Water Vapor Profiles from the Orbiting Carbon Observatory-2

Abstract:

The NASA Orbiting Carbon Observatory-2 (OCO-2) was launched in 2014 with the goal of precisely measuring carbon dioxide. In order to fit the measured near infrared radiances properly, other physical parameters must be included in the state vector. One such parameter is atmospheric water vapor, which has absorption features in OCO-2’s weak CO₂ and strong CO₂ bands. Up until now, the NASA Atmospheric Carbon Observations from Space (ACOS) retrieval simply solved for a multiplicative scaling factor on an a priori water vapor profile. However, preliminary synthetic studies have indicated that water vapor has 2-3 degrees of freedom in the vertical column and that solving for a properly constrained profile can lead to an error reduction in column-mean CO₂ of around 0.3 ppm. Here, we present results using real measurements and assess the error statistics. We also evaluate the performance of the retrieved water vapor profiles themselves by comparing them to Integrated Global Radiosonde Archive (IGRA) observations.