With the BASTA project "Turbulence investigations in the urban boundary layer" it is intended to make a contribution to pure research of the turbulent exchange processes in the urban boundary layer. The current state of research in that area shows that in general, due to the special properties of the urban infrastructure, there is a lack of suitable data for the development and the verification of corresponding models. Here the requested project shall start.
A city consists of building structures of varying material and varying horizontal and vertical length scales. The interaction between this heterogeneous urban surface and the atmosphere modifies the surface layer and produces an urban boundary layer. The turbulent exchange processes will vary strongly due to the inhomogenous physiognomy of a city both in time and space. Strictly speaking the conventional methods to describe the characteristics of turbulent exchange of momentum, heat and mass, as developed for smooth, homogeneous surfaces, can not be applied to the urban boundary layer. Due to the lack of better knowledge dispersion- and flow-models still apply the semi-empirical Monin-Obukhov similarity framework for the parametrisation of the urban boundary layer. Therefore the present model calculations are normally based on very simplified methods.
For modelling purposes it is important to know the complex structure of the turbulent characteristics above urban areas and how they compare to the better known values of other surfaces types. The dimensionless variances of the three wind components are of special interest because they play a dominant role in pollution dispersion models.
In the framework of the SNF-project 20-40621.94 BASTA (BAsler STAdtklima) the turbulent fluxes of momentum and heat are measured for the first time continuously over a long period on a innercity tower at 32 m above ground in addition to the measurement of the basic meteorological parameters temperature, humidity, wind and radiation. These measurements at the BASTA-tower of the MCR Lab, Departement of Geography, are complemented by a supporting measurement campaign from July 1995 to the end of February 1996 at an antenna-tower on the roof of the Messe Basel. This tower is located 24 m above street level and reaches a height of 76 m above ground. The turbulent fluxes were measured at three different heights. This allows a better characterization and analysis of the vertical structure of turbulence. Simultanuously a SODAR was operated from July 1995 to September 1995 close to the antenna tower. This additional measurement campaign at the Messe Basel was carried out in cooperation with the Geographisches Institut of the ETH Zürich.
The BASTA-project is closely connected to the international projects REKLIP (Regio Klima Projekt) and ERSCliP (ERS-1 Climate Project), which are carried out at the MCR Lab. The data of these projects are fully available to the BASTA-project. By this, the possibility to extend the basic knowlegde of the turbulent structures in selected urban locations is given.
Keywords: Boundary layer meteorology, urban climatology, atmospheric turbulence, urban heat island, measurement technique for atmospheric turbulence.