The current release of OMCLDRR is version 1.4.0.
OMCLDRR v1.4.0 is the updated release of the software designed to produce effective cloud pressure and fraction from OMI UV-2 channel data using rotational Raman (RR) scattering.
This section describes significant issues for the OMCLDRR v1.4.0 product.
The OMCLDRR software applies adjustments to the radiance measurements for wavelengths between 346 and 354 nm used by the OMCLDRR algorithm. The OMCLDRR algorithm uses the solar irradiances measured by OMI on a single day (orbit 2327 on 22 December, 2004), and assumes that neither the true solar irradiance (at 1 AU) nor the instrument calibration change with time. The solar irradiance is corrected for the Sun-Earth distance effect. Any significant change in the instrument calibration over time may introduce errors, including across-track biases (striping), in the product.
The current version of OMCLDRR uses the spectral range 346-354 nm, where ocean Raman scattering contribution is negligible. The sun glint issue still exists but is significantly reduced as compared with the previous version because of enhanced Rayleigh scattering in the spectral fitting window.
Errors in the retrieved cloud pressures increase rapidly as the cloud fraction tends to zero. For cloud fractions < 5%, the retrievals are flagged and the effective cloud pressure is not retrieved. Instead, an effective scene pressure is retrieved for diagnostic purposes.
Large transient events may produce unreliable cloud pressures at the scan position hit by radiation until the elevated dark current is corrected.
Absorbing aerosol above or within a cloud reduces the retrieved cloud pressure. The reduction depends on aerosol optical properties and cannot be specified a priori.
An average scene pressure is retrieved over snow/ice because the cloud fraction is assigned to 1.
Other algorithm features are documented in the OMCLDRR README file.
From the OML1B
README :
1) Anomaly
at binned row numbers (across-track positions) 53-54 in the UV2 channel (zero
based) in the global earth measurements (binning factor 8) causes anomalies in
the cloud fractions and pressures. Between 25 and 27 June 2007, a change
occurred in the OMI instrument in the rows mentioned above. A decrease in the radiance signal of about
20-30% under illuminated conditions is observed over the complete illuminated
part of the orbit. All columns (wavelengths) in the UV2 (and VIS) channels
are affected by this effect. The anomaly seems to be more or less stable after
27 June 2007. The data before 25 June
2007 is much less affected by the row anomaly.
The anomaly is currently under investigation to examine whether
correction for the effect can be implemented in the level 1b data. No correction
has been implemented at the moment. It is recommended not to use the affected
rows for now.
2) Anomaly
at binned row numbers (across-track positions) 38-41 in the UV2 channels (zero
based) in the global earth measurements (binning factor 8) causes anomalies in
the cloud fractions and pressures. Since 11 May 2008, a signal suppression has
been observed in the above rows for the UV2 (and VIS) channels, but only for
northern latitudes, at Solar Zenith Angles (SZAs) larger than 44.0 degrees for
the affected rows (though data at lower SZAs is also suspect). The anomaly is
currently under investigation to examine whether a correction for the effect
can be implemented in the level 1b data.
No correction has been implemented at the moment. It is recommended not to use the affected
rows at all latitudes and SZAs for now.
Release History
First release was: v1.0.0
V1.1.0.1 moved the fitting window from 395 nm in VIS channel to 350 nm in the UV-2 channel
V1.2.0 fixed problem with Processing Quality flag
V1.4.0 uses collection 3 radiances with a new soft
calibration and a surface albedo climatology from TOMS in place of the
previously fixed value of 15%. The data fields CloudPressure and CloudFraction
are removed. The relevant products are named CloudPressureforO3 and
CloudFractionforO3.