Low-level jet modeling investigation in the metropolitan region of São Paulo, Brazil

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.