Observational and numerical study of the time evolution of the planetary boundary layer at the urban area of São Paulo, Brazil

The MCITY project, conducted by the Group of Micrometeorology at the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) of the University of São Paulo (USP), intends to investigate the micrometeorological features of the urban climate of the cities of São Paulo and Rio de Janeiro. This knowledge is crucial to better understand the atmosphere dynamics and consequently air quality, weather conditions and Urban Heat Islands phenomenon in these two cities. The present work analyses part of the data that are being collected during the project: an experiment that comprises a series of atmospheric soundings performed during the austral summer and winter at the Campo de Marte airport, near the center of the city of São Paulo. The two periods were chosen because observations indicate that the climate in São Paulo is divided in two predominant seasons: a wet and hot summer, with monthly averaged maximum values of temperature and precipitation in February (22.5 °C and 255 mm, respectively), and of humidity from December to January and from March to April (80 %), and a dry and cold winter, with monthly averaged minimum values of temperature and humidity from July to August (16 °C and 74 %, respectively) and of precipitation in August (35 mm). During the experiment, that lasted 10 days in the summer (from February 19th to February 28th of 2013) and in the winter (from August 6th to August 16th of 2013), a sounding was launched every 3 hours, night and day, totalizing 80 vertical profiles of temperature, humidity and wind speed and direction for each campaign. Then, experiments with the Weather Research and Forecast (WRF) model were performed to simulate the same days and hours of the soundings. The WRF model is an atmospheric numerical model, suitable for scales ranging from meters to thousands of kilometers, that is being used worldwide in a broad range of applications. It is designed to be a flexible, state-of-the-art atmospheric simulation system, that is portable and efficient on available parallel computing platforms. The goal of this work is to investigate the time evolution of the Planetary Boundary Layer at the city of São Paulo during the period of the two campaigns: austral summer and winter, by analyzing the soundings and comparing with the results of the WRF model. So far, only the summer soundings have been analyzed and simulated. The soundings showed that the PBL height reached over 2000 m during the day and then collapsed to no more than 250 m during the night, but there is evidence of a residual layer. Another interesting feature is the wind vertical profile. The wind reaches over 12 m s-1 almost every night, around 800 m of height. The results from the model do not entirely agree with the soundings, but some patterns are present, indicating that it is an important tool to better understand the physical processes occurring in the atmospheric boundary layer