The retrieval of carbon dioxide above clouds - sensitivity studies

In 2008 the Orbiting Carbon Observatory (OCO) is planned to be launched, aiming to determine the global distribution and trend of atmospheric carbon dioxide. Highly resolving spectrometers on board OCO are going to measure solar radiation reflected by the earth-atmosphere-system in three spectral bands in the near infrared. Measurements in two carbon dioxide bands (around 1.6 and 2.06 microns, respectively) will mainly contain information about the amount of CO2 in the column under consideration. Additional measurements within the oxygen A band around 763nm provide information about clouds and thick aerosol layers in the optical path. As only clear sky soundings can be used to assess the amount of CO2 over the whole atmospheric column, those data affected by clouds are separated and excluded from the operational data processing. We conduct several sensitivity studies, in order to determine the usability of these measurements for the retrieval of carbon dioxide above clouds. We study the radiance errors introduced by sensor noise, by the uncertainty of the spectral location of the OCO channels, and by approximations used in the forward radiative transfer. The results are compared to the found sensitivities for changes of CO2. This information will be used for observation error characterization in the framework of a planned optimal estimation algorithm for cloud properties and carbon dioxide above clouds.