Observations of urban boundary layer structure during a strong urban heat island event

It has long been known that the urban surface energy balance is different to that of a rural surface, and that heating of the urban surface after sunset gives rise to the Urban Heat Island (UHI). Less well known is how flow and turbulence structure above the urban surface are changed during different phases of the urban boundary layer (UBL). This study presents new observations above both an urban and rural surface and investigates how much UBL structure deviates from classical behaviour. Measurements were taken as part of the ACTUAL (Advanced Climate Technology Urban Atmospheric Laboratory) project. Fluxes and meteorological conditions were monitored in central London, UK, using two identical eddy covariance systems with weather stations, operating at 190m and 18m agl. A Doppler lidar operating in two modes (continuous stare mode and Doppler Beam Swinging mode) observed the vertical profiles of turbulence and wind speed, and was used to derive mixing heights. The rural boundary layer was observed at Chilbolton Observatory (100 km south-west of London) using an eddy covariance system and a Doppler lidar which measured the vertical turbulence structure. A five-day, low wind, cloudless, high pressure period was chosen for analysis, during which there was a strong UHI. Boundary layer evolution for both sites was determined by the diurnal cycle in sensible heat flux, with an extended decay period of c. 4 hours for the convective UBL. This is referred to as the “Urban Convective Island” as the surrounding rural area was already stable at this time. Mixing height magnitude depended on the combination of regional temperature profiles and surface temperature. Given the daytime UHI intensity of 1.5 °C, combined with multiple inversions in the temperature profile, urban and rural mixing heights underwent opposite trends over the period, resulting in a factor of three height difference by the fifth day. Nocturnal jets undergoing inertial oscillations were observed aloft in the urban wind profile as soon as the rural boundary layer became stable: clear jet maxima over the urban surface only emerged once the UBL had become stable. This was due to mixing during the Urban Convective Island reducing shear. Analysis of turbulent moments (variance, skewness and kurtosis) showed “upside-down” boundary layer characteristics on some mornings during initial rapid growth of the convective UBL. During the “Urban Convective Island” phase, turbulence structure still resembled a classical convective boundary layer but with some influence from shear aloft, depending on jet strength. These results demonstrate that the UBL is clearly the result of processes driven not only by local surface conditions but also regional atmospheric structure.