Evaluation of simulated clear sky O2 A-band measurements from GOSAT over different surfaces - a sensitivity study

We present a follow-up study on previous investigations of photon path lengths distributions in cloudy atmospheres using O₂ A-band measurements from the GOSAT TANSO-FTS satellite instrument (Kremmling, B., Investigation of photon path length distributions derived from oxygen A-band measurements of the GOSAT satellite instrument, PhD thesis, 2018). The original study used TANSO-FTS measurements of high spectral resolution over cloud covered ocean areas and compared them to radiative transfer simulations using the Monte Carlo model McArtim. The comparison is based on a fitting process, allowing spectral alignment as well as an adjustment of the simulated O₂ absorption. A systematic overestimation of 5-10% of the simulated O₂ absorption was found for the considered case studies. Despite the investigation of different sensitivity studies, the cause of this overestimation remained unresolved.

The consequence of these finding was the thorough investigation of clear sky measurements from TANSO-FTS between 2009 and 2015. The analysis includes the retrieval of the surface albedos and their comparison to those included in the TANSO-FTS data products as well as the subsequent fitting results of the simulated spectra. The analysis is applied to two datasets, both consisting of measurements passing different clear sky and quality criteria. Dataset 1 additionally has information from independent lidar measurements of CALIOP (CALIPSO) and is limited to the northern hemisphere due to the spatial and temporal collocation criteria. Dataset 2 has no independent collocation measurements but a more uniform distribution in space and time.

While the retrieved surface albedos compare well, an overestimation of the simulated O₂ absorption by about 5% is found for measurements over ocean. Good agreement is found for the land cases.

In order to better understand these observations, different sensitivity studies as well as fit settings are investigated. The sensitivity studies include parameters such as SZA, surface albedo, NDVI values as well as the polarization of the TANSO-FTS radiances. The presentation shows the outcome of these studies.