atmospheric temperature dataset can be seen even more clearly in the variations of the upwelling LW flux at SRF because the TOVS variations introduce spurious variations of the surface temperatures retrieved from the satellite infrared radiances in clear scenes. Most of the variations in this figure can be associated with changes in the TOVS product. Many, but not all, of the large variations of downwelling LW flux at SRF are also associated with changes in the atmospheric temperature dataset used in the calculations: the downwelling LW flux is more sensitive to these changes that is the upwelling LW flux at TOA. In particular the general slow decrease of LW flux from the beginning of the record until 2000 is not likely to be correct nor is the sudden increase in 2001.
In the second row, the prominent peak in upwelling SW flux at TOA at around 1992 is caused by the volcanic aerosols from the Mt. Pinatubo eruption; the large values at the very beginning of the record may be due to the remnants of the El Chichon volcanic aerosol. However, the magnitude of the upwelling SW flux perturbation is exaggerated in these calculations (as shown in the comparison with ERBS) because the aerosol effect is included explicitly by using the SAGE II stratospheric aerosol record in the calculations and implicitly through the ISCCP cloud properties, which were not corrected to account for the extra aerosol. The sudden decrease of upwelling SW flux at TOA near the end of 1988 and its generally lower values until the end of 1994 (except for the Pinatubo event) may indicate a low bias of visible radiance calibration for NOAA-11; calibration of this satellite against the other polar orbiters was made difficult by the Pinatubo event. There is a brief increase of upwelling SW flux at TOA at the end of 1994 that appears to be caused by a high bias in the visible radiance calibrations of some satellites for a few months when no "afternoon" polar orbiter was available to normalize the calibrations. The overall slow decrease of upwelling SW flux from the mid-1980's until the end of the 1990's and subsequent increase from 2000 onwards appear to caused, primarily, by changes in global cloud cover (although there is a small increase of cloud optical thickness after 2000) and is confirmed by the ERBS measurements. The variations of upwelling SW flux at TOA are shown in terms of planetary albedo variations in the last figure. The main variations in the upwelling SW flux at SRF are very muted versions of the features in the upwelling SW flux at TOA except for the sudden decrease in the last couple of months of data: this appears to be an error and is being investigated. The features in the downwelling SW flux at SRF mirror those seen in upwelling SW flux at TOA.
In the third row, the planetary cooling events (negative global total net flux at TOA) at the beginning of the record and around 1992 are very likely real, if exaggerated in magnitude, and caused by volcanic aerosol events. The cooling at the end of the record is probably exaggerated because of the inhomogeneity of the atmospheric temperature dataset used for the calculations. The overall slight rise (relative heating) of global total net flux at TOA between the 1980's and 1990's is confirmed in the tropics by the ERBS measurements and exceeds the estimated climate forcing changes (greenhouse gases and aerosols) for this period. The most obvious explanation is the associated changes in cloudiness during this period. The variations of the total net flux at the surface reflect the variations in the upwelling LW flux for the most part. Although some smaller magnitude features may be realistic, the larger variations are likely to be spurious (except for the Pinatubo decrease). The total net fluxes in the atmosphere (positive values imply heating) are also mostly distorted by the spurious changes in the atmospheric temperature dataset.
FD On-line Data
Monthly, seasonal and annual means of selected FD variables are available here for the period July 1983 to December 2004 and may be viewed as GIF images or downloaded to your local disk. If you are downloading data, please read the format description. Variables are net fluxes (difference of upward and downward fluxes) with positive values indicating energy input or heating and negative values indicating energy loss or cooling. Shortwave (SW) fluxes are for wavelengths from 0.2 to 5 microns and longwave (LW) fluxes are for wavelengths from 5 to 200 microns. Even though the wavelength ranges overlap, radiation in SW comes solely from the sun and in LW comes solely from Earth.
If you have downloaded any of these data sets prior to October 23, 2002, please note that the file formats for Equal-Area grid and/or Square grid have changed. Prior to the above date, boxes were numbered sequentially starting at 180W and the South Pole. The boxes of the current FD data are now numbered sequentially starting at 0 degrees longitude, beginning at the South Pole.
Below you will find two sets of data corresponding to Net Flux (left column) and Flux (right column). There are no restrictions when choosing the parameters of the Net Flux dataset. However, there are restrictions that apply to the flux dataset when choosing the spatial/vertical position "at Top of Atmosphere" (e.g. TOA) parameter as follows:
- The flux component "Downwelling" is restricted to the spectral range "SW": all flux variables have the same value, except for "Cloud Effect" whose values are zeroes and are therefore not available.
- The flux component "Downwelling" is not available for the spectral range "LW" since all values are zeroes for this variable.
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