SCIAMACHY calibration: - direction of polarisation - reflectance verification

1
SCIAMACHY calibration - direction of
polarisation- reflectance verification

• Gijs Tilstra, Piet Stammes
• KNMI
• SCIAMACHY meetings, DLR, Bonn, 6-8 Oct. 2003

2
Direction of polarisation The assumed direction
of polarisation c ( 1/2 arctan U/Q) affects
SCIAMACHYs radiometric calibration. POLDER
measurements vs single Rayleigh scattering theory
3
for different wavelengths
Conclusion a single Rayleigh scattering
calculation is in general just as good or more
accurate than a measured value. The spread of
cPOLDER can be explained by POLDER measurement
errors. For SCIAMACHY, no PMD7 measurement is
needed! (Schutgens, Tilstra, Stammes and Breon,
submitted, 2003)
4
Reflectance verification Strategy l lt 400
nm compare with radiative transfer model,
because the surface albedo is low in the UV, and
Rayleigh scattering and ozone absorption are well
known. l gt 400 nm compare with colocated GOME
reflectance.
5
l lt 400 nm We focus on a particular, cloud-free
SCIAMACHY state over the Sahara (orbit 2509, 23
August 2002, state 910 UTC)
6
Comparison between reflectance reported by
SCIAMACHY and a polarised radiative transfer code
(DAK)
7
Relative error found
Tilstra et al., Envisat Validation proceedings,
Dec. 2002
8
l gt 400 nm Comparison between SCIAMACHY and GOME
reflectances for nadir pixels. Important step
Tiling GOME pixels (40x320 km) with SCIA pixels.
In blue 5 scans of GOME nadir pixels.
9
The small boxes are 0.25-sec (30x60 km) SCIA
pixels. The SCIA pixels are counted for their
coverage fraction.
10
Result for cloudfree Sahara state
11
Relative difference (SCIA-GOME) GOME problems
(a) etalon effect (solar irradiance is from Oct.
2001) (b) GOME degradation must be accounted for.
12
Cloudy scene over Atlantic Ocean, west of Africa
13
Result for cloudy ocean state
14
Relative difference (SCIA-GOME)
15
Comparison for both cases

16
Conclusions on GOME SCIA comparison

• The different solar angles for SCIA and GOME (due
  to 30 min difference in overpass time) causes
  only lt 1 difference in reflectance.
• For the various states in orbit 2509 the
  difference between SCIA and GOME reflectance is
  varying per state, and lies in the range of about
  0 to 20 .
• There does not seem to be a constant
  multiplication factor between the SCIA and GOME
  reflectances.
• GOME degradation complicates the absolute
  accuracy of the comparison.