Observationally-Constrained Estimates of Effective Radiative Forcing from Aerosol Radiation Interactions

We will use fine-mode fraction from AERONET, MISR and MODIS, aerosol vertical profile from CALIPSO and cloud from CERES to constrain aerosol direct radiative forcing, in addition to MODIS AOD and AERONET SSA and ASY that were used in the previous semi-empirical direct forcing estimates.  We will therefore give the most observationally-constrained estimate of aerosol direct forcing and also evaluate its uncertainty.  Our observationally-constrained aerosol direct forcing estimate will then be used to estimate the rapid adjustment (i.e., semi-direct forcing) and quantify its uncertainty by incorporating our aerosol direct forcing estimates into several GCMs, including NCAR CAM4, GFDL AM2.1, GISS ModelE, and MPI ECHAM5.  Furthermore, we will investigate the reasons for the large range of existing semi-direct forcing estimates, by evaluating the difference between the semi-direct forcing from our observationally-constrained aerosols versus model default aerosols.

PROGRAM MANAGERPRINCIPAL INVESTIGATORFUNDED BY
Marc Pitchford Eddy Chung NSF

Project Title: Observationally-Constrained Estimates of Effective Radiative Forcing from Aerosol Radiation Interactions

PROJECT  DESCRIPTION

Objectives

Significantly increase the use of observations to estimate the effective radiative forcing from aerosol radiation interactions (ERFari = direct forcing + rapid adjustments), and thus reduce the uncertainty in estimated ERFari.

Determine the most observationally-constrained estimate of aerosol direct forcing (using AERONET, MISR, MODIS, CALIPSO and CERES) and evaluate its uncertainty.

Use our observationally-constrained estimate of aerosol direct forcing to estimate the rapid adjustment (semi-direct forcing) and quantify its uncertainty using a suite of GCMs.

Expected Significance

  • Reduce the uncertainty in estimated ERFari.
  • Validate model estimates of ERFari and improve regional-scale climate simulations.
  • Reduce the uncertainty in climate sensitivity and future global warming projections.

Introduction

The burden of tropospheric aerosols has increased since preindustrial times due to anthropogenic activities (Smith et al., 2004; Bond et al., 2007). These aerosols affect climate in several ways, including scattering and absorbing solar radiation. Sulfate aerosols, which reflect solar radiation, cause cooling of the earth-atmosphere system. Conversely, black carbon (BC), the strongest absorbing aerosol species, primarily absorbs solar radiation which warms the atmosphere (Ramanathan et al., 2001). Most aerosol-climate studies have focused on these direct effects, yielding a radiative forcing of −0.35±0.5 W m−2 in the latest IPCC report (Boucher et al., 2013). As Figure 1 shows, the uncertainty range (i.e., 1.0 W m−2  in the 5th  IPCC report) has persisted in the last 25 years.

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