Flux and turbulence measurements at a densely built‐up site in Marseille: Heat, mass (water and carbon dioxide), and momentum

Abstract

[1] Eddy covariance (EC) observations above the densely built-up center of Marseille during the Expérience sur site pour contraindre les modèles de pollution atmosphérique et de transport d'émissions (ESCOMPTE) summertime measurement campaign extend current understanding of surface atmosphere exchanges in cities. The instrument array presented opportunities to address issues of the representativeness of local-scale fluxes in urban settings. Separate EC systems operated at two levels, and a telescoping tower allowed the pair to be exposed at two different sets of heights. The flux and turbulence observations taken at the four heights, stratified by wind conditions (mistral wind and sea breeze), are used to address the partitioning of the surface energy balance in an area with large roughness elements. The turbulent sensible heat flux dominates in the daytime, although the storage heat flux is a significant term that peaks before solar noon. The turbulent latent heat flux is small but not negligible. Carbon dioxide fluxes show that this central city district is almost always a source, but the vegetation reduces the magnitude of the fluxes in the afternoon. The atmosphere in such a heavily developed area is rarely stable. The turbulence characteristics support the empirical functions proposed by M. Roth.

Modeling the energy balance in Marseille: Sensitivity to roughness length parameterizations and thermal admittance

“…Combining these previous steps results in an energy flux from traffic of 6.3 W m À2 per 10 mmol m À2 s À1 CO 2 . Following the CO 2 flux measurements of Grimmond et al [2004], the diurnal CO 2 flux values range from 10 mmol m À2 s À1 during the night to 30 mmol m À2 s À1 , which corresponds to a range of Q Fv between 6.3 (night) to 18.9 W m À2 (day). To determine hourly energy flux values from traffic it is then fitted (within its measurement errors) to the diurnal evolution of the measured CO 2 flux, resulting in a traffic induced anthropogenic heat of 5.3 W m À2 between 20 and 5 h GMT, a linear increase (decrease) between 5 and 8 h GMT (18 -20 h GMT) from 5.3 W m À2 to 18.9 W m À2 and a constant flux between 8 and 18 h GMT of 18.9 W m À2 , resulting in a daily mean of 12.3 W m À2 .…”

“…The absorption of CO 2 by vegetation during the day results in a negative (downward) CO 2 flux, which should be taken into account while interpreting the CO 2 measurements of Grimmond et al [2004] to obtain the ''pure'' traffic signal. Therefore we use the water use efficiency, which returns the amount of assimilated CO 2 per kg evaporated water.…”

“…[48] The total anthropogenic heat flux of 18.6 W m À2 is calculated for the CAAM site using the CO 2 fluxes measured by Grimmond et al [2004]. Following Lemonsu et al [2004] and Hamdi and Schayes [2005] the plan area of vegetated surfaces per total plan area for this urban measurement site adds up to 14%.…”

“…In order to be able to compare our simple approach to the results from LUMPS, we use the observation done by Grimmond et al [2004] to calculate all fluxes. A full explanation of LUMPS is given by Grimmond and Oke [2002], and hereby, only a short description of its configuration is given.…”

Trade-offs and responsiveness of the single-layer urban canopy parametrization in WRF: An offline evaluation using the MOSCEM optimization algorithm and field observations

“…Using measurements of CO 2 fluxes as an indication of human activity in the area, Grimmond et al (2004) Figure 7(b)). In a related study using data from the meso-NH mesoscale model and two intensive observation periods (21-23 and 24-26 June), Pigeon et al (2007) have shown that for a particular set of synoptic conditions (e.g.…”