Aerosols and the Hydrological Cycle

Aerosols are key players in Earth’s energy balance and atmospheric composition. They can have a cooling or warming effect, contribute to acid deposition, and influence the rate of snow melt and the Earth’s albedo. Particularly in areas such as Asia and the Indian Ocean, where human aerosol emissions contain high amounts of black carbon, aerosols can influence a local drying and warming of the atmosphere. This workshop considered existing studies on aerosols, field experiments, laboratory experiments, and models to identify areas of valuable future research.

Keywords: aerosols, atmospheric composition, models and modeling

Relevance:
Recent field programs have shown that aerosols from Asia absorb a significant amount of solar radiation in the atmosphere before it can reach the Earth’s surface. These international campaigns include the Indian Ocean Experiment (INDOEX) and the Aerosol Characterization Experiment (ACE-Asia). The results from these programs suggest that Asian anthropogenic aerosols contain a substantial amount of black carbon. In terms of radiative forcing, the traditional metric of climate change, black carbon is a relatively small term compared to other anthropogenic forcings. In the northern Indian Ocean, however, its effects on the surface and atmosphere are an order of magnitude larger than the effects of greenhouse gases during winter. The implication is that redistribution of solar radiation within the climate system may be much more important than the additional reflection of solar radiation by aerosols.

Modeling studies of these effects are very limited, but the few available studies indicate the potential importance of aerosol effects on the hydrological cycle. When absorbing aerosols are introduced into climate models, atmospheric circulation and the hydrological cycle are both decelerated. The regional shifts in rainfall are large and could have major implications for water supplies and agricultural practices. One of the main reasons for the decelerated hydrological cycle is a reduction in surface evaporation caused by aerosols. Water vapor introduced by evaporation is one of the principal energy sources for atmospheric circulation and essentially the only source for precipitation. Another reason is the reduction in convective activity due to increased atmospheric heating and stabilization by aerosols. Atmospheric convection drives the main circulation systems in the tropics and subtropics. This second effect has been omitted from many of the previous modeling studies of aerosols used for international climate assessments. If the new modeling studies (as yet unpublished but reported in scientific meetings) are correct, the Atmospheric Brown Cloud (ABC) may already be altering precipitation patterns and surface temperatures over the more densely populated regions of India, China, and south-east Asia. A modeling study published recently concluded that the Sahelian drought of the 1970s could be attributed to the aerosol effects on the hydrological cycle. It is also important to recognize that essentially no operational weather forecast systems include the radiative effects of aerosols, although some centers are now revisiting this issue. Aerosols could certainly affect forecasts of diurnal temperature cycles and other important meteorological fields.

Design and sub-topics:
The workshop is designed to help stimulate new national and international research to understand the links between aerosols and the hydrological cycle.

In order to cover this broad range of issues in adequate depth, the workshop should last five to seven days. The topics covered would include:
•  Summary of recent field and satellite observations of aerosols;
•  New observational studies of the effects of aerosols on surface insolation, evaporation, and thermal structure of the lower atmosphere;
•  Discussion of new methods and ideas to improve the fidelity of aerosol models compared to observations;
•  Analysis of ongoing modeling studies of interactions between aerosols and the hydrological cycle on regional and global scales;
•  Discussion of the linkage between aerosol radiative effects and atmospheric circulation deduced from theories of atmospheric dynamics;
•  Investigation of changes in regional precipitation and temperature that could be related to local or remote effects of aerosols; and
•  Discussion of the implications for water supplies and agriculture.

Disciplines needed:
The topics covered by the workshop would include atmospheric dynamics, global modeling, hydrology, and climate monitoring. In order to foster an interdisciplinary discussion of the links between aerosols and the hydrological cycle, we anticipate inviting participation from:

•  Global climate modelers specializing in the direct and indirect effect of aerosols;
•  Chemical transport modelers investigating aerosol transport and aerosol/cloud interactions;
•  Aerosol investigators collecting field observations of the optical, chemical, and physical properties of aerosols;
•  Specialists in the remote sensing of aerosols from satellites and from surface-based radiometric systems;
•  Atmospheric dynamicists, with an emphasis on tropical circulation systems and coupled interannual variability;
•  Scientists investigating the formation and evolution of monsoons;
•  Specialists in climate monitoring, with an emphasis on regional patterns and trends in temperature and precipitation; and
•  Scientists modeling the effects of solar insolation and aerosol forcing on crops.