AeroCom Prescribed Experiment

Introduction

The simulated AeroCom forcings show significant diversity that can partly be attributed to processes in the host models. To isolate the host model contribution we propose a simple AeroCom experiment with prescribed aerosol fields derived from the median of the AeroCom models.

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, AGU poster - AeroCom page down, will upload links later) 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. The magnitude of these effects cannot be estimated from the diagnostics of the first AeroCom forcing experiment.

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