Vertical profiles of carbon dioxide, temperature, and water vapor within and above a suburban canopy layer in winter
Ryo Moriwaki, Tokyo Institute of Technology, Tokyo, Japan; and M. Kanda
In this study we focused on highly accumulated carbon dioxide (hereafter CO2) within a suburban canopy under nocturnal stably stratified conditions. The results were derived from the wintertime field measurements of the vertical profiles of CO2, air temperature, and turbulent flows, which were conducted in a residential area (the mean height of canopy is 7.3 m) of Tokyo, Japan.
In the daytime especially under very windy condition, CO2 concentration measured at a reference height (29-m height a.g.l.) is almost same level with background CO2 concentration (=380 ppmv), while in the nighttime the CO2 concentration increased. The increase of CO2 concentration was significantly associated with the stably stratified condition. We therefore examined the ensemble mean vertical profile of CO2 concentration using bulk Richardson number (Rb) as a stability index.
Under stably stratified conditions (Rb > 5), the CO2 concentration above the canopy decreased with height (the difference within and above the canopy reaches 40 ppmv). On the contrary, the CO2 concentration within the canopy kept almost same level, which indicates that the CO2 emitted from the houses accumulated within the canopy. This was also visible in the snap-shot of height-temporal contour map of CO2 concentration. Such behavior was not found in H2O profile. The vertical profile of air temperature and additional observations on the surface temperature using a thermal infrared camera suggested that the cold air generated at roof level moved down to the ground level, causing the minimum of air temperature to occur at ground surface level. The effect of ‘cold air subsidence' within the canopy is the most plausible reason for the dynamical behavior of scalars. The cold air generated at the roof-top takes down the high CO2 emitted from ventilation fan towards the ground level. For the case of H2O, there is no significant source of H2O within the canopy except for the soils in the backyard. Therefore, the subsidence flow from the roof level takes the less humid air down to the ground and decreases the H2O concentration within the canopy except the region closest to the ground.
Under unstable conditions (Rn < -1), the CO2 concentration for the unstable case during daytime is almost homogeneous within and above the canopy, although the CO2 flux was upward even in the daytime. The locations of the ventilating fans are usually in the middle or upper part of the canopy. Therefore, the emitted CO2 from the houses were easily dispersed by larger turbulent mixing. The shape of the H2O profile for the unstable case was quite different from that of CO2. The H2O concentration was highest near the ground and there were differences within and above the canopy. This is probably because the emission of H2O was located in the vicinity of the ground surface. The turbulent kinetic energy within the canopy is less than that above or in the upper part of the canopy, and thus the emitted H2O near the ground would not be well-mixed. The present results and discussion indicate that urban micro-climate models should include both the three-dimensional turbulent flow around the building and the source distributions to accurately describe the dynamical behavior and diffusion processes of the scalars within and above urban canopies.
Extended Abstract (752K)
Supplementary URL: http://www.cv.titech.ac.jp/~kandalab/
Joint Session 8, Urban Turbulent Transport And Dispersion Processes III (Cosponsored by BL&T committee) (Joint With The 6Th Symposium On The Urban Environment And The 14Th Joint Conference On The Applications Of Air Pollution Meteorology With The A&WMA)
Thursday, 2 February 2006, 1:30 PM-2:45 PM, A315
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