Observational investigation of the low-level jets in the metropolitan region of são paulo, brazil

In this work the main features and evolution of Low-Level Jets (LLJs) in the metropolitan region of São Paulo, Brazil, are investigated using two sets of radio soundings carried out: (a) every three hours and during 10 consecutive days in the summer and winter field campaigns of the MCITY BRAZIL Project and (b) regularly two per day between 2009 and 2013. Visual inspections of wind speed and wind direction profiles indicate the presence of LLJs in 85 % of days, predominantly at nighttime during undisturbed conditions, with mean intensity and height of 8.4 ± 0.32 m s and 541 ± 26 m, respectively. The predominant wind directions at the maximum wind speed level are from east, north and northeast during field campaigns of 2013. Most of the LLJs observed during both field campaigns (76 %) are associated to the inertial oscillation mechanism and modulated by the baroclinicity induced by mesoscale features of topography and land-ocean thermal contrast. The remaining cases are associated to synoptic disturbances baroclinicity. The monthly variations of LLJs properties between 2009 and 2013 were estimated using an objective method and the results showed that the LLJs are very frequent throughout the year, exceeding 65 % of days in all months. The maximum LLJs occurrence was observed in October (86 % of days) and the minimum frequency occurs in July (69 % of days). The impact of LLJs in the intensity of urban heat island and surface inversion layer was assessed during field campaign of August, 2013 and the results indicate that the shear generated by the LLJs plays an important role in reducing the SI layer and urban heat island