Forcing mechanisms governing diurnal, seasonal, and interannual variability in the boundary layer depths: Five years of continuous lidar observations over a suburban site near Paris

The atmospheric boundary layer (ABL) depth, z_iis a fundamental variable of ABL and a climatologically important quantity. The exchange of energy between the Earth’s surface and the atmosphere is governed by turbulent mixing processes in the daytime ABL, and thus, z_i is important for scaling turbulence and diffusion in both meteorological and air quality models. A long-term data set of z_i was derived at the Site Instrumental de Recherche par Télédétection Atmosphérique (SIRTA) observatory near Paris, using measurements obtained from a ground-based vertically pointing aerosol lidar and an autonomous algorithm STRAT+. Using multiparameter observational data sets covering a 5 year period (October 2008 to September 2013), this study aims to explore two interconnected ABL research topics: brief climatology involving multiscale temporal zi variability (diurnal, seasonal, annual, and interannual) and the relationship between zi and near-surface thermodynamic parameters to determinemeteorological processes governing z_i variability. Both the zi and the growth rate over SIRTA showed large seasonal variability with higher mean values in spring (1633m and 225mh_1) and summer (1947m and 247mh^–1) than in autumn (1439m and 196mh^–1) and winter (1033m and 149mh^–1). Seasonal variability of daytime maximumz_i is found to be strongly and linearly correlated with downwelling solar radiation at the surface (r = 0.92), while the dependence between daytime maximum z_i and sensible heat flux (SHF) at seasonal scales is not fully linear, in particular, for summer months. Interannual variability is studied using deseasonalizedmonthly-mean anomalies of each variable. Conditional sampling and linear regression analyses between the anomalies of deseasonalized SHF and daytime maximum z_ishow (1) stronger correlation between the two parameters for the soil conditions compared to the wet soil conditions, (2) that z_i anomalies were more dependent on SHF anomalies for negative than for positive boundary layer wind speed anomalies, and (3) in the summer season, z_i anomalies varied more consistently with SHF anomalies for conditions with negative cloud cover anomalies than in conditions with positive cloud cover anomalies.