The Airborne Backscatter Lidar (ABL) was commissioned in the late 1980s for deployment aboard the NASA Dryden Flight Research Center DC-8 Airborne Laboratory and was designed to provide calibrated aerosol and cloud backscatter data over a wide dynamic range, with emphasis on the measurement of atmospheric backscatter in relatively clean regions of the troposphere at a midinfrared wavelength near 9 µm. The lidar transmitter was a 1J class pulsed CO2 laser operated in a heterodyne detection configuration. The lidar was capable of viewing in either the upward (zenith) or downward (nadir) observing mode.
ABL flights on the NASA DC-8 research aircraft during Fall 1989 and Spring 1990 have been used to study aerosol effects over the Pacific Ocean, in both hemispheres, and also to study cloud backscatter and extinction properties. The image below contains a false-color backscatter time series from the 1990 Global Backscatter Experiment (GLOBE) airborne field campaign.
This particular data sample was acquired during a flight segment between Darwin, Australia and Tokyo, Japan on May 31, 1990 during the second GLOBE deployment. It shows the increasing prominence of a strongly-scattering air mass as the northward traverse proceeds. Post-mission trajectory analyses revealed the presence of a powerful westerly jet over this portion of the Pacific during the time frame in question, while onboard in situ sampling instruments determined that the aerosol composition was refractory in nature. Collating these various data sources, the observed plume has been interpreted as originating from either the Gobi Desert or the central China loess plateau.
Each of the vertical strips appearing in this display represents a backscatter profile computed from an 80-shot average (corresponding to an integration time of about 20s). The blank areas denote regions where the inferred signal is indistinguishable from system noise, so that the recovered information is therefore meaningless. This format allows us to view very clearly the structural features of the atmospheric volume under study, but does not provide sufficient signal-to-noise ratio to extract the very low backscatter values that are of central interest to the GLOBE program. However, such a display does permit us to assess the horizontal homogeneity over a given track, which knowledge we may then use to judge the validity of integrating over longer intervals to yield correspondingly greater effective sensitivity.
The GLOBE/ABL flight data have also been used to establish a link between opacity at the lidar wavelength and cloud liquid-water content, which is an important parameter for radiative and hydrological cycle studies. The false-color backscatter below shows a distinct mid-level cloud layer at an altitude of 4.5 km, surmounted by multiple uniform aerosol bands. Beneath this display can be seen the corresponding variation of inferred cloud optical thickness along the flight track:
Work with the GLOBE dataset is ongoing, with an emphasis on characterizing the observed clouds and aerosols by intercomparing absolute backscatter measurements of selected airmasses obtained contemporaneously by the JPL and NASA Goddard Space Flight Center pulsed lidar systems.
References
R. T. Menzies and D. M. Tratt: Airborne CO2 coherent lidar for measurements of atmospheric aerosol and cloud backscatter. Appl. Opt., 33(24), 1994, 5698-5711.
R. T. Menzies, D. M. Tratt, and P. H. Flamant: Airborne CO2 coherent lidar measurements of cloud backscatter and opacity over the ocean surface. J. Atmos. Oceanic Technol., 11(3), 1994, 770-778.
D. R. Cutten, R. F. Pueschel, D. A. Bowdle, V. Srivastava, A. D. Clarke, J. Rothermel, J. D. Spinhirne, and R. T. Menzies: Multiwavelength comparison of modeled and measured remote tropospheric aerosol backscatter over Pacific Ocean. J. Geophys. Res., 101(D5), 1996, 9375-9389.
R. T. Menzies and D. M. Tratt: Airborne lidar observations of tropospheric aerosols during the GLOBE Pacific circumnavigation missions of 1989 and 1990. J. Geophys. Res., 102(D3), 1997, 3701-3714.
D. R. Cutten, J. D. Spinhirne, R. T. Menzies, D. A. Bowdle, V. Srivastava, R. F. Pueschel, A. D. Clarke, J. Rothermel: Intercomparison of pulsed lidar data with flight level CW lidar data and modeled backscatter from measured aerosol microphysics near Japan and Hawaii. J. Geophys. Res., 103(D16), 1998, 19649-19661.
