GACP Meetings
First Science Team Meeting
Notes from Discussion Group C: Surface, In Situ, and Field Experiments: Validation and Constraints
Moderator: Brent N. Holben ; Rapporteur: John A. Ogren
Surface and in-situ aerosol measurements made locally, regionally or globally during GACP's proposed twenty-year retrospective time frame are largely disorganized and incomplete, particularly as we move back in time. The focus of the Within Atmosphere Measurement (WAM) working group was to raise issues related to:
- Approach for WAM retrospective inventory.
- Strategies for combined use of satellite and field measurements.
- constraints versus validation versus uncertainties
- statistical and scaling issues
- radiative versus climate forcing
- Suggestions for new measurement programs.
The WAM data set is largely to be organized to constrain, validate and impose error bars on the satellite and model generated aerosol climatology. The group emphasized the importance of separating the WAM data used to constrain the aerosol models employed in satellite aerosol retrievals from the WAM data used to validate the retrievals. Additionally, WAM data will be used to validate derived aerosol forcing from chemical transport and radiative transfer models. Closure experiments from various intensive field programs (IFP) are designed to establish uncertainties in 'critical' parameters.
The specifications for the WAM data base must be developed jointly by the measurement community (satellite and WAM) and modelers (transport and GCM), and should address the resolution (both spatial and temporal) and types of aerosol regimes needed. A clear specification of criteria for choice of WAM parameters and their acceptable uncertainties for inclusion in the WAM data base is required. Several broad measurement categories were identified:
- In-situ (chemical composition, radiative properties, and microphysical properties)
- Passive remote sensing systems, including spectral and broadband radiative fluxes, optical depth, and sky radiance.
- Active remote sensing (LIDAR).
Each of these categories includes measurements made both at the surface and from aircraft.
Several strategies for integration of WAM data into GACP were discussed. Closure experiments, from IFP's, test the combined ability of models and measurements to quantify key aerosol properties. Intercomparisons of long-term WAM data, satellite-derived aerosol properties, and results from chemical transport models can address questions related to scaling of point measurements to larger spatial scales and longer temporal scales. Calculating radiative fluxes from radiances is another method for integrating different types of WAM data.
Although the parameters from the retrospective data base have not been fully identified new measurements and programs were suggested to enhance the utility of WAM to the GACP team.
| Parameter | Systems | Programs |
|---|---|---|
| Single scattering albedo, as function of wavelength and relative humidity |
Improved/coordinated LIDAR |
Asian Dust |
| Particle shape & size | Repeat airborne measurement systems |
EOS validation |
| DMS fluxes | Enhanced co-location of WAM systems |
|
| Sulfur speciation | ||
| Aerosol composition aloft |
The group emphasized the need for co-location of long term measurement systems — particularly surface radiation budget, aerosol optical depth, LIDAR profiles of aerosol backscattering and extinction, and in-situ aerosol chemical, radiative, and microphysical properties - and for establishing criteria for choosing sites and coordination with existing measurement programs.
The WAM working group ended with several action items and recommendations listed as follows:
- Develop task-oriented working groups to cover the following topics:
- Aerosol optical depth and related products.
- Long-term, in-situ aerosol measurements
- Radiative fluxes
- IFP's.
- LIDAR measurements.
- Establish a time frame for the inventory
- Develop, in collaboration with the entire GACP team, the critical parameters for inclusion into the retrospective data base.
- Identify comprehensive supersites, such as the ARM CART site, where long-term WAM data are available for multiple parameters.
- Develop a data inventory questionnaire as a possible follow-on to step 3 and 4.
The retrospective data sets archived will follow the development of current technology. This will require geocoding with adequate meta data for retrieval from web based browser systems. The group concurred with other sessions that an infrastructure needs to be established to facilitate communication and collaboration between team members, including web-based systems.
