The main features of low‐level jet (LLJ) in the metropolitan region of São Paulo (MRSP), Brazil, are assessed using rawinsondes carried out: (a) every 3‐hr during 10‐consecutive days in summer and winter field campaigns of the MCITY BRAZIL Project in 2013, (b) at 0900 and 2100 local time, from September 2009 to August 2013. These observations indicate that the LLJ is a typical feature of the MRSP, observed in 85% of the 20 days of the field‐campaigns and 77.6% of the 1,446 days of regular rawinsonde period. The fine temporal and spatial resolution soundings indicate that most of the LLJs occur during nighttime and early in the morning, with mean intensity of 8.5 ± 0.3 m s⁻¹, height of 539 ± 26 m, and mostly (52.5%) from east and north. The coarse resolution soundings indicate the LLJ display a seasonal variation with maximum intensity in October (8.6 ± 0.3 m s⁻¹) and a minimum in February (7.1 ± 0.2 m s⁻¹), a maximum height in March (703 ± 151 m) and a minimum in June (577 ± 151 m). During MCITY campaigns about 76.4% of LLJ events show inertial oscillation and 35.3% of them are combined with sea breeze. Only 17.6% of LLJ events are associated with cold fronts and post‐frontal high pressure system. The urban heat island intensity, surface inversion layer strength, particulate matter 2.5 and carbon monoxide concentrations are negatively correlated with the LLJ intensity, suggesting the jet‐induced turbulent mixing may contribute to reduce them.

This work presents the use of two elastic lidar systems to assess the horizontal variation of the PBL height (PBLH) and aerosol layer aloft in the São Paulo Megacity. These two lidars performed simultaneous measurements 10.7 km apart in a highly urbanized and relatively flat area of São Paulo for two winter months of 2019. The results showed that the PBLH differences display diurnal variation that depends on the PBL during daytime growth phases. Cloud and sea breeze effects control most of PBLH variation. In the absence of cloud and sea breeze, the maximum difference (~300 m) occurs in the rapid development stage and is due to topographic effects. When the PBL approaches its maximum daily value, it tends to level off with respect to the topography. In addition, it was presented a method that combines elastic lidar (to detect an aerosol layer) and satellite data (to classify such a layer from Aerosol Optical Depth (AOD) and Aerosol Index (AI) information) for the detection of biomass burning events. This methodology demonstrated that the variations caused by Biomass Burning in AOD and AI enable both the detection of aerosol plumes originating from biomass burning and the identification of their origin.

ρ air density (kg m-3); cp, specific heat at constant

Main properties of the Low-Level Jet (LLJ) in the metropolitan region of São Paulo (MRSP) are numerically simulated with the WRF (Weather Research Forecasting) model for two 10-day field campaigns of the MCITY BRAZIL Project, carried out in February 19-28 (Summer) and August 6-15 (winter) 2013. The WRF model was able to simulate 66.6% of the observed LLJ events, displaying a high degree of agreement with the observed main properties. These modeling results confirmed that the presence of an Upper Tropospheric Cyclonic Vortex northeast MRSP during summer and South Atlantic Subtropical High circulation in the MRSP during winter favor the LLJ formation. On average, the simulated LLJ is 2.9 m s-1 more intense in the rural area and 189 m higher in the urban area. The direction of the LLJ does not vary much between the urban and rural areas. These differences can be attributed to the urban- rural contrast in the roughness and thermal properties at the surface, which together influence the intensity of turbulence in the urban boundary layer. The simulations indicated that LLJ is part of a shallow flow reminiscent of the daytime sea-breeze circulation produced by shallow baroclinicity associated by the daytime land-ocean thermal contrast. The WRF-model simulation indicates that the LLJ observed in the MRSP spreads westward for more than 300 km over the interior of the State of São Paulo, becoming more intense. The behavior of the rural LLJ can be explained in terms of the mechanical blocking effect produced by the Serra de Cantareira high hills to the easterly flow from ocean caused by a combination of sea breeze and large-scale circulations in the MRSP.