Drivers of wind profile variability in Bristol, UK: the role of synoptic flow, complex terrain, and the urban atmosphere

Cities strongly modify the surface energy balance, increase aerodynamic roughness, and release anthropogenic emissions, thereby significantly affecting atmospheric boundary layer (ABL) processes from local to regional scales. For cities located in complex terrain, these effects are further modulated by orography: thermal forcing depends on slope and shading, while mechanical forcing strongly deflects, accelerates or decelerates the wind field. High-resolution observations of the ABL wind profile over urban areas in complex terrain are therefore essential to provide evaluation data to test if models are able to represent the interaction between ABL flow and heterogeneous surfaces, with implications for urban heat, air quality, and numerical weather prediction.

Within the ASSURE-urbisphere observational campaign in Bristol, UK, a network of five Doppler wind lidars (DWLs) was deployed across the urban–rural interface. This mid-sized city is located in complex terrain, providing a testbed for investigating coupled urban–terrain effects. A key orographic feature of Bristol is the Avon Gorge, located in the northwest of the city, with a length of approximately 3 km, a typical width of 100 m and an average depth of 80 m.

The five DWLs were operated with different scanning strategies and were deployed across six sites, providing continuous wind and turbulence profiles. This unique dataset enables characterization of how enhanced urban buoyancy and surface roughness modify wind and turbulence, as well as how terrain-induced thermal structures control near-surface atmospheric stability.

We have found that the synoptic wind direction is the primary driver of the formation of a valley-exit jet from the Avon Gorge, with direction determining whether the jet flows into or exits the city. Near-surface atmospheric stability, largely influenced by urban buoyancy, modulates the jet intensity. Under anticyclonic conditions, a mesoscale nocturnal low-level jet (LLJ) is formed. These stable conditions, together with cold-air pooling in the urban basin, also favour the development of a weak and partially decoupled near-surface flow that drains downslope within the gorge. The LLJ core characteristics (intensity, direction, and height) influence the formation (or suppression) and intensity of the valley-exit jet.

These results provide new insights into how coupled urban–terrain interactions modulate ABL dynamics, with direct implications for urban air quality, and human thermal comfort.