Diploma Thesis

Scintillometer Measurements above the Urban Area of London

Lukas Pauscher (03/2009-06/2010)

Support: Thomas Foken, Johannes Lüers

Partly advised by Prof. Sue Grimmond King's College London (University of London) and Dr. Jennifer Salmond University of Auckland.

The spatial heterogeneity of urban surfaces presents a particular challenge to the measurement of turbulent fluxes. This is particularly true close to the urban surface (in the roughness sublayer). Scintillometery, which offers the ability to make path-averaged measurements of turbulent fluxes of heat and momentum, provides an approach to obtaining more spatially representative data sets in urban areas. In this study three Scintec displaced beam small aperture scintillometers (DBSAS) of the type SLS 20 were used to measure the sensible heat flux at a densely built up site at Strand Campus, King’s College London, UK. One of the DBSAS was aligned just atop a courtyard, while the other two were set up in two different heights above a rooftop line. As a reference an eddy-covariance system, located nearby, was used. Different methods were used to estimate the aerodynamic measurement height above the rooftop. For the courtyard top scintillometer only morphometric approaches could be applied. The uncertainty associated with aerodynamic measurement height is large, especially close to the urban canopy. For the set-up used in this study it ranges from approx. 10 % (highest DBSAS) to approx. 30 % (lowest DBSAS). The differences in the sensible heat flux derived using different urban modifications of the universal functions were surprisingly large. They ranged from an average underestimation of the rural references of about 20 % to an average overestimation of about 25 %. The fluxes atop the courtyard showed small differences between day and night. In contrast, the fluxes above the rooftop line exhibited a marked diurnal cycle. During nighttime both measurement sites showed similar sensible heat fluxes. Generally, the rooftop DBSAS showed a smoother pattern than the eddy-covariance measurements, where short time variability was high. Also, in contrast to the eddy-covariance measurement, the diurnal pattern of the sensible heat flux recorded by the rooftop DBSAS showed almost no dependence on wind direction. These observations indicate that the DBSAS measurements might be able to better capture the patterns of the sensible heat flux of a rooftop surface. Finally, suggestions are given, how the performance of DBSAS in urban environments could be improved and how future applications of DBSAS in urban areas should be designed.

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last modified 2011-09-30