Automating Boundary Layer Detection for Aerosol Lidar
J. Richards/J. Welton
October 26, 2012 15:25:26
Description of Problem
During March and April 2008, the ICEALOT research cruise traveled into the ice-free Arctic as far as 81° N. The campaign observed many properties of the 2008 Arctic haze, including the aerosol backscatter with altitude that was provided by an MPLNET lidar instrument. Because of the shared focus between many instruments, the lidar backscatter profiles came with a greater context of observations with which to validate conclusions about the chemistry and transport of the Arctic haze. The observations were also supported by satellite data from CALIPSO and MODIS, and by trajectory modeling. This information was valuable for studying the behavior of the planetary boundary layer over the marine Arctic and the transport of aerosol plumes into it from the midlatitudes. With several potential sources of validation, the cruise observations were also useful for evaluating methods to detect the planetary boundary layer in lidar profiles.
Scientific Objectives and Approach
A wavelet covariance transform technique [Davis et al. 2000; Brooks 2003] was adapted to detect the boundary layer in backscatter profiles from ground-based aerosol lidar such as those that make up the MPLNET network. The resulting PBL heights were compared visually to the raw backscatter data, and to thermodynamic data from the sonde launches that were made up to four times per day during the ICEALOT cruise. Changes to the algorithm aimed to reduce errors due to confusion with high clouds and elevated aerosol plumes, and also to minimize the computing time necessary to process a day’s worth of backscatter data. Two aerosol plumes that were observed in the free troposphere during the cruise were analyzed using back-trajectories, satellite data, and the MPLNET lidar at the University of New Hampshire to determine their origin. Later, the algorithm was applied to detect PBL heights in springtime data from the longer-serving MPLNET lidar at Ny-Ålesund, Svalbard (Figure 1).
The Ny-Ålesund PBL heights demonstrated the performance of the wavelet covariance transform technique over a variety of weather conditions. The PBL height information made it possible to observe entrainment of elevated aerosol plumes into the boundary layer, where the pollutants become part of the Arctic haze (Figure 2). While PBL detection using lidar backscatter data still has room for further work, it is an improvement on modeled PBL heights. The lidar-derived PBL heights can be used to answer questions about the PBL, aerosol transport, and climate.
Other Publications and Conferences
Sawyer, V.R., R.K. Varner, P.J. Kelly, E.J. Welton, D.C. Vandemark, D.E. Wolfe, and R.W. Talbot. Height of the planetary boundary layer during ICEALOT 2008. AGU Fall Meeting, San Francisco.
Sawyer, V.R., Z. Li, and E.J. Welton, 2010. Automated boundary layer detection for micropulse lidar (MPL). ARM Science Team Meeting, Bethesda, MD.