Mixed layer depth determination in the Barcelona coastal area from regular lidar measurements: methods, results and limitations

(2005) Mixed layer depth determination in the Barcelona costal area from regular lidar measurements: methods, results and limitations. Boundary-Layer Meteorology (in press) ABSTRACT Regular aerosol backscatter measurements using an elastic-backscatter lidar were performed understand the physical and chemical processes taking place in the low troposphere, was one of the major parameters to be retrieved. Three derivative methods have been tested in this complex coastal area using the range squared-corrected lidar signal: (1) the minimum of its first derivative, (2) the minimum of its second derivative, and (3) the minimum of the first derivative of its logarithm. The second method was found to give statistically the best results when compared to radiosoundings and it was used to process the whole dataset. A number of 162 days and 660 profiles averaged over 30 minutes have been examined. The mixed layer depth was inferred in cases such as Saharan dust events and sea breeze and mountain-induced re-circulations. It oscillates between 300 and 1450 m in summer and between 390 and 1420 m in winter from 1000 to 1500 UTC. The standard deviation for this portion of the day is 180 and 256 m, respectively in summer and winter. In summer, low heights (mainly limited to 400-800 m) are associated to large mesoscale compensatory subsidence over the sea and to the thermal internal boundary layer formation. The strong coastal and orographic influences and the climatological settling of Barcelona determine the complexity of its atmospheric boundary layer dynamics and the high heterogeneity of the lidar signals. In many cases, single lidar analyses do not allow an unambiguous determination of the mixed layer depth. Two diurnal cycle measurements are discussed together with synoptic maps, backtrajectories and radiosoundings in order to outline the complexity of the area and the limitations of the methods.