Observational and Numerical Characterization of the Planetary Boundary Layer at the Urban Area of São Paulo, Brazil

São Paulo is the largest city of Brazil and has a population of more than 11 millions and more than 7 millions of vehicles, including cars, buses, motorcycles, trucks and others. It also is a host city for the 2014 FIFA World Cup of football, one of the greatest international sport event of the world. In this scenario, the weather conditions, air quality and aircraft turbulence are important topics of concern for all its inhabitants and the tourists that are coming for the event. Unfortunately, despite all of the efforts of the scientific community, that are still much to understand on these topics.

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). Up until now, only the summer experiment was already performed. During the experiment, that lasted 10 days (from February 19th to February 28th of 2013), a sounding was launched every 3 hours, night and day, totalizing 80 vertical profiles of temperature, humidity and wind speed and direction.

The goal of this work is to investigate the time evolution of the Planetary Boundary Layer during the period of the two campaigns: austral summer and winter. The soundings are used to estimate the PBL height and to calculate the momentum, latent and sensible heat fluxes vertical profiles. The synoptic conditions during the experiments are also analyzed in order to help the understanding of the PBL evolution. For the austral summer, the preliminary results show PBL heights that can reach 2000 m during the day. During the period of the experiment, two passages of cold fronts are observed and most of the days presents thunderstorms in the late afternoon. At night, the PBL height has lower values, around 250 m. An interesting feature is the presence of a low level jet, that appeared some nights during the campaign. The winter campaign will start at August 6th of 2013 and the soundings will be analyzed the same way as the summer campaign data. As future work, the observational data will be compared to numerical simulations using the WRF model, to better understand the physical mechanisms that characterize the atmospheric features of this important urban region.