Intermittent exchange of heat and pollutants between the urban canopy and above-roof boundary layers at night

The vertical exchange processes which couple the urban canopy layer (UCL) to the overlying urban boundary layer (UBL) are poorly understood. This is particularly true at night when the release of heat stored within the urban fabric plays an important role in the maintenance of a weakly convective boundary layer in urban areas. Previous studies in Marseille show discrete convective structures in the nocturnal UBL that are consistent with the intermittent venting of heat and pollutants from within the street canyons of the UCL. However, their path from the UCL into the UBL has not been properly traced. Here we draw upon the rich data collected in a densely urbanized district in central Basel, Switzerland, as part of the Basel Urban Boundary Layer Experiment (BUBBLE) in the summer of 2002. Measured profiles of the turbulent fluxes of heat and carbon dioxide extend from within the street canyon, up through the roughness sublayer to above the blending height, at more that twice the building height. Wavelet analysis of the turbulent data is used to determine the origin and development of convective structures observed in the nocturnal UBL.