Water vapor retrieval over ocean using near‐infrared radiometry

Abstract

A methodology is presented and evaluated to retrieve vertically integrated water vapor content over the ocean in any viewing geometry from POLDER data. The methodology is based on differential absorption by water vapor in the near‐infrared. Over the ocean, except in sun glint conditions, surface reflectance is small, and interaction between aerosol scattering and water vapor absorption is exploited to estimate total water vapor content. A sensitivity study performed with an accurate radiative transfer code (GAME) shows that a determination of total water vapor content is theoretically possible if the optical thickness and scale height of aerosols are known. An inaccuracy of 0.1 to 0.6 g cm−2 is expected, depending on water vapor content. The aerosol optical thickness δa at 865 nm is available from POLDER standard products. The aerosol scale height Ha can be estimated from the surface pressure derived from POLDER oxygen channels at 763 and 765 nm. A retrieval scheme is devised using parameterizations calculated using GAME for water vapor and surface pressure. The inversion scheme is applied to 20 POLDER orbits with observed water vapor contents ranging from 0.2 to 6 g cm−2. Comparisons with SSM/I estimates indicate a RMS error ranging from 0.21 to 0.58 g cm−2, depending on water vapor content, with a mean RMS error of 0.44 g cm−2. This result is slightly larger than deviations obtained between POLDER and SSM/I during the POLDER standard product validation over ocean in sun glint conditions (mean RMS error of 0.42 g cm−2).