Direct forcing

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AeroCom working group Direct forcing

Goals

To complement individual estimates of the direct aerosol radiative effects by multi-model / satellite-retrieval intercomparison studies.

Status and Progress

A number of new assessments of the direct aerosol radiative effects have been published since the 2006 AeroCom meeting, most notably the fourth IPCC assessment report (FAR). In Chapter 2 of FAR, Foster et al. estimate the annual-mean anthropogenic direct aerosol radiative forcing at the top-of atmosphere as -0.50±0.40 Wm-2, partly based on results of the AeroCom Forcing experiment (Schulz et al., 2006). As evident from the large error bar, the uncertainty in current aerosol radiative forcing estimates remains large and demands for future research.

Individual studies on the global direct aerosol radiative effects since AeroCom 2006 include work by Charlson et al. (2007) “On the climate forcing consequences of the albedo continuum between cloudy and clear air”, Chen et al. (2007) on “Future climate impacts of direct radiative forcing of anthropogenic aerosols, tropospheric ozone, and long-lived greenhouse gases”, Stier et al. (2007) on “Aerosol absorption and radiative forcing”, Mishchenko et al. (2007) on “Accurate monitoring of terrestrial aerosols and total solar irradiance - Introducing the glory mission”, Liu et al. (2007) on “Uncertainties in global aerosol simulations: Assessment using three meteorological data sets”, Jones and Christopher (2007) on “Statistical variability of top of atmosphere cloud-free shortwave aerosol radiative effect”, Bauer et al. (2007) on “Do sulfate and nitrate coatings on mineral dust have important effects on radiative properties and climate modeling?”, Koch et al. (2007) on “Linking future aerosol radiative forcing to shifts in source activities”, Naik et al. (2007) “On the sensitivity of radiative forcing from biomass burning aerosols and ozone to emission location”, Koch et al. (2007) on “Global impacts of aerosols from particular source regions and sectors”, Feng and Penner (2007) on “Global modeling of nitrate and ammonium: Interaction of aerosols and tropospheric chemistry”, Balkanski et al. (2007) on “Reevaluation of Mineral aerosol radiative forcings suggests a better agreement with satellite and AERONET data”, Schulz et al. (2006) on “Radiative forcing by aerosols as derived from the AeroCom present-day and pre-industrial simulations”, Bond et al. (2007) on “Limitations in the enhancement of visible light absorption due to mixing state”, Myhre et al. (2007) on “Modelling of nitrate and ammonium-containing aerosols in presence of sea salt”, Takemura et al. (2006) “Time evolutions of various radiative forcings for the past 150 years estimated by a general circulation model”, Stier et al. (2007) on the “Impact of nonabsorbing anthropogenic aerosols on clear-sky atmospheric absorption”, Ghan and Easter (2006) on the “Impact of cloud-borne aerosol representation on aerosol direct and indirect effects”.

Proposed AeroCom experiments

Introduction

Aerosol radiative forcing estimates of global aerosol models and satellite retrievals show considerable diversity that can partly be attributed to differences in the aerosol radiative properties but partly also to processes and assumptions in the host models (e.g. surface albedos, clouds). The contribution of these host model processes to the total forcing uncertainty is entirely unclear. We propose a simple AeroCom model/satellite intercomparison study with prescribed aerosol fields that will facilitate their quantification. [edit] Motivation

Even for the case of identical aerosol emissions, the simulated direct aerosol radiative forcings show significant diversity among the AeroCom models (Schulz et al., 2006). Our analysis of the absorption in the AeroCom models (Presentation at the 2006 AeroCom meeting , Poster at the AGU Fall Meeting 2006) indicates a larger diversity in the translation from given aerosol radiative properties (absorption optical depth) to actual atmospheric absorption than in the translation of a given atmospheric burden of black carbon to the radiative properties (absorption optical depth). The large diversity is caused by differences in the simulated cloud fields, radiative transfer, the relative vertical distribution of aerosols and clouds, and the effective surface albedo. This indicates that differences in the host model (GCM or CTM hosting the aerosol model) parameterizations contribute significantly to the simulated diversity of atmospheric absorption and consequently of the TOA forcing.

However, similar issues equally apply to models used to retrieve the total and anthropogenic aerosol radiative effects from satellite data. Recent retrieved forcing estimates, all based on the MODIS satellite data, show considerable diversity in the resulting aerosol radiative forcings. These diversities indicate that differences in host model (GCM or CTM hosting the aerosol modules or model to retrieve the aerosol effects from satellite data) parameterisations contribute significantly to the diversity of the simulated and retrieved aerosol radiative forcing. The magnitude of these effects cannot be estimated from the diagnostics of the first AeroCom forcing experiment.

To quantify the contribution of differences in host models (global aerosol models and satellite retrievals) to the estimated aerosol radiative forcing and absorption we propose a simple AeroCom experiment with prescribed aerosol fields. The simulated forcing variability among the models and satellite retrievals is then a direct measure of the host model contribution to the uncertainty in the assessment of the aerosol radiative effects.

Please find more information - and contribute to the discussion - on the AeroCom Prescribed page.

Data to look at

• Schwarz et al. (in press) on “Coatings and their enhancement of black-carbon light absorption in the tropical atmosphere”
• Bellouin et al. (submitted) on an “Updated estimate of aerosol direct forcing from satellite observations and comparison against the Hadley Centre climate model”
• Kinne et al. (submitted) on “Aerosol forcing with an AERONET touch”
• Myre et al. (in preparation) on the “Use of multi observations to estimate the radiativeforcing due to the direct aerosol effect“