Scintillometer Observations of Sensible Heat Flux in Central London

There is need for spatially-integrated measurements of sensible heat flux (QH) in urban areas to match the spatial resolution of urban climate models and satellite observations. Scintillometers have been used successfully in urban environments to measure at these spatial scales at high temporal resolution, though there remain challenges with making measurements and interpreting results from heterogeneous urban surfaces. This work presents a full year of observations from a network of three scintillometers in central London with pathlengths of 1.1-3.2 km and measurement source areas with length scales of up to 6 km. Scintillometer data are compared with simultaneous eddy-covariance (EC) measurements from a flux tower operating in the long-term scintillometer source area. Scintillometer measurements are comparable to EC observations, though scintillometer calculations are sensitive to estimates of the effective sensor measurement height and displacement height of the urban canopy. Scintillometer observations show that QH dominates the surface energy balance in central London throughout the year, with expected diurnal courses and seasonal trends in QH magnitude related to solar radiation input. Measurements also reveal a clear anthropogenic component of QH with summertime weekday QH values on average significantly higher than weekend values. Scintillometer observations are also linked with surface geospatial datasets using turbulent source area models to investigate underlying physical controls on QH spatial variability. When QH is conditionally sampled to control for synoptic conditions, vegetation and building land cover fraction in the measurement source areas are a strong control on QH magnitude. This work demonstrates the viability of scintillometers to provide spatially-integrated measurements of QH in urban areas. Results also highlight the importance of anthropogenic heat as an energy source in dense urban areas and the role of vegetation in modifying the urban surface energy balance.