Surface Attached Aerosol Layer product
The Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations, or CALIPSO, mission was developed as part of the NASA Earth System Science Pathfinder (ESSP) program in collaboration with the French space agency CNES, with the goal of filling existing gaps in our abilities to observe the global distribution and properties of aerosols and clouds (Winker et al., 2010). CALIPSO was launched (together with the CloudSat satellite) in April 2006. CALIPSO's Cloud-Aerosol LIdar with Orthogonal Polarization (CALIOP) is a nadir-viewing two-wavelength polarization-sensitive lidar that is ideal for the study the global distributions of aerosols near the surface. This study is a collaboration between the University of Wisconsin-Madison and the NASA Langley Research Center.
Traditionally the horizontal and vertical structure of the planetary boundary layer (PBL) has been measured by in situ instrumentation on aircraft, towers, or balloons. Boundary layer properties can be inferred via remote sensing instrumentation such as lidar, radar, sodar, etc. The launch of the CALIPSO satellite in 2006 provides a new opportunity to measure boundary layer clouds and aerosols with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument, a two-wavelength (532 and 1064 nm) polarization sensitive lidar.
The CALIOP lidar does not observe the boundary layer but rather is sensitive to the aerosols confined to that layer. If there are no aerosols embedded in the boundary layer, then the CALIOP observations will not be sensitive to the PBL height. Therefore, we refer to the retrieved parameter as the Surface Attached Aerosol Layer, or SAAL.
During the day, with solar heating, thermal circulations create strong turbulence and cause aerosols and potential temperature to be well mixed. Air in the free-troposphere is entrained into the mixed layer, causing the mixed-layer depth to increase during the day, and forming an entrainment zone. The mixed layer, surface layer and bottom portion of the entrainment zone are statically unstable. Aerosols trapped in the mixed layer cannot escape through the entrainment zone, the lidar being sensitive to aerosol scattering can observe the scattering contrast between the generally ‘clean’ free-atmosphere and the mixed layer. The top of the surface-aerosol layer is used as a proxy for the PBL height (PBLH). A few methods have been developed to retrieve the PBLH from lidar data, and ours uses a wavelet covariance transform analysis technique similar to that found in Davis et al (2000). Ours is novel in that it has been developed to work with the lower SNR data provided by CALIOP, and is intended to work autonomously, that is applied to a global data without intervention. The retrieval methodology is validated through comparison with TAMDAR observations and HSRL observations from aircraft platforms.
Data Example - global averages
We have run the algorithm on 6 years of CALIPSO lidar data from June 2006, to November 2012 the results are shown below. The global results are reasonable, for example the boundary layer is higher over the deserts in summer than in the winter, though the SAAL of some deserts seems a bit low in altitude. The retrieved SAAL are reasonable over the oceans, being generally less than 2 km.
A publication is being prepared and will be available from this web site that describes the algorithm, the validation apporach and applications.
DJF |
MAM |
JJA |
SON |
Global mean SAAL height (above ground level) for 6 years (June 2006, November 2012), 2x3 degree grid boxes.
References
Davis, K.J., N. Gamage, C.R. Hagelberg, C. Kiemle, D.H. Lenschow and P.P. Sullivan, “An Objective Method for Deriving Atmospheric Structure from Airborne Lidar Observations,” J. Atmos. Sci., AMS, 17, pp. 1455-1468, November 2000.
Vaughan, M. A., S. A. Young, D. M. Winker, K. Powell, A. Omar, Z. Liu, Y. X. Hu, and C. Hostetler, 2004: Fully automated analysis of space-based lidar data: an overview of the CALIPSO retrieval algorithms and data products. SPIE, 16-30.
Winker, D. M., W. H. Hunt, and M. J. McGill, 2007: Intial performance assessment of CALIOP. Geophysical Research Letters, 34.
Winker, D. M., J. Pelon, J. A. Coakley, Jr., S. A. Ackerman, R. J. Charlson, P. R. Colarco, P. Flamant, Q. Fu, R. Hoff, C. Kittaka, T. L. Kubar, H. LeTreut, M. P. McCormick, G. Megie, L. Poole, K. Powell, C. Trepte, M. A. Vaughan, and B. A. Wielicki, 2010: The CALIPSO Mission: A Global 3D View of Aerosols and Clouds. Bull. Amer. Met. Soc. 91, no.9, 2010, pp1211-1229.
