Atmospheric stability and transport processes in the urban boundary layer from ground-based remote sensing profile observations

Micro-climate conditions and air quality in the urban environment are strongly influenced by anthropogenic activities and local characteristics of the surface canopy. However, larger-scale flow dynamics also have an important impact on surface-atmosphere exchange processes and their spatio-temporal variations. While “background weather conditions” may to some extent be described through rather general categories (e.g. clear sky; low wind speed), the latter rarely portray the complexity of relevant dynamics and limit investigations of process-understanding. To better characterise the joint implications of synoptic-scale (or regional-scale) dynamics and surface-driven processes, a better monitoring of transport processes and atmospheric stratification across the urban atmospheric boundary layer is required.

 

Thanks to advances in atmospheric ground-based remote sensing technology and product development, vertical profiles of different key variables (wind, temperature, aerosol) are increasingly gathered by distributed sensor networks across urban areas. In the Paris Region (France), a dense monitoring network is being developed by the PANAME initiative, including a network of remote sensing instruments. Here we exploit observations from Doppler wind lidars (DWL) and microwave radiometers (MWR) to derive high-resolution data of wind, turbulence and air temperature throughout the vertical extent of the atmospheric boundary layer. The continuous profile observations in suburban and urban settings allow for a detailed mapping of atmospheric stability and dynamic mixing. Comparing a range of indicators (lapse rate, bulk Richardson number, vertical velocity variance, wind shear, mixed-layer height) and their spatial and temporal variations, it is shown that the diurnal evolution of the urban boundary layer and its vertical structure can effectively be monitored continuously.

 

Highlighting the importance of atmospheric boundary layer dynamics and atmospheric stratification for street level micro-climates and ventilation, the work suggests that such processes should be better incorporated in the assessment and mitigation of e.g. heat-risk or air pollution episodes.