Characteristics of Vertical Velocity Fluctuations in a Convective Urban Boundary Layer

Abstract

Vertical air motions within the convective boundary layer over Toronto, Canada, have been observed using a monostatic Doppler echosonde. Data were acquired on cloudless days with strong insolation and light winds, when turbulence dynamics were dominated by buoyancy. Data were analyzed only for time periods with nearly constant wind and mixed-layer depth. Scaling parameters of mixed-layer similarity theory were estimated from the amplitude of the acoustic echo. Minimum urban fetch was 7.5 times the boundary-layer depth. Nondimensional mean vertical profiles of vertical velocity variance, and of its vertical turbulent transport, of the dissipation rate of turbulence kinetic energy, and of the spectrum of vertical velocity were computed. These data support the validity of mixed-layer similarity theory in flow over an urbanized, inhomogeneous land surface, and thus suggest that adjustment to boundary changes in a convective layer is rapid (relaxation time less than about four times the convective scaling time). Indications of the importance, even during light winds, of turbulence generated by urban surface roughness were found in the large magnitude and high frequency of the peak in the variance spectrum. Vertical motions were vertically coherent, in the form of updrafts and downdrafts, and the alongstream dimensions of these currents were log-normal in distribution. Mean dimensions scaled with the mixed-layer depth, and the median updraft separation lay close to the wavelength at the spectral peak. No representative updraft profile or cross section could be isolated because of large stochastic fluctuations in plume structure.