Group of Micrometeorology - IAG/USP

Planetary Boundary Layer over complex surfaces

Planetary boundary layer (PBL) is the turbulent region of atmosphere in direct contact with the surface where most of human activities are performed. In the PBL is also located the main natural and anthropogenic sources of pollutant gases and particulate matter.

The PBL has its origin connected to the exchange of energy, momentum and mass between atmosphere and surface, which in turn is modulated by turbulence within time scales of one hour or less. In this time scale, the vertical turbulent fluxes of sensible and latent heat determine the mean structure of the PBL temperature and humidity fields, while the vertical turbulent flux of momentum establishes the mean structure of the PBL winds over continental rural e urban) and oceanic regions.

Among all the micrometeorological processes relevant to the PBL, turbulence is the most important and also the more complex one. Turbulence is still an unsolved phenomenon in classic physics despite the big effort carried out in the last 100 years. This difficulty reflects dramatically on the development of related scientific areas. Up to now, it was not possible to develop a general theory that could describe the turbulence behavior with the required precision for meteorological, environmental and technological applications.

Despite the inherent difficulties, the main motivation for micrometeorological studies is that only with a better understanding of the atmospheric turbulence will be possible to improve (i) the weather and climate prediction skills; (ii) the assessment of the environmental impact of atmospheric pollutant emissions; (iii) the precise evaluation of the surface energy components; (iv) the surface water and momentum balance and (v) the knowledge of the dynamic of the winds to accurately estimate the wind effects over buildings and structures.

This project aims to develop a methodology of atmospheric turbulence investigation using both numerical and analogical modeling of turbulent flows over regions covered by complex topography and heterogeneous land use in order to be applied without distinction to characterize the PBL atmospheric processes over urban, rural and oceanic regions. It will allow a significant progress in the understanding of the PBL turbulent processes.
The present PBL knowledge is based, in most of the cases, on information gathered from laboratory investigations of turbulent boundary layers over rigid surfaces. According to these studies, turbulent flows can be vertically divided in adjacent layers where turbulence behaves in different ways.

Considering the vertical spatial distribution of the turbulence production, the tradition PBL – the one where turbulence is produced at the surface and transferred vertically upwards – can be divided in one internal boundary layer adjacent to the surface where are located all instability mechanisms that generate and sustain the turbulence and another external boundary layer where the turbulence production is smaller.

In general, the internal boundary layer over most of the surfaces can be divided in canopy, roughness and surface layers. The canopy layer is the region immediately above the surface where most of the roughness elements are located. Roughness layer is the region above the canopy where the turbulence field responds individually to the roughness elements and finally, the surface layer is the region where the turbulence is in equilibrium with the surface and therefore is valid the Monin Obukhov theory.

These three layers are affected directly by the surface features and compose the internal turbulent boundary layer. One major goal of this project is to characterize the turbulence structure within these three layers using analogical simulation of turbulent flows. It is also intend to evaluate using numerical simulations the impact of thermal stratification over the turbulence structure in the internal boundary layer. It is important to emphasize that there is no an universal theory applicable to describe the turbulence behavior in the internal boundary layer valid for heterogeneous surface.

The general objective of the project is to study, by numerical and analogical simulation, the dynamic and thermodynamic structure of the PBL over continental (urban and rural) and oceanic surfaces. The simulations will be performed to quantify the turbulence properties in the internal layer of the PBL.

Three regions were chosen to carry out the project: (i) Metropolitan region of São Paulo, MRSP (urban PBL); (ii) Iperó (rural PBL) e (iii) Atlantic ocean (maritime PBL).
As representative of continental areas it will be studied Iperó, SP, (rural region) and a pilot urban area of the MRSP (urban region). As representative of oceanic region it will be investigated the São Pedro and São Paulo Archipelago, located at the tropical open sea and the southeast continental platform of Brazil.

Publications related to the PBL project

Marciotto E.R., 2008: Estudo da influência de um dossel urbano sobre o balanço de energia na superfície e implicações na estrutura vertical da camada limite atmosférica. Tese de Doutorado. Departamento de Ciências Atmosféricas, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, SP, Brasil, 120 pp.

Codato G., 2008: Simulação numérica da evolução diurna do monóxido de carbono na camada limite planetária sobre a RMSP com modelo LES. Dissertação de Mestrado. Departamento de Ciências Atmosféricas, Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, SP, Brasil, 94 pp.

Codato G., Oliveira A.P., Soares J., Marques Filho E.P. e Rizza U., 2008: Investigação do monóxido de carbono na RMSP usando LES. Anais do XV Congresso Brasileiro de Meteorologia, São Paulo, SP.

Codato G., Oliveira A.P., Soares J., Marques Filho E.P. and Rizza U., 2008: Investigation of carbon monoxide in the city of São Paulo using large eddy simulation. Proceedings of 15th Joint Conference on the Applications of Air Pollution Meteorology with the A&WMA, 88th Annual Meeting, 20-24 January 2008, New Orleans, LA (CDROM).