Urban modifications of boundary layer heights along a ceilometer transect through Paris

Abstract. Atmospheric boundary layer (ABL) processes over cities impact the distribution and advection of pollutants, moisture and heat. To investigate urban modification of BL dynamics we deploy seven automatic lidar-ceilometers along a 100 km transect upwind, across and downwind of Paris, for clear days with consistent synoptic flow conditions. The average growth rate (GR) varies between days, but on average consistently increases over the city (+ 21 %) and downwind (+ 23 %). The effect is most pronounced for the maximum GR. We observe an enhancement in the maximum mixed-layer height (MLH) over the city (+ 14 % and +28 %) and downwind (+ 15 % and +19 %) in summer and winter, respectively. Seasonal differences in the spatial extent of the downwind effect indicate they are greatest in summer (>60 km). The MLH at the start of the morning transition (MT) is deeper over the city (+ 12 %). However, the onset of the MT and the time of maximum GR show no clear link to their location on the transect. A key driver for the elevated MLH is found in the observed turbulent sensible heat fluxes, which are relatively stronger over the city, especially in winter and during nighttime.