Turbulent transfer in a tropical city

Objectives

Field study of energy balance fluxes and first measurements of CO2 fluxes in a tropical city plus analysis of the turbulent transfer mechanisms.

Background

Energy balance and CO2 flux measurements were carried out during a few days between March 18 - April 2, 2003 in Singapore (1deg 17min N, 103 deg 51min E). The location of the observations was in a predominately industrial area characterized by mostly square-shaped warehouse-type buildings of mean height 8 m in Jurong (SW Singapore). Streets are generally lined by mature trees with canopy height of up to 14 m.

Methodology

The eddy correlation approach was used for the measurement of the turbulent fluxes of sensible heat and momentum (using a CSI-CSAT) and latent heat and CO2 (using a LI-7500 open-path gas analyzer). The fluctuations were recorded at 20 Hz on a CSI-CR500 data logger. The turbulence sensors were attached to the side of a basket at the end of a manlift which was extended to 25 m, i.e. well above the roughnes sublayer (see photos to the right). The basket housed the data logging equipment and one researcher and was rotated in order for the turbulence sensors to be directed into the mean wind. Net radiation and standard meteorological variables (wind speed and direction, temperature and humidity, surface temperature) were also measured. Post procesing of the data included rotation of the sonic anemometer into the mean wind and application of the WPL correction to the latent heat and CO2 fluxes.

The photograph on the left shows the Li-7500 H2O/CO2 gas analyzer (left) and 3D-CSAT (right) at 25 m above ground looking towards NW, the predominant daytime wind direction. The tall buildings in the background at a distance of about 4 km are part of the new Boon Lay HDB development.

First results

The 2 graphs below show the normalized spectra of vertical velocity (on the left) and the cospectra of sensible heat (on the right) for a few selected cases representing stabilities between -0.75 < z'/Lv < 0.003. The agreement with similar observations over rural and other urban surfaces is very good in both cases in terms of location of peaks and the high-frequency roll-off in the inertial subrange.

The normalized CO2 spectra (on the left) and CO2 flux cospectra (on the right) shown below are the first measured in a tropical urban environment. Overall the (co)spectral shapes generally follow "expected" behaviour, but further analysis is needed to determine the extent of similarity or differences to other urban and rural reference data.