An Examination of the Clear-Sky Solar Absorption over the Central Equatorial Pacific: Observations versus Models

Abstract

Measurements of downward surface solar radiation (global radiation) and albedo taken during the Central Equatorial Pacific Experiment (CEPEX) are used to obtain baseline estimates for two quantities concerning the radiation budget of the tropical oceans: 1) surface absorption of solar radiation in the central equatorial Pacific under cloud-free conditions, and 2) the corresponding absorption by the atmosphere. These values are then compared to two state-of-the-art radiative transfer models to determine if the models are accurately partitioning solar absorption between the atmosphere and the ocean. The paper develops an independent approach to obtain a clear-sky signal from 10-s resolution surface pyranometer data that is in excellent agreement with upper envelope methods. Over a diurnal average, the ocean absorbs 70.9% ± 1.3% of the solar radiation incident at the top of the atmosphere (TOA). The data, measured from ship and low-flying aircraft platforms, also yield the zenith angle dependence of the surface absorption. The clear-sky data are representative of dry regions east of the date line during March 1993. Likewise, a combination of tropopause albedo measurements from the ER-2 aircraft and Earth Radiation Budget Experiment (ERBE) clear-sky TOA albedos are used to find the absorption of solar radiation by the atmosphere (integrated from the surface to the TOA). Clear-sky TOA albedo is computed from the ER-2 tropopause measurements using a radiative transfer model and measurements of stratospheric aerosol and ozone. The computed TOA albedos agree with ERBE at about 6% for overhead sun. The diurnal average fractional atmospheric column absorption is 20.2% ± 1.6%. Two multispectral radiation models agree to within 5 W m?2 of the observed daily average clear-sky oceanic solar absorption when the atmospheric profile is constrained by measurements and the observed TOA albedo is used as a boundary condition.