There is a visible and common artifact in much of the data set period, due to a
lack of coverage from geostationary satellites over an area centered on 70
degrees east longitude. This situation, commonly called the Indian Ocean gap,
occurs for all of the July 1983 - June 1998 time period, except for April 1988 -
March 1989, when data from the INSAT satellite is available to cover the gap.
In July of 1998, Meteosat-5 was moved over the gap area, eliminating the gap.
When the Indian Ocean gap occurs, the gap area is covered by polar orbiting
satellites, which can result in only one or two daytime overpasses per day.
Geosynchronous temporal sampling during the daytime is 3-5 times per daytime
depending upon the latitude (between 55 degrees North and South) and the time of
year. In addition, the limbs of the geostationary satellites which bound the
gap may suffer from spuriously high cloud amounts, due to large view angles.
This results in an abrupt drop-off of cloud fraction in the gap as compared to
the gap boundary.
Downward shortwave radiation is therefore higher in the gap, creating an
appearance of a flux discontinuity. The discontinuity approaches 60
Wm-2 on a 3-hourly basis, raising the uncertainty of the fluxes in
this region. Downward longwave radiation is lower in the gap, creating an
appearance of a flux discontinuity. For 3-hourly averaged fluxes, a
discontinuity of magnitude up to 20 Wm-2 for TOA and 5
Wm-2 surface fluxes may appear in the Indian Ocean gap region.
The SRB algorithms rely heavily on radiances and cloud properties from
ISCCP. Great care is taken by the ISCCP team to produce a well-calibrated,
homogeneous dataset from dozens of satellites over the course of several
decades. However, there are a few known issues and discontinuities in the
ISCCP products which are likely to be reflected in SRB products.
ISCCP uses a reference afternoon polar orbiter as a calibration standard for
the other satellites (geostationary and polar orbiting) in the
constellation. The afternoon orbiters are subject to orbital drift and are
replaced every few years. The dates of transition from one reference
satellite to the next are particular dates where small discontinuities in
the SRB products are possible. The transition dates are as follows:
For the October 1994-January 1995 timeframe there was no reference orbiter
available, so an interpolation between NOAA-11 and NOAA-14 is used. SRB
results show noticeable anomalies in this period, some of which are likely
artifacts of the calibration situation.
From NOAA-16 onward, the visible calibration is bi-linear, which has led to
some changes from the previous linear calibrations. The 2001 transition to
NOAA-16 is accompanied by fairly strong radiance increases over ice, which
has led to polar values of SRB surface albedo and cloud optical thickness
which are probably anomalously high, and surface downward fluxes which may
be too low. The NOAA-18 calibration appears to be raising overall reported
visible radiances, especially reducing the frequency of very low radiance
scenes, such as clear skies over ocean near the day-night terminator. The
effect on SRB products from 2006 onward has been mainly to cause a jump in
surface albedo over much of the planet. Surface downward fluxes are less
Care should be taken when computing long term time series from SRB data,
with special notice taken of known transition dates, including those noted.
The operational TOVS algorithm changed on October 1, 2001. This affects the
surface skin temperature from the ISCCP project used in the primary longwave
algorithm. While the algorithm normally uses the GEOS-4 surface skin
temperature, the ISCCP surface skin temperature is used in place of the GEOS-4
temperature over land with total cloud amounts less than 50% and also under the
condition of ice or snow coverage greater than 80%. Long-term deseasonalized
trends of clear-sky and all-sky upward surface fluxes show an abrupt increase in
the fluxes at the time of the change that continues to the end of the data