CO 2 dispersion modelling over Paris region within the CO 2-MEGAPARIS project

Lac, C.; Donnelly, R. P.; Masson, V.; Pal, S.; Riette, S.; Donier, S.; Queguiner, S.; Tanguy, G.; Ammoura, L.; Xueref-Remy, I.

Accurate simulation of the spatial and temporal variability of tracer mixing ratios over urban areas is a challenging and interesting task needed to be performed in order to utilise CO 2 measurements in an atmospheric inverse framework and to better estimate regional CO 2 fluxes. This study investigates the ability of a high-resolution model to simulate meteorological and CO 2 fields around Paris agglomeration during the March field campaign of the CO 2-MEGAPARIS project. The mesoscale atmospheric model Meso-NH, running at 2 km horizontal resolution, is coupled with the Town Energy Balance (TEB) urban canopy scheme and with the Interactions between Soil, Biosphere and Atmosphere CO 2-reactive (ISBA-A-gs) surface scheme, allowing a full interaction of CO 2 modelling between the surface and the atmosphere. Statistical scores show a good representation of the urban heat island (UHI) with stronger urban–rural contrasts on temperature at night than during the day by up to 7 °C. Boundary layer heights (BLH) have been evaluated on urban, suburban and rural sites during the campaign, and also on a suburban site over 1 yr. The diurnal cycles of the BLH are well captured, especially the onset time of the BLH increase and its growth rate in the morning, which are essential for tall tower CO 2 observatories. The main discrepancy is a small negative bias over urban and suburban sites during nighttime (respectively 45 m and 5 m), leading to a few overestimations of nocturnal CO 2 mixing ratios at suburban sites and a bias of +5 ppm. The diurnal CO 2 cycle is generally well captured for all the sites. At the Eiffel tower, the observed spikes of CO 2 maxima occur every morning exactly at the time at which the atmospheric boundary layer (ABL) growth reaches the measurement height. At suburban ground stations, CO 2 measurements exhibit maxima at the beginning and at the end of each night, when the ABL is fully contracted, with a strong spatio-temporal variability. A sensitivity test without urban parameterisation removes the UHI and underpredicts nighttime BLH over urban and suburban sites, leading to large overestimation of nocturnal CO 2 mixing ratio at the suburban sites (bias of +17 ppm). The agreement between observation and prediction for BLH and CO 2 concentrations and urban–rural increments, both day and night, demonstrates the potential of using the urban mesoscale system in the context of inverse modelling



Lac, C. / Donnelly, R. P. / Masson, V. / et al: CO2 dispersion modelling over Paris region within the CO2-MEGAPARIS project. 2013. Copernicus Publications.


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