The
current release of OMCLDRR is version 1.9.1.
OMCLDRR
v1.9.1 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 v1.9.1 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 the solar irradiance (at 1 AU)
does not 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.
To address increases in striping and drifts in the product, version 1.9.1
updates the soft calibration once per year. Therefore, at the start of a year,
there may be a discontinuity.
The
current version of OMCLDRR uses the spectral range 346-354 nm, where ocean
Raman scattering contribution is negligible. Cross-track variations in cloud
optical centroid pressure still exist but have been reduced as compared with
previous versions as improvements have been made in the characterization of the
ocean surface. In addition, we have applied an additional adjustment in v1.9.1
to remove remaining cross track biases.
Errors
in the retrieved cloud pressures increase rapidly as the effective cloud
fraction approaches zero. For effective 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.
Both
cloud pressures and fractions are affected by the so-called OMI row anomaly.
Please consult the OMCLDRR README file for more information.
Other algorithm
features are documented in the OMCLDRR README file.
Detailed information on the OMI row anomaly can be found here.
From
the OMI Data Products webpage:
Several row anomalies
have occurred in the recent past. These anomalies affect the quality of the
Level 1B and Level 2 data products. Please read this information carefully
prior to using OMI data. Please respect the dates mentioned as the anomalies
have occurred recently.
1) Anomaly 1: Since June
25th, 2007, cross-track scenes 53-54 (0-based).
2) Anomaly 2: Since May
11th, 2008, cross-track scenes 37-44 (0-based)
3) Anomaly 3: Since
January 24th, 2009, cross-track scenes 27-44 (0-based).
4) Anomaly 4: Since July
5th, 2011, cross-track scenes 42-45 (0-based)
5) Anomaly 5: Since Aug,
2011, cross-track scenes 41-45 (0-based)
6) Since then, the row
anomaly shows fairly small changes.
No corrections have been
implemented in the operational Level 1B and Level 2 data. It is recommended not
to use the affected cross-track scenes. Affected rows can be identified using
the XTrackQualityFlags and documentation found here.
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”.
V1.8.0
uses a new specification of the ocean surface reflectivity based on the
Cox-Munk approach with climatological monthly estimates of water leaving
radiance based on TOMS. The data field “XTrackQualityFlags”
(copied from the level 1b product) was added and the field “UVAerosolIndex”
was removed.
V1.9.0 updates the soft calibration coefficients yearly
V1.9.1 adds a cross-track bias adjustment