Observed structure of the nocturnal urban boundary layer and its evolution into a convective mixed layer

The turbulence properties of the nocturnal urban boundary layer (UBL) and its transition to the convective mixed layer have been observed over the urban center of Sapporo, Japan. The nocturnal UBL was characterized by an elevated inversion layer and a near-neutral mixed layer below it. The inversion layer varied with time and showed a wavy pattern. Kelvin-Helmholtz instability (billow) was observed within the elevated inversion layer, the horizontal and vertical scales of this billow being evaluated to be of the order of the vertical scale of the elevated inversion layer. The transition from the nocturnal UBL to the convective mixed layer occured rapidly, within 1-1.5 h over the center of the urban area. At this time, the downward transport of sensible heat below the inversion base became important. The normalized turbulence intensities in the convective mixed layer were also examined.