Integrated role of the urban canopy on turbulent transfer with the roughness sub-layer from observations

Abstract

About 50% of the world population nowadays lives in urban areas. With the fast growing trends of urbanization, understanding meteorological processes in such environments is rapidly gaining importance. A complex process that still needs thorough

investigation is turbulent transfer of sensible heat across the urban canopy and the roughness sub-layer. It is particularly important to understand how individual sections of the canopy contribute to its integrated role. In an attempt to address this problem, sonic

anemometer and scintillometer measurements were organized in three field campaigns.

The first two were designed to evaluate and calibrate the scintillometer. The third and core campaign consisted of a one-year-long experiment carried out in an urban environment. The campaign was prolonged to address temporal representativeness issues

present from the briefness of previous campaigns. Spatial representativeness was addressed with scintillometer measurements since they represent path-integrated

quantities.

A limitation of scintillometers is their dependence on Monin-Obukhov Similarity Theory, which does not apply in heterogeneous surfaces and non stationary flows. Studies have yet showed good agreement between scintillometer and sonic anemometer

sensible heat fluxes over complex environments, attained by modifying empirical coefficients to approximate Monin-Obukhov Similarity quantities. Although this was attempted, it was found that correcting errors in the measurement of the inner scale of

turbulence and the structure function of refractive index fluctuations produced much better results. Corrective expressions were thus proposed and the applicability of scintillometers over the highly complex urban environment was then explored.

Urban campaign results revealed several complex interactions present inside the urban canopy. The distribution of sensible heat fluxes was found to exhibit high threedimensional complexity, and to be mostly regulated by surface properties and their interaction with solar radiation, local advection and turbulent mixing. Sensible heat

fluxes in the RSL decrease across street canyons during periods of stronger mixing with low-momentum and low-sensible heat flux in-canyon air. Scintillometers were found skillful for measuring turbulent heat fluxes over the canopy but only for some wind directions. Problems were identified when measurements

were placed within the wake of large roughness elements, which suggest that under some wind directions the measurements were made too close to the surface. Yet, scintillometer measurements in the urban environment depend on a large number of corrections. From these, the most critical is the determination of an appropriate value of zero-plane displacement height, which needs to be considered to account for flow modification by the roughness elements. The dependence on a large number of factors and limitations on the determination of appropriate values of zero-plane displacement height suggest that at this point in time scintillometer measurements made that close to the canopy should be considered with care.