The GLObal Backscatter Experiment (GLOBE)

Justification

GLOBE was initiated in 1986 as a cooperative interagency and international research program to characterize the spatial, temporal, and spectral variability of atmospheric aerosol backscatter coefficients. This research was needed to provide realistic inputs to design studies and performance simulations for satellite Doppler wind lidars. This information addressed three major issues:
  1. How do regional-scale to global-scale aerosol properties affect the design selection, baseline performance, and spatial/temporal coverage of a satellite wind lidar?
  2. How do cumulus-scale to regional-scale aerosol properties affect the accuracy and interpretation of wind measurements from satellite lidars?
  3. How do aerosol properties in general affect the science value of satellite-based wind measurements for studying the transport of climatically relevant quantities, such as heat, moisture, momemtum, and trace aerosols or gases?
All of these issues are relevant regardless of the size of the wind lidar, because the atmosphere provides a natural continuum of scientifically useful scattering targets over a dynamic range of at least eight orders of magnitude. However, the relative weight given to each issue depends very strongly on the size of the lidar and the nature of its primary scattering targets. The methodology and results are also applicable to any earth-based or space-based lidar that uses atmospheric aerosols as passive scattering targets for the measurement of primary atmospheric quantities.

Methodology

Consequently, GLOBE was designed on a very simple premise, which also remains valid regardless of the size of the wind lidar. That is: Aerosol backscatter studies for any given satellite wind lidar concept should concentrate on the detailed backscatter physics, the spatial/temporal variability, and the frequency of occurrence of the "e;marginal scattering targets"e;, i.e, those targets that provide signal strengths near the detection threshold for that design. Studies of scattering targets that are well above or well below the marginal backscatter at the primary design wavelength should be limited primarily to assessing their preferred location, typical structure, and frequency of occurrence, to the extent that these properties affect the overall measurement capabilities of that design. Initial studies should be directed toward the timely development of realistic empirical models, while at the same time providing the foundation for later development of predictive theoretical models. The initial wind lidar concept called for a large lidar optimized for global circulations in the middle and upper troposphere. The marginal scattering target for this lidar was the middle and upper tropospheric aerosol backscatter system. GLOBE efforts from 1986 to 1994 were therefore designed to characterize the backscatter properties of that background system, which was very poorly understood. GLOBE research activities were based on the coupled hypothesis that: The strategy that was developed to characterize this challenging target involved a diverse set of sensors, measuring various aerosol properties, ideally simultaneously, over a broad range of spatial and temporal scales, producing large aerosol data sets. The diverse sensors were used to provide empirical links to long-term or global-scale aerosol climatologies, or other potential backscatter surrogates, and to ensure that aerosol backscatter could be modeled at any desired lidar wavelength. The scale coverage established the preferred scale domain for the background, and guarded against meteorological or geographical bias. The large data sets were used to increase the probability of detecting the background and to increase the significance of derived background statistics. The program that was developed to implement this strategy involved six major research thrusts:
  1. Long-term lidar backscatter climatologies at several strategic sites;
  2. Short intensive field programs, usually involving multiple sensors over large spatial scales;
  3. Long-term global climatologies of aerosol extinction from satellite-based solar occultation sensors;
  4. Careful instrument calibrations, fully traceable to absolute radiometric standards, for highly sensitive aerosol instruments;
  5. Development of a centralized database to compile and archive the comprehensive aerosol data, review it for strict quality control standards, and distribute the data to other interested research groups;
  6. Detailed data analysis, leading to the development of empirical, and eventually theoretical, global-scale models of aerosol backscatter at the desired laser wavelengths.

Results

The GLOBE program has been highly successful in achieving most of these objectives. The multi-faceted research strategy was instrumental in confirming the existence of the middle and upper tropospheric background aerosol system, determining the typical values and upper/lower bounds of background backscatter, characterizing its physical and chemical properties and its backscatter wavelength dependence, delineating its global coverage, demonstrating its general uniformity, and observing its remarkable resilience to large external perturbations. These results established the background aerosol as a historically reliable baseline upon which to design future satellite wind lidars. An unexpected result was the discovery of aerosol genesis at high altitudes in otherwise pristine air near deep convective clouds. This result provided important insights on the physical processes that develop and maintain the background aerosol throughout its life cycle. It also led to the discovery of the importance of background free tropospheric aerosols to the properties of boundary layer aerosols, and consequently on clear air and cloudy air albedo, especially over the remote oceans. This information has created substantial interest in the external atmospheric aerosol and chemistry communities; so much so, in fact, that other research groups are planning to retrace much of the GLOBE flight tracks with aerosol and gas chemistry sensors. These results also led to the development of a simple aerosol physicochemical model, which, when combined with standard Mie theory, predicts to first order the observed backscatter magnitudes and wavelength dependence of the background aerosol system. In summary, GLOBE has been highly successful in developing empirical diagnostics for the background middle and upper tropospheric aerosol system for input to satellite wind lidar design studies.

GLOBE Flight 14 Results

Mission Emblems

GLOBE 1

GLOBE 2

References

David A. Bowdle
Institute for Global Change Research and Education
Earth Systems Science Laboratory, University of Alabama in Huntsville, Huntsville, AL
Aerosol/Lidar Group, NASA Marshall Space Flight Center, Huntsville, AL
Phone: 205-922-5955
EMAIL: david.bowdle@msfc.nasa.gov

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Last Updated: Feb 22, 1995