Bringing Climate Models into Agreement with Observations of Atmospheric Absorption

Abstract

A comparison of the output of two data assimilation models with a quasi-global, multiyear set of monthly mean observations shows that the models underestimate the amount of solar energy absorbed in the atmosphere by 15?30 W m?2, out of a total of ?80 W m?2. In addition, observations show a much stronger dependence of absorption on column water vapor than models. Here the author analyzes absorption measured between two aircraft on a clear day during the Atmospheric Radiation Measurement (ARM) Enhanced Shortwave Experiment (ARESE) and finds a similarly strong dependence of absorption on water vapor. This common feature, in disparate types of observations, suggests the possible existence of appreciable continuum absorption in the water vapor spectrum. Various formulations of continuum absorption are tested against the aircraft observations and against the monthly mean dataset. In both cases, the addition of continuum absorption brings the models substantially closer to the observations, especially in the dependence of absorption on column water vapor. Of the two data assimilation models considered, introducing either an e-type continuum or one based on dimers?with the overall magnitude of the absorption coefficient adjusted to obtain agreement with the aircraft measurements?the overall discrepancy with respect to the monthly mean observations is reduced from 28 to about 17 W m?2 for the Goddard Earth Observing System-1 model, and from 18 to about 7 W m?2 for the National Centers for Environmental Prediction model.