Abstract
Understanding the physical processes that affect the turbulent structure of the nocturnal urban boundary layer (UBL) is essential for improving forecasts of air quality and the air temperature in urban areas. Low-level jets (LLJs) have been shown to affect turbulence in the nocturnal UBL. We investigate the interaction of a mesoscale LLJ with the UBL during a 60-h case study. We use observations from two Doppler lidars and results from two high-resolution numerical-weather-prediction models (Weather Research and Forecasting model, and the Met Office Unified Model for limited-area forecasts for the U.K.) to study differences in the occurrence frequency, height, wind speed, and fall-off of LLJs between an urban (London, U.K.) and a rural (Chilbolton, U.K.) site. The LLJs are elevated (approx 70 m) over London, due to the deeper UBL, while the wind speed and fall-off are slightly reduced with respect to the rural LLJ. Utilizing two idealized experiments in the WRF model, we find that topography strongly affects LLJ characteristics, but there is still a substantial urban influence. Finally, we find that the increase in wind shear under the LLJ enhances the shear production of turbulent kinetic energy and helps to maintain the vertical mixing in the nocturnal UBL.
Highlights
Urban boundary layers (UBLs) differ substantially in their depth and vertical structure from their rural counterparts (Pal et al 2012; Barlow 2014) due to differences in the surface energy balance (Arnfield 2003; Barlow et al 2015)
Nocturnal UBLs exhibit increased turbulent mixing due to greater turbulence kinetic energy (TKE, variable notation as e) produced from buoyancy, the nocturnal release of heat stored in buildings during daytime, anthropogenic activities, and increased surface drag
Unravelling the physical processes that affect the nocturnal production of TKE is essential to better understand and represent the evolution of the nocturnal UBL, which can be beneficial for air quality and weather forecasts in cities
Summary
Urban boundary layers (UBLs) differ substantially in their depth and vertical structure from their rural counterparts (Pal et al 2012; Barlow 2014) due to differences in the surface energy balance (Arnfield 2003; Barlow et al 2015). Low-level jets are super-geostrophic wind-speed maxima that occur near the surface, usually at the top of the nocturnal boundary layer (Blackadar 1957; Baas et al 2009). They are formed through: a) inertial oscillations due to the collapse of turbulent mixing after sunset (Blackadar 1957) and b) baroclinicity from local topographic differences or largescale synoptic forcing (Holton 1967; Kotroni and Lagouvardos 1993). Previous studies have employed in situ observations (i.e. Doppler lidar, sodar) (Banta et al 2003; Wang et al 2007; Baas et al 2009; Kallistratova and Kouznetsov 2012; Barlow et al 2015; Banakh and Smalikho 2018) and/or numerical models (i.e. numerical weather prediction, NWP, and large-eddy simulation, LES) (Storm et al 2009; Park et al 2014; Vanderwende et al 2015) to study LLJs
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