Inter-comparison of lidar methods for obtaining planetary boundary-layer height from a July 2012 monitoring campaign over the Iberian Peninsula in the framework of EARLINET

The depth of the planetary boundary-layer (PBL) is defined as the height of the inversion level separating the free troposphere (FT) from the boundary-layer (Stull, 1988). Reliable representation of PBL height is important in applications ranging from climate studies to air quality modeling. Convective turbulent mixing processes are dominant in the mixing layer of the PBL and have a major influence on the growth and transport of atmospheric pollutants. In recent years, lidar (laser radar) has proven to be a useful operational tool for nearly continuous monitoring of the lowest levels of the atmosphere with high spatial (~ 3.75 m) and temporal (< 5 min) resolutions. Four Raman-elastic multi-wavelength lidar stations from EARLINET (European Aerosol Research Lidar Network) conducted a 72-hr campaign of continuous observations over Spain (Barcelona, Granada, Madrid) and Portugal (Evora) in early July 2012. This study systematically exploits 1-min averaged, range-squared-corrected lidar signals (RSCS) from the 532 nm analog reception channel of the instruments. Several methods that have been applied in previous literature to derive PBL height from vertical aerosol backscatter profiles are compared. Most widely used are derivative techniques such as the gradient method (GM), inflection point method (IPM), and logarithm gradient method (LGM) and covariance techniques such as the wavelet covariance transform (WCT) method using a Haar wavelet. The methods function by detecting steep gradients in the aerosol backscatter profile, a proxy for the transition zone between the PBL and FT. It is found that all the methods provide comparable results. However, it is determined that WCT is an optimal method as it is more computationally efficient than the derivative techniques. In summer, PBL heights over the Iberian Peninsula are typically between 1-3 km. In addition, spatial patterns and diurnal variation of the PBL height and an analysis of the meteorological situation over the study area are also conducted. Backward trajectories from the NOAA HYSPLIT model indicate aerosols arrived from tropical maritime origins over the eastern Atlantic Ocean in the previous 24-48 hours of the campaign. Overall, it is shown that lidar can be an effective means of obtaining accurate PBL heights on a nearly continuous basis