Chapter 3: Evaluating the impacts of carbonaceous aerosols on clouds and climate

• Main Authors: Menon, Surabi, Del Genio, Anthony D.
• Other Authors: Lawrence Berkeley National Laboratory. Environmental Energy Technologies Division.
• Format: Book
• Language: English
• Published: Lawrence Berkeley National Laboratory 2007
• Subjects: Precipitation; Biomass; Aerosols; Clouds; 54; Greenhouse Gases; Stratosphere; Heating; Phase Shift; Permafrost; Climates; Northern Hemisphere; Convection; Arctic Regions; Troposphere; Climate Models; Polar Regions; Arctic; Beck; black carbon; Ice; permafrost
• Online Access: http://digital.library.unt.edu/ark:/67531/metadc896996/

Any attempt to reconcile observed surface temperature changes within the last 150 years to changes simulated by climate models that include various atmospheric forcings is sensitive to the changes attributed to aerosols and aerosol-cloud-climate interactions, which are the main contributors that may well balance the positive forcings associated with greenhouse gases, absorbing aerosols, ozone related changes, etc. These aerosol effects on climate, from various modeling studies discussed in Menon (2004), range from +0.8 to -2.4 W m{sup -2}, with an implied value of -1.0 W m{sup -2} (range from -0.5 to -4.5 W m{sup -2}) for the aerosol indirect effects. Quantifying the contribution of aerosols and aerosol-cloud interactions remain complicated for several reasons some of which are related to aerosol distributions and some to the processes used to represent their effects on clouds. Aerosol effects on low lying marine stratocumulus clouds that cover much of the Earth's surface (about 70%) have been the focus of most of prior aerosol-cloud interaction effect simulations. Since cumulus clouds (shallow and deep convective) are short lived and cover about 15 to 20% of the Earth's surface, they are not usually considered as radiatively important. However, the large amount of latent heat released from convective towers, and corresponding changes in precipitation, especially in biomass regions due to convective heating effects (Graf et al. 2004), suggest that these cloud systems and aerosol effects on them, must be examined more closely. The radiative heating effects for mature deep convective systems can account for 10-30% of maximum latent heating effects and thus cannot be ignored (Jensen and Del Genio 2003). The first study that isolated the sensitivity of cumulus clouds to aerosols was from Nober et al. (2003) who found a reduction in precipitation in biomass burning regions and shifts in circulation patterns. Aerosol effects on convection have been included in other models as well (cf. Jacobson, 2002) but the relative impacts on convective and stratiform processes were not separated. Other changes to atmospheric stability and thermodynamical quantities due to aerosol absorption are also known to be important in modifying cloud macro/micro properties. Linkages between convection and boreal biomass burning can also impact the upper troposphere and lower stratosphere, radiation and cloud microphysical properties via transport of tropospheric aerosols to the lower stratosphere during extreme convection (Fromm and Servranckx 2003). Relevant questions regarding the impact of biomass aerosols on convective cloud properties include the effects of vertical transport of aerosols, spatial and temporal distribution of rainfall, vertical shift in latent heat release, phase shift of precipitation, circulation and their impacts on radiation. Over land surfaces, a decrease in surface shortwave radiation ({approx} 3-6 W m{sup -2} per decade) has been observed between 1960 to 1990, whereas, increases of 0.4 K in land temperature during the same period that occurred have resulted in speculations that evaporation and precipitation should also have decreased (Wild et al. 2004). However, precipitation records for the same period over land do not indicate any significant trend (Beck et al. 2005). The changes in precipitation are thought to be related to increased moisture advection from the oceans (Wild et al. 2004), which may well have some contributions from aerosol-radiation-convection coupling that could modify circulation patterns and hence moisture advection in specific regions. Other important aspects of aerosol effects, besides the direct, semi-direct, microphysical and thermodynamical impacts include alteration of surface albedos, especially snow and ice covered surfaces, due to absorbing aerosols. These effects are uncertain (Jacobson, 2004) but may produce as much as 0.3 W m{sup -2} forcing in the Northern hemisphere that could contribute to melting of ice and permafrost and change in the length of the season (e.g. early arrival of Spring) (Hansen and Nazarenko, 2004). Besides the impacts of aerosols on the surface albedos in the polar regions, and the thermodynamical impacts of Arctic haze (composed of water soluble sulfates, nitrates, organic and black carbon (BC)), the dynamical response to Arctic haze (through the radiation-circulation feedbacks that cause changes in pressure patterns) is thought to have the potential to modify the mode and strength of large-scale teleconnection patterns such as the Barrents Sea Oscillation that could affect other climate regimes (mainly Europe) (Rinke et al. 2004). Additionally, via the Asian monsoon, wind patterns over the eastern Mediterranean and lower stratospheric pollution at higher latitudes (Lelieveld et al. 2002) are thought to be linked to the pollutants found in Asia, indicating the distant climate impacts of aerosols.