Host model uncertainties in aerosol radiative forcing estimates: results from the AeroCom Prescribed intercomparison study

Abstract

Simulated multi-model "diversity" in aerosol direct radiative forcing estimates is often perceived as a measure of aerosol uncertainty. However, current models used for aerosol radiative forcing calculations vary considerably in model components relevant for forcing calculations and the associated "host-model uncertainties" are generally convoluted with the actual aerosol uncertainty. In this AeroCom Prescribed intercomparison study we systematically isolate and quantify host model uncertainties on aerosol forcing experiments through prescription of identical aerosol radiative properties in twelve participating models. <br><br> Even with prescribed aerosol radiative properties, simulated clear-sky and all-sky aerosol radiative forcings show significant diversity. For a purely scattering case with globally constant optical depth of 0.2, the global-mean all-sky top-of-atmosphere radiative forcing is −4.47 Wm⁻² and the inter-model standard deviation is 0.55 Wm⁻², corresponding to a relative standard deviation of 12%. For a case with partially absorbing aerosol with an aerosol optical depth of 0.2 and single scattering albedo of 0.8, the forcing changes to 1.04 Wm⁻², and the standard deviation increases to 1.01 W⁻², corresponding to a significant relative standard deviation of 97%. However, the top-of-atmosphere forcing variability owing to absorption (subtracting the scattering case from the case with scattering and absorption) is low, with absolute (relative) standard deviations of 0.45 Wm⁻² (8%) clear-sky and 0.62 Wm⁻² (11%) all-sky. <br><br> Scaling the forcing standard deviation for a purely scattering case to match the sulfate radiative forcing in the AeroCom Direct Effect experiment demonstrates that host model uncertainties could explain about 36% of the overall sulfate forcing diversity of 0.11 Wm⁻² in the AeroCom Direct Radiative Effect experiment. <br><br> Host model errors in aerosol radiative forcing are largest in regions of uncertain host model components, such as stratocumulus cloud decks or areas with poorly constrained surface albedos, such as sea ice. Our results demonstrate that host model uncertainties are an important component of aerosol forcing uncertainty that require further attention

An AeroCom assessment of black carbon in Arctic snow and sea ice

“…We use data from 12 models contributing to the AeroCom phase I intercomparison project (e.g., Kinne et al, 2006;Schulz et al, 2006;Textor et al, 2006Textor et al, , 2007Koffi et al, 2012) and 13 models contributing to the more recent phase II project (e.g., Myhre et al, 2013;Samset et al, 2013;Stier et al, 2013). Table 1 summarizes the names and descriptions of these models.…”

“…The Aerosol Comparisons between Observations and Models (AeroCom) project was initiated for the aerosol observation and modeling communities to synthesize results in order to improve aerosol simulation skills Schulz et al, 2006;Textor et al, 2006Textor et al, , 2007Koffi et al, 2012;Myhre et al, 2013;Samset et al, 2013;Stier et al, 2013). A large number of global aerosol models have contributed to the AeroCom archive.…”

The Radiative Forcing Model Intercomparison Project (RFMIP): experimental protocol for CMIP6

“…3 are aimed at quantifying the degree to which parameterization weaknesses and errors impact estimates of radiative forcing -that is, the degree to which approximations and errors in radiative transfer parameterizations, as assessed against reference models, drives differences in radiative changes when the physical perturbation is fully specified. Although RFMIP seeks to understand this error for anthropogenic aerosols it is clear that the largest contributor to model differences in aerosol radiative forcing arises from distributions of anthropogenic aerosols (themselves the result of different prescriptions of aerosol precursors and processes), further modulated by varying distributions of clouds (Penner et al, 2006;Stier et al, 2013). As a result the temporal and spatial distribution of ERF caused by anthropogenic aerosols varies widely, greatly hindering attempts to identify and explain robust responses to aerosol perturbations including how anthropogenic aerosols affected 20th century climate.…”

A pathway analysis of global aerosol processes

“…Aerosol Comparisons between Observations and Models (AERO-COM) is a community effort to intercompare global aerosol models and evaluate them against observations Schulz et al, 2006;Textor et al, 2006Textor et al, , 2007Koffi et al, 2012;Myhre et al, 2013;Samset et al, 2013;Stier et al, 2013). This work has led to many valuable insights into what models can and cannot do.…”