Radiative flux and forcing parameterization error in aerosol-free clear skies

Abstract

Radiation parameterizations in GCMs are more accurate than their predecessorsErrors in estimates of 4 ×CO₂ forcing are large, especially for solar radiationErrors depend on atmospheric state, so global mean error is unknown.

Keywords: Parameterization; Radiation; Radiative forcing.

Figure 1

(left) Errors in top‐of‐atmosphere net downward broadband longwave flux under present‐day conditions and (right) error in forcing from CO₂ concentrations quadrupled from present‐day values for a variety of reference models (squares, including one high‐resolution k‐distribution model) and parameterizations used in climate models (circles). Calculations for four atmospheric profiles are ordered by column‐integrated water vapor ranging from 0.32 to 4.85 cm. Reference values are noted. Histograms along the right edge of each panel show the error by model type (black for reference models, dark grey for current parameterizations, and light grey for parameterizations used in CMIP5; normalized across model categories) in intervals of 1 (Figure 1, left) or 0.5 (Figure 1, right) W/m²) centered on 0. Reference models agree to within fractions of a percent in most calculations. Parameterization error is larger but still modest, with error in some parameterizations depending strongly on the water vapor path. Parameterizations that have been updated since CMIP5 are more accurate than their predecessors (compare filled circles and dark grey histogram bars to open circles and light grey bars.)

Figure 2

Errors in surface net downward (left) longwave and (right) shortwave flux in the present day as a function of errors in net top‐of‐atmosphere flux, along with (diagonal) isolines of constant error in atmospheric absorption inferred from fluxes at the boundary. All four cases are shown. Axis tick marks are every 1 W/m² and absorption isolines every 5 W/m². Errors in longwave boundary fluxes and radiative divergence are generally within a few W/m², especially for current parameterizations, while errors in shortwave fluxes and absorption are twice as large.

Figure 3

(left) Errors in surface net downward shortwave forcing from quadrupled present‐day CO₂ concentrations as a function of error in net top‐of‐atmosphere forcing, along with (diagonal) isolines of constant error in atmospheric absorption forcing. Axis tick marks are every 1 W/m² and absorption isolines every 5 W/m². For many parameterizations errors in top‐of‐atmosphere forcing are smaller than errors in surface forcing, implying errors in absorption forcing with impacts on the host model's hydrologic cycle. (right) Error in solar absorption forcing as a function of error in the present‐day estimates of solar absorption. Parameterizations have characteristic relationships among present‐day and forcing errors that outweigh the impact of atmospheric state. Errors in the present day are a poor predictor of errors in forcing.