Screening artefacts in infrared measurements

Applicability of infrared limb sounding is limited by high-reaching clouds. Since cloud signal in the spectra causes retrieval artefacts, cloud-contaminated spectra are rigorously discarded. Thus, measurements do no longer represent the atmosphere with its entire variability but only the cloud-free atmosphere. This causes a screening bias in averaged data. The figure shows the estimated H₂O cloud screening bias for MIPAS.

One could argue that less rigorous cloud screening would reduce the screening bias. The price to pay, however, would be a retrieval bias, which is considered much more harmful and less controllable. Contrary to the screening bias, a retrieval bias would affect also analysis of single (non-averaged) data. The effect of the screening bias in model-observation comparisons can easily be remedied by emulating the cloud screening in the model data as well: Only data of modeled cloud-free scenes should be used for calculation of the mean values.

For futher details of the analysis method, see https://doi.org/10.5194/acp-18-4187-2018.