Aerosol Radiative Forcing of Asian Continental Outflow
Research Staff: Rudolf F. Pueschel and Anthony W. Strawa
Anthropogenic aerosols are a major uncertainty in climate prediction. Current estimates of the global annually averaged direct radiative forcing of sulfates, soot (black carbon), mineral dust and biomass smoke range from -0.3 to -1.0 W m-2, with an uncertainty factor of about two. To reduce this uncertainty via better characterization of aerosol properties, integrated field experiments have been conducted (e.g. SCAR-A, SCAR-B, INDOEX, TARFOX, SAFARI). Here we present aerosol radiative forcing data for the western Pacific Ocean region (not covered by any of the above-mentioned deployments) with simulations based on in situ measurements of particle characteristics in elevated Asian outflow.
As an illustration of our findings, Figure 1 compares the clear-sky daily averaged aerosol forcing of Asian outflow (column 1) per unit optical depth with other types of forcings, namely biomass burning (ScB and Zam), forcing by dust (Ace2), forcing associated with urban-industrial pollution (Tfox, ScA, Indo) and forcing by enhanced stratospheric aerosol (Pin). Results are presented for solar broadband fluxes at the surface and at the top of the atmosphere, and for infrared broadband fluxes at the top of the atmosphere. It follows from Figure 1 that 1) the daily averaged forcing varies between -60 and 100 W m-2 irrespective of the geographic location of the measurements; 2) the averaged forcing at the top of the atmosphere is usually significantly smaller than at the surface; and 3) infrared heating at the top of the atmosphere in Asian continental outflow is similar to that of Saharan dust.
In spite of only a relatively small change in the net flux at the top of the atmosphere, generally looked at as a measure of the climatic impact of aerosols, Asian outflow strongly reduces the solar radiation reaching the surface. Possible consequences are dynamical feedbacks (e.g., suppressed convection, weakening of the hydrological cycle, and changes in local heating rates.) Heating rates of +10 K/d (per unit mid-visible extinction) were calculated, mainly due to sub-micron particles. Particles exceeding one micrometer contributed only about 30% to this rate due to reduced solar heating
because of infrared cooling. This atmospheric heating by aerosols could cause low-level clouds to evaporate. This is an opposing effect to increase in cloud cover that can be attributable to more cloud condensation nuclei forming smaller drops at higher concentration.
Although stratospheric background aerosol is non-absorbing, the particles injected by the Pinatubo volcanic eruption were large enough to cause infrared warming (Fig. 1). In the Arctic stratosphere, the existence of soot aerosol (arguably from aircraft) increases heating rates from a fraction of a degree per day to about two degrees per day with possible implications to dynamics and chemical reaction rates.
The presence of clouds is critical for an assessment of aerosol forcing. In the presence of clouds at altitudes higher than the aerosol layer, the aerosol forcing is similar to a cloud-free scenario, albeit at a reduced rate. If the clouds are below the aerosol layer, aerosols primarily reduce cloud-associated solar flux losses such that cooling is not only reduced but even can change to warming. Thus the removal of lower-level clouds turned weak net flux gains (warming) into weak net-flux losses (cooling).
Collaborator: S. Kinne, NASA Goddard Space Flight Center
Point of Contact: Rudolf F. Pueschel, (650) 604-5254, firstname.lastname@example.org