Validation of SCIAMACHY polarisation using POLDER as a reference

(bonus: reflectance validation)

Tiger Team Meeting 23, SRON, 23 May 2006

POLDER• imager• CCD detector matrix• bandfilter carousel• wide FOV lens

• resolution: 6 × 7 km2

• polarisation at 443,

670, and 865 nm• well-calibrated

443, 443, 490, 565, 670, 763, 765, 865, 910 nm

Intercomparison: SCIAMACHY versus POLDER look for collocated data (position, time) with identical viewing geometry

a. POLDER image (reflectance @ 670 nm) + previous images

b. POLDER viewing angles (in green). In black: SCIAMACHY nadir pixels of 1.0 s IT

c. POLDER viewing angles for data where the viewing geometry is similar to SCIAMACHY’s

d. For the same measurement data again the reflectance @ 670 nm. In white: SCIAMACHY nadir pixels of 0.25 s IT

Data can be compared

Reflectance: spectral averaging --- IT = 0.25 s

--- IT = 1.00 s

transform SCIAMACHY reflectance into 9 broadband values for comparison with POLDER broadband reflectance measurements

Results for the reflectance: (SCIAMACHY s/w version 5.01)

× = cloudy

+ = cloud-free

Cloudy data are less reliable; fit results for cloud-free data

Slope ≠1 (known) calibration problem of SCIAMACHY

Reflectance: comparison with other “sources”

10–20% error in radiometric calibration….

Results (polarisation) (SCIAMACHY s/w version 5.01)

× + = water

× + = land

• PMD 2: something is wrong, bad correlation

• PMD 3: looks better, but look at the slope

• PMD 4: a lot of serious errors

polarisation product of bad quality

much better than version 4.01, though

IT = 0.25 s

PMD 4: simple check

IT = 0.25 s

- zero-point problem

- “unphysical” values

Conclusions

• radiometric calibration SCIAMACHY contains errors• reflectance 10-20% too low in visible wavelength range• for the first time successful polarisation intercomparison• polarisation errors for PMD 2 and 3• polarisation retrieval of PMD 4 is in a very bad shape

EXTRA SLIDES: PMD 4 versus single scattering value: Q/I and U/I

IT = 0.25 s

IT = 1.00 s