| Summary: | Abstract The main features of the urban boundary layer in the metropolitan region of São Paulo are estimated based on rawinsondes carried out (a) every 3 hr in two 10‐day field campaigns of the MCITY BRAZIL Project during the summer and winter of 2013 and (b) regularly once per day and continuously for 4 years from 2009 to 2013. On average, the boundary layer height showed a daytime maximum of 1476 ± 149 m in summer and 1122 ± 168 m in winter campaigns. The differences are related to seasonal variations in the (a) buoyancy flux at the surface, which was 30% larger in summer (4.7 ± 0.6 MJ m−2 day−1), and the (b) static stability of the free atmosphere, which was 15% smaller in summer (3.3 ± 0.1 K km−1). The average nighttime boundary layer height, estimated from equilibrium empirical expression, indicated maximum of 126 ± 13 m in summer and 122 ± 10 m in winter campaigns. The presence of a low‐level jet was identified in 80% of the field campaign nights, with intensity varying from 2.7 to 14 m s−1 and height ranging from 95 to 962 m. The nighttime residual‐mixing layer is well estimated using the air temperature gradient method and is a good indication of the maximum vertical evolution of the previous daytime boundary layer. The monthly average maximum boundary layer height varied from a minimum of 1,061 ± 77 m in September to maximum of 1,632 ± 96 m in May.
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