Aerosol Workshop — June 2-3, 1997
Panel C: Strategy
J. Hansen (Facilitator), M. Prather (Recorder), A.Clarke, R. Kahn, V. Ramaswamy
Recorder and Facilitator Notes
Hansen reviewed objectives: How can we use satellite data, global models and analysis to advance our understanding of aerosol (direct and indirect) climate effects?
The focused workshop target is quantitative definition of the global distribution of aerosol (direct and indirect) radiative forcing for the period of satellite data (1979-present-near future) using satellite data, modeling/analysis and whatever else is available.
The short-term product of the workshop will be guidance for an NRA (NASA Research Announcement) for research/analysis to achieve this objective.
Desired longer-term outcomes include:
- Bright ideas (strategy) on how to quantify (and confirm) aerosol climate forcing.
- Productive interactions/cooperations between specialized groups [satellite <=> global modeling <=> in situ data <=> small scale modeling].
- Interagency coordination/cooperation.
Hansen also mentions the "events" or, more accurately, the "events in global decadal context" strategy (see following viewgraph). This is based on the hypothesis that several aerosol forcings may be sufficiently large to have detectable influences on regional or global climate. Examples of these forcings are large volcanos such as Pinatubo, soil dust outbreaks, biomass burning, and regional industrial plumes. Thus the approach in this strategy is to quantify such aerosol distributions and their impact on climate parameters. This must be done in the context of a climate record of sufficient length to define the influence of unforced (chaotic) variability of climate parameters; also the record length should force models to be general, i.e., avoid curve-fitting to a single event. The period of satellite data, now approaching 20 years, provides an excellent opportunity for this strategy. The single most important data set for this period that is not yet available is the 4-D temperature field, which can be extracted from infrared meteorological sounding data (TOVS).
Ramaswamy said that much can be done with existing satellite data, and specifically seconds the importance of the 4-D temperature from sounders. We should try to put together a first order record of climate forcings and climate parameters, including temperature, water vapor, clouds and aerosols, for this period. He suggests that we begin with a focus on "dust", which clearly is visible to satellites, occurs in large "events", and is perhaps simpler than some other aerosols. Can we document the change in climate forcing by dust over the past two decades? Ramaswamy also mentions the need to try to separate natural and anthropogenic contributions to the soil aerosol forcing, as IPCC, for example, wants to define the anthropogenic component.
Clarke recommended focusing on specific regions, after establishing goals and objectives that help to identify appropriate study regions. An example is the schematic region used by Hobbs [see Panel B] to discuss a possible experiment to study indirect aerosol effects. Clarke suggests that this might be done before global studies. Prather pointed out that it will be important to include chemistry and related source gases in such regional studies. Clarke's suggestions regarding strategy are summarized on the following chart.
Kahn discussed elements that he felt should be included in the strategy. His main point was that the goal of obtaining an aerosol climatology requires an ISCCP-like project that would integrate satellite and in situ data into the best possible constraints on aerosol amount and type. This could be done with retrospective data, and should also be done with new data. The project would need a PI with the responsibility of making tough decisions about how to weight different observational constraints, and how to combine them into a meaningful product. The PI would need to have an appreciation for statistics and data handling issues, and a deep understanding of aerosol modeling and observations. He mentioned that we should make use of related ground-based records such as atmospheric extinctions measured at astronomical observatories and aerosol depositions in ice cores, establish a "best" albedo (and changes) for the globe, and establish a column water vapor "climatology". He also noted the problems in aggregating fractal scales and comparing non-aligned co-data sets. Laboratory experiments on aerosols and clouds are a potentially very valuable source of information, which unfortunately is being largely ignored. His discussion is summarized on the following viewgraph.
Prather suggests consideration of the following strategy sequence:
- Define homogeneous record of aerosols and clouds for 1980-present. Store as "radiances" or a similar quantity that is independent of the aerosol-model assumptions used in retrievals. For example, do not report aerosol "optical depth" based on irradiance measured at a single wavelength. [The comment was made that yes, it is desirable to avoid a model-dependent "archive", but users will want more geophysical quantities than just the spectral irradiance. Another comment was that yes, we need to have aerosols and clouds together, for aerosol production/loss terms and for analyzing indirect effects.] Although the emphasis should be on a long (approximately 20 year) record, it would be useful to have subperiods with more intensive detailed data, for example, the period with POLDER data.
- Develop models with all aerosol (and cloud) components in order to predict the "record" of radiances, aerosols, clouds, etc. This requires a parallel record of analyzed meteorological fields to run chemistry/aerosol transport models. Climate models (e.g., forced by observed SSTs and aerosol/cloud radiative forcings) also may be useful.
- Use these model and data records to test simulation of: a) long term changes over the period of record, b) variability (synoptic to seasonal), c) examine for notable "goofs", e.g., under or over prediction of aerosols.
- Use the above comparison to identify locales for "process" studies for today's atmosphere, checking identification of sources and mechanisms.
Other specific comments by Prather re an "events" strategy
Focus should be on radiative properties in the above comparison of models and data records; other physical properties are needed to understand mechanisms, but not for the climate forcing.
Is it possible to measure directly the radiative forcing over some of the "hot spots" of aerosol activity?
Interactions between gas-phase chemistry and different aerosol types is important in planning "process" studies.
The upper troposphere was neglected in our discussions, as we did not discuss the potential of SAGE (or CCOSM) for the upper troposphere. Perhaps this was because relevant people were at a concurrent "cirrus" workshop, but this neglect may need to be redressed.