The OCO‐2 oxygen A‐band response to liquid marine cloud properties from CALIPSO and MODIS

Abstract

AbstractSpectra of reflected sunlight in the oxygen A‐band contain information about cloud properties such as cloud top pressure, optical depth, and pressure thickness. Here we show, for the first time, that high‐spectral‐resolution A‐band Orbiting Carbon Observatory‐2 (OCO‐2) spectra respond largely as simulated to the optical properties of water clouds over ocean during November 2015 (N = 184,318) using input cloud properties from the Moderate Resolution Imaging Spectroradiometer (MODIS) on Aqua and the Cloud‐Aerosol Lidar with Orthogonal Polarization on Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). In A‐band continuum channels the standard deviation of simulated minus observed radiance is ±37%. Selecting horizontally homogeneous clouds to mitigate three‐dimensional cloud effects and collocation error with the other satellites, the standard deviation of the residuals is reduced to ±18%. Using a look‐up table developed from simulations, OCO‐2's estimated cloud top pressure for low clouds (Ptop > 680 hPa) has a standard deviation of ±61 hPa relative to CALIPSO retrievals, and bias is dependent on assumed cloud pressure thickness, with our smallest value being −5 hPa. Versus MODIS optical depth, the standard deviation is ±9.0 and the bias is −2.0, although these shrink for clouds of τ < 30. These values include collocation error between the different satellites, meaning that they place an upper bound on the OCO‐2 retrieval uncertainty. The theoretical precision limit from OCO‐2's instrumental uncertainty is shown to be ±2.4 hPa in above‐cloud path and ±0.2% in optical depth for a two‐channel retrieval. Options for retrieving cloud optical depth, cloud top pressure, and pressure thickness are discussed in the context of a formal OCO‐2 cloud property retrieval.