Simple Optical Models for Diagnosing Surface–Atmosphere Shortwave Interactions

A technique is developed for diagnosing effective surface and atmospheric optical properties from climate model shortwave flux diagnostics. These properties can be used to distinguish the contributions of surface and atmospheric optical property changes to shortwave flux changes at the surface and top of the atmosphere. In addition to the four standard shortwave flux diagnostics (upward, downward, surface, and top of atmosphere), the technique makes use of surface-down and top-up fluxes over a zero-albedo surface obtained from an auxiliary online shortwave calculation. The simple model optical properties, when constructed from the time-mean fluxes, are effective optical properties, useful for predicting the time-mean response to optical property changes. The technique is tested against auxiliary online shortwave calculations at four validation albedos and shown to predict the monthly mean surface absorption with an rms error of less than 2% over the globe. The reasons for the accuracy of the technique are explored. Less accurate techniques that make use of existing shortwave diagnostics are presented and compared.