Surface Energy Balance Observations and Carbon Dioxide Exchange in the Centre of an European Metropolitan City

The urban heat island effect and the relation between surface energy balance components has a large impact on the health and thermal comfort of the urban population. Hence, knowledge of the distribution of energy sources and the energy partitioning into the different turbulent fluxes represents a crucial input to urban city planning strategies. Urban micrometeorology studies that have longer-term surface flux measurements denote crucial steps in the understanding of energy exchange processes in urban areas. But still they are scarce, mainly due to high labour costs and permission restrictions. As inter-urban differences in energy partitioning tend to be large due to differing surface characteristics (e.g. shape/height of roughness elements, surface materials and urban vegetation) and site locations (e.g. latitude, climate conditions), the need for these studies is large. The current study presents the first data set of turbulent surface fluxes in Central London comprising more than a year worth of observations. The measurement site is located in the very centre of London. Instruments on a rooftop tower at King's College London allow for observations of varying source areas within the highly urbanized surroundings. Besides a climatology of the energy balance components of net all-wave radiation, sensible heat flux and latent heat flux, the annual variation of the turbulent flux of carbon dioxide is presented. The results indicate strong coherency of traffic volume and carbon dioxide transport, expressed in a significant difference between weekday and weekend observations. Furthermore, sensible heat flux results stress the importance of anthropogenic heat sources in the metropolitan city. During the whole observation period, nighttime upward transport of sensible heat exceeds radiative input to the surface energy balance. During winter months even daytime turbulent sensible heat flux values are of the order of magnitude of the net all-wave radiation. The relation of latent heat flux to vegetative fraction of the source area is also analysed.