Impact of Unresolved Cloud Variability on Cloud Retrievals for Sentinel 5 Precursor
Gimeno Garcia, Sebastian1; Loyola, Diego1; Schuessler, Olena1; Heinze, Rieke2; Trautmann, Thomas1
1DLR-IMF, GERMANY; 2Leibniz Universitšt Hannover, GERMANY

In order to guarantee high signal-to-noise ratios, the design of spectrometers aboard spaceborne platforms is a tradeoff between spectral and spatial resolution. Since molecular absorption is highly spectrally dependent, atmospheric composition instruments favor spectral over spatial resolution.

The TROPOMI instrument aboard Sentinel 5 Precursor (S5P) will have a spatial resolution of about 7x7 km2 at nadir, which clearly outperforms the resolution of previous atmospheric missions (320x40 km2 for GOME/ERS-2, 60x30 km2 for SCIAMACHY/ENVISAT and 80x40 km2 for GOME-2/Metop-A,B). However, inside a TROPOMI ground pixel there may still be a considerable amount of unresolved cloud variability.

Traditionally, the retrieval algorithms of cloud parameters using atmospheric composition instruments have exploited the spectral region around the oxygen A-band centered at 760 nm. The new version of the ROCINN_CAL cloud retrieval algorithm is also based on the A-band, with the novelty that it explicitly accounts for scattering within cloud layers.

In this work, we aim to study the sensitivity of cloud retrieval algorithms to unresolved cloud variability under different solar illumination conditions. For this purpose, we will simulate virtual measurements by a TROPOMI-like instrument in high-resolution cloudy scenes by means of Monte Carlo (MC) three-dimensional radiative transfer modeling. The synthetic MC spectra will be used as input by ROCINN_CAL and the retrieved parameters compared to the original ones.