Probing the sensitivity of polarimetric O2 A-band measurements to clouds with emphasis on potential OCO-2 and GOSAT retrievals

Clouds play a crucial role in the Earth's radiative budget, yet their climate feedbacks are poorly understood. The advent of space-borne high resolution spectrometers probing the O2 A-band, such as the GOSAT and OCO-2 satellites, could make it possible to simultaneously retrieve several cloud parameters that play a vital role in the Earth's radiative budget, thereby allowing a reduction of the corresponding uncertainty due to clouds. The hyperspectral, polarimetric response of the O2 A-band to optical thickness, top height and droplet size has been studied, revealing a different sensitivity to each for the varying atmospheric absorption strength within the A-band. Cloud optical thickness finds greatest sensitivity in both intensity and polarization measurements at non-absorbing wavelengths. Cloud height had a negligible effect on intensity measurements at non-absorbing wavelengths, but finds maximum sensitivity at an intermediate absorption strength, which increases with cloud height. The same is found to hold for cloud geometric thickness, except that the sensitivity is weaker. Sensitivity to droplet size is generally weaker than to cloud optical thickness to top height at non-absorbing wavelengths and diminishes further with increasing absorption strength. It has been shown that significantly more information on droplet size can be drawn from multiangular measurements. Our results show that, in the absence of sunglint, the backscatter direction is richer in information on droplet size, especially in the glory and rainbow regions. It has been shown that I and Q generally have differing sensitivities to cloud parameters. Thus, accurate measurements of both orthogonal components Ih and Iv (as in GOSAT) are expected to contain more information than measurements of only I, Ih or Iv (as in the case of OCO-2).