A review of measurement-based assessments of the aerosol direct radiative effect and forcing

Abstract

Abstract. Aerosols affect the Earth's energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and, thereby, affecting cloud properties. However, large uncertainties exist in current estimates of aerosol forcing because of incomplete knowledge concerning the distribution and the physical and chemical properties of aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing from largely model-based to increasingly measurement-based. Our goal is to assess current observational capabilities and identify uncertainties in the aerosol direct forcing through comparisons of different methods with independent sources of uncertainties. Here we assess the aerosol optical depth (τ ), direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements supplemented by global chemical transport modelCorrespondence to: H. Yu (hyu@climate.gsfc.nasa.gov) (CTM) simulations. The multi-spectral MODIS measures global distributions of aerosol optical depth (τ ) on a daily scale, with a high accuracy of ±0.03±0.05τ over ocean. The annual average τ is about 0.14 over global ocean, of which about 21%±7% is contributed by human activities, as estimated by MODIS fine-mode fraction. The multiangle MISR derives an annual average AOD of 0.23 over global land with an uncertainty of ∼20% or ±0.05. These high-accuracy aerosol products and broadband flux measurements from CERES make it feasible to obtain observational constraints for the aerosol direct effect, especially over global the ocean. A number of measurement-based approaches estimate the clear-sky DRE (on solar radiation) at the top-of-atmosphere (TOA) to be about −5.5±0.2 Wm −2 (median ± standard error from various methods) over the global ocean. Accounting for thin cirrus contamination of the satellite derived aerosol field will reduce the TOA DRE to −5.0 Wm −2 . Because of a lack of measurements of aerosol absorption and difficulty in characterizing land surface reflection, estimates of DRE over land and at the ocean surface are currently realized through a combination of satellite Published by Copernicus GmbH on behalf of the European Geosciences Union. 614H. Yu et al.: Measurement-based aerosol direct forcing retrievals, surface measurements, and model simulations, and are less constrained. Over the oceans the surface DRE is estimated to be −8.8±0.7 Wm −2 . Over land, an integration of satellite retrievals and model simulations derives a DRE of −4.9±0.7 Wm −2 and −11.8±1.9 Wm −2 at the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that on average are smaller than the measurement-based DRE by about 30-40%, even after accounting for thin cirrus and cloud contamination.A number of issues remain. Current estimates of the aerosol direct effect ...

Climate Feedback on Aerosol Emission and Atmospheric Concentrations

“…Boundary layer stability impacts vertical mixing of aerosols from the surface, controlling heights of aerosol layers, which in turn influence the impacts of aerosols on clouds and radiation (e.g., [69]). Aerosol particles are transported from their surface sources to higher altitudes by turbulent mixing and convection.…”

A long-term study of aerosol modulation of atmospheric and surface solar heating over Pune, India

“…Many investigations have been conducted to characterise atmospheric aerosols and their radiative effects around the world, as recently reviewed by Yu et al (2006). Studies focusing on Asia include, among others, the Indian Ocean Experiment (INDOEX) (Ramanathan et al, 2001b), the *Corresponding author.…”

On the twilight zone between clouds and aerosols

“…This transition zone is currently classified as cloud-free by most observing systems, but since algorithms that invert aerosol properties are sensitive to cloud contamination, most retrieval algorithms will be biased toward data that is far from clouds and therefore, will miss some of the contribution of this zone. Furthermore, the possible effects of this zone are not included in chemical transport models used to estimate the aerosol direct forcing, possibly leading to discrepancies between models and measurements [Yu et al, 2006].…”