Spatial variations in mixed-layer height along a rural-urban transect in Paris in relation to surface heat fluxes

A process-based understanding of urban atmospheric boundary layer (ABL) dynamics and surface-atmosphere feedback is central to improving the modelling and forecasting of weather, air quality, and thermal comfort, particularly in densely populated urban areas. The year-long urbisphere-Paris measurement campaign generated extensive in-situ and remotely sensed observations of the ABL. Seven Vaisala CL61 automated lidar-ceilometers (ALC) were operated on a 120 km transect along the predominant wind direction (SSW-NNE), from the rural periphery both up- and downwind of central Paris. The high signal-to-noise ratio CL61 observations are used to derive mixed layer height (MLH).  We find significant changes in MLH between locations along the transect that exceed inter-CL61 differences. 

With along-transect wind, the observed ABL is modified as air moves into, across, and beyond the metropolitan area. To understand surface-atmosphere feedbacks, the differences in MLH between urban and rural locations along the transect are explained with differences in turbulent heat fluxes measured by multiple eddy-covariance towers at contrasting urban, peri-urban and rural sites along the transect.

Spatiotemporal differences in MLH are analysed for different synoptic conditions. During typical clear–sky summer days an elevated MLH over both the urban area and downwind in comparison to upwind. MLH differences are most pronounced in afternoon/evening, with a maximum difference of 12 % for upwind vs urban/downwind. Over the city, the MLH grows earlier and faster, and collapses later than upwind. MLH growth and collapse at downwind sites varies with increasing distance from the city. At the most remote downwind site, MLH growth rates are similar to upwind sites, but evening collapse is even later than at the urban sites, suggesting that excess buoyancy is advected downwind.