Turbulent transfer within and above a street canyon

Objectives

To investigate the turbulent transfer mechanism within and above an urban canyon in a densely-built European city based on single-point and spatially-averaged statistics.

Background

An international project (BUBBLE) to study the regional climate, air flow, turbulence characteristic and dispersion in the urban boundary layer has been organised by the Institute of Meteorology, Climatology and Remote Sensing, University of Basel and Institute for Atmospheric and Climate Science, ETH in Zürich, Switzerland. The local organizers under the leadership of Roland Vogt put together an exemplary experiment which was featured on national TV. In particular, a micrometeorological tower was installed in a street canyon and equipped with several levels of slow and fast response sensors. This specific site was the focus of an IOP in June/July, 2002 to which various international research groups contributed additional resources and expertise. The main interest of the NUS group together with Jenny Salmond (University of Birmingham) was on the vertical variation of fluxes within and above the canyon and the measurement of line-averaged fluxes out of the canyon and above the roof-tops.

Methodology

Observations during the IOP were done from a tower erected near the North wall at the mid-point of a canyon typical of European residential city areas (joined apartment houses with one central court-yard making up one street block unit) (see photograph on the right). The tower was installed in the canyon and reached to about two times the height of the buildings. Additional instruments were installed on the roofs of buildings to the North and South of the canyon. Two scintillometers were used to measure line-averaged fluxes out of the canyon and above the roof-tops. Lines connecting respective transmitters and receivers are show in yellow.

One scintillometer was installed at canyon height (just below the height of the average roof-line). The photograph on the left is a view from the transmitter towards the receiver (about 150 m away) located on a balcony of a house on the opposite side of the canyon (yellow arrow).

The photograph on the right shows the receiver of the scintillometer installed a few meters above the average height of the roofs. The laser emitted by the transmitter can be seen in the far distance about 170 m away.

The top of the tower (at 31.7 m) installed at the site reaches to about two times the height of the buildings (photograph on the left). Fast response sensors were available at 6 heights - 3 within and 3 above the roof-line. Mean temperature, humidity and CO2 concentration is measured at 10 levels and net radiation at 2 levels (near canyon ground and at tower top).

Within the canyon, the turbulence sensors on the tower were located between the canyon wall and canyon center (left edge on photograph to the right). The exception was a RMY 3-D sonic anemometer which was placed just below the roof-line less than 0.5 m from the North wall to capture the flow along the canyon wall.

The scintillometer provides line-averaged statistics. To get an appreciation of the horizontal variability of fluxes and also to be able to compute corrections to the MOS theory implied in conventionally calculated scintillometer values, a CSI 3-D sonic anemometer was installed at the mid point of the roof-top scintillometer (photograph on the left).

The flux of carbon-dioxide over a city is of particular interest. The photograph on the right shows a LI-7500 open-path CO2/H20 analyzer (center, mounted at an angle) with a RMY 3-D sonic anemometer (left) installed in the center of the canyon at roof-level. Using the eddy-correlation approach it is possible to determine the fluxes of CO2, momentum, sensible and latent heat with this sensor combination. A similar system was installed at the top of the tower