OMI - Knmi

Preliminary comparison of OMI and
OMPS rotational-Raman cloud
optical centroid pressures
Alexander Vasilkov1, Joanna Joiner2,
and Colin Seftor1
1. Science Systems and Applications, Inc.
2. NASA Goddard Space Flight Center
OMI Science Team Meeting 18
March 11-13, 2014
1
Rotational-Raman scattering cloud
algorithm basics
• Cloud pressures (a.k.a. OCP) are retrieved from the highfrequency structure of TOA radiance caused by RRS.
Mixed Lambertian Equivalent Reflectivity (MLER) model
Fitting window 345.5-354.5nm. Reflectivity/Effective cloud fraction at 354.1nm
Fixed solar irradiance of Day1 to calculate normalized radiances
Lookup tables generated with a synthetic high-resolution solar spectrum (OMPS
solar data did not work well)
– Spline interpolation of TOA normalized radiance over the lookup table wavelength
grid. Linear interpolation did not work at all (OMPS less well sampled than OMI)
–
–
–
–
• Soft calibration is important: Over Antarctic Plateau the scene
pressure is assumed be equal to the surface pressure.
– Compute spectral residuals (observed - calculated radiances) at each swath
position
– Use calculated residuals (determined from Dec. 2012) to correct TOA radiances
A. Vasilkov, J. Joiner, C. Seftor, “First results from the Suomi National Polar-orbiting
Partnership (NPP) Ozone Mapping Profile Spectrometer (OMPS) nadir mapper
2
rotational-Raman scattering cloud algorithm”, submitted to AMTD.
OMPS solar flux
Expected solar flux based on
convolution of high resolution
spectrum with OMPS band passes
Actual measured OMPS solar flux
(Day 1)
Dots are for different cross track positions (total 36). Each cross track position has a
different set of wavelengths and a different band pass
The allowed pre-launch specification in radiance and irradiance calibrations are 8%
and 7% respectively. The normalized radiance errors are generally less than 2%, well
within instrument specification (Seftor et al. 2014)
3
Rotational-Raman scattering effect on
TOA radiances
OMPS less sensitive
to effects of
rotational-Raman
scattering (RRS)
than OMI owing to
its lower spectral
resolution
Good for SO2 & O3
retrievals, not so
good for RRS cloud
pressure retrievals
Percentage effect of RRS on OMPS radiances at the total ozone algorithm wavelengths
Wavelength (nm)
317.62
331.34
360.15
filling-in (%)
- 0.29
0.42
- 0.79
4
Soft calibration of TOA radiance
Corrections to
TOA radiances
are within ± 1%
which is quite
similar to OMI.
Depend on
wavelength and
cross-track
position
Spectral corrections that are applied to measured TOA OMPS
radiances as derived from data over Antarctica. Each curve is
for a particular OMPS cross-track position.
5
OMPS
Comparison of OMPS and OMI
Cloud pressure retrievals
of Jan 07, 2013 (ECF>0.05)
OMI
•
Most cloud OCP patterns
are same (northern
Pacific, Mexico, northern
Atlantic, northern China)
•
OMI OCP retrievals are
somewhat lower than
OMPS particularly in the
tropics
6
PDFs of effective cloud fraction
Comparison of PDFs
calculated for different
latitude & ECF bins.
A direct comparison
cannot be
accomplished because
of (1) different spatial
resolution, (2)
observations not at the
same time.
PDFs are practically same. Not necessarily expected
because of different sizes of the OMI and OMPS footprint.
7
Comparison of PDFs of cloud pressure
Southern mid-latitudes
Tropics
Northern mid-latitudes
•In general, OMI retrieves somewhat lower cloud OCPs than
OMPS does. Differences most pronounced in the tropics.
•Not understood yet. The use of experimental OMPS stray light
correction does not improve the agreement.
8
Comparison of PDFs of cloud
pressure, OMI O2-O2 added
Southern mid-latitudes
Tropics
Northern mid-latitudes
Differences between OMI RRS and OMPS cloud pressures appear
to be similar to differences between OMI RRS and OMI O2-O2
except for the differences in the tropics.
9
Total column ozone: OCP climatology
vs retrieved OCP
Ozone differences:
- mostly positive
- up to 5%
- well correlated with
bright clouds
Caused by deviation
of the actual OCP
from the monthly
climatology.
Reflectivity
10
OCP climatology
Comparisons with OMI
Retrieved OCP
Retrieved OCPs slightly improve OMPS – OMI comparison: the width
of the OMPS-OMI difference distribution becomes slightly less
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Conclusions
• OMPS less sensitive to rotational-Raman scattering
than OMI owing to lower spectral resolution
• OMPS retrieves somewhat higher cloud OCPs than
OMI, particularly in the tropics (not understood)
• PDFs of ECF retrieved from OMI and OMPS are
practically same, good confidence in the OMPS ECF
• Monthly gridded OMI OCP climatology captures
much of the variability in cloud OCP
• The use of actual cloud OCPs improves total column
O3 estimates
12
Backup
13
Wavelength shift
Wavelength shift as a function
of OMPS cross-track position
Wavelength shift as a function
of OMPS orbit scan position
- Typical OMI wavelength shift is much smaller: it is about
0.002 nm.
OMPS wavelength shift agrees well with that reported by
NOAA.
14
Surface pressure – Retrieved cloud
pressure: cross track dependence
OMPS
OMI
Bad cross track position #36 (seen on
the OMPS cloud pressure map).
15
Tropical total ozone:
Comparison of the use of retrieved OCP (OMI), OCP
climatology (NASA OMPS), & CTP (1st 1.5 years IDPS)
The use of cloud-top
pressures (red curve)
leads to unphysical
increase of total ozone
for bright deep
convective clouds
Monthly gridded
OMI OCP
climatology
captures much of
the variability in
cloud OCP
16
Stray light correction
Comparison of PDFs of cloud pressures retrieved with and
without the stray light correction shows insignificant
differences only.
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