Structure of the atmosphere in an urban planetary boundary layer from lidar and radiosonde observations

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Abstract
The planetary boundary layer (PBL) over Mexico City was probed with a scanning backscatter lidar to characterize and evaluate the multidimensional structure of the atmosphere. Comparisons were made between radiosonde and lidar‐derived PBL heights which showed the two techniques to be in close agreement. The spatial properties of the free atmosphere‐PBL interface were found to be approximately the same size as the entrainment zone thickness. Below the interface the lidar observed spatially resolved structures, such as thermal plumes, convective eddies, low‐level jets, and entrainment into the PBL. These structures were spatially correlated with the local diabatic condition and wind stress. One highly unstable atmosphere contains a lidar‐visualized convective structure rising to a height of 0.45 the inversion base, which was predicted from earlier turbulence models. Other features, such as low‐level jets, were found to be associated with neutral atmospheres in the mixing layer. The analysis indicates that the transport of pollutants is not a continuous and gradient‐driven process, but low frequency and spatially discontinuous. The high spatial and temporal resolution afforded by the scanning lidar depicts surface‐atmosphere interactions which are neither spatially homogeneous nor horizontally uniform.