Modelling urban area in Alpine complex terrain in winter time

Urban area which are situated at the intersection of Alpine valleys are submitted to complex atmospheric patterns with significant impacts on air quality. Local dynamics develop systems of slope and valley winds which interact ones with the others. Grenoble, a 400 000 inhabitant town in the west French Alps is typical of such an area. Summer conditions with ozone peak values were extensively considered in previous studies [2],[3]. Several systems of numerical models were successfully used to efficiently describe the predominantly convective activity. Winter time is considered here because generating also health hazards associated to high level of particulate matter. Stable flow conditions induce low mixing by damping kinetic turbulent energy and reducing vertical length scales. Even in winter, sun radiation plays a major role in inducing some convective activity while at night soil cooling contributes to increase stability. The configuration of the urban site under interest together with Alpine valley narrowness create various space heterogeneities from changes in canopy (forest, rural, peri urban, urban) to slope orientation and development (gentle slope in the south versus cliffs on the west side).

Because of the interest in health impact, there is a need of refining model grid size, down to few hundreds meters in the urban area. Larger length scales have also to be considered in order to take full account of the interactions between synoptic winds and atmosphere local to the valleys. With regard to time evolution, morning and sunset transition periods are critical in building up a convective mixing layer and a stable layer respectively. A non hydrostatic model with appropriate turbulent 3D formulation is run in order to cover a variety of flow regimes, from stable layers to fully thermally convective layers. The model is operated on nested grids to take account of the different scales of the terrain, from the Alpine massif to fine details in the valley. The Meso-NH model [1] is used here down to a 300 meters horizontal mesh size with 4 nested domains. It includes the SURFEX module for ground boundary condition with a special handling of town canopy through the TEB model [4].

February 2005 episode was selected because of significant pollution with particulate matter [2]. Temperature distribution near ground is analysed as it evolves with time, focus is put on the transitions in the morning and at night. Heat fluxes at ground level are diagnosed at the same time in order to identify terrain induced contributions (orography, soil cover) and urban effects. Urban heat island effect is assessed from temperature difference between the town center and a rural station in Charavines Vertical distribution of air mass in each of the three contributing valley is considered.

Focus is put here on flow transport and mixing properties. Heterogeneous chemistry is not yet taken into account. Particulate matter, at least, the locally emitted contribution, can be, as a first step, modelled with a passive scalar as in [3].

References

[1] Bougeault, P. Mascart, P, et al, 2001, . The Meso-NH Atmospheric Simulation System: Scientific Documentation, http://mesonh.aero.obs-mip.fr/mesonh/dir_doc/book1_14dec2001/book1_14122001.html

[2] Chaxel E., 2006, Photochimie et aérosol en région alpine: mélange et transport, Thèse de l'Université Joseph Fourier, Grenoble, France [http://tel.archives-ouvertes.fr/].

[3] Chemel, C., Chollet, J.P., Chaxel, E., 2007, On the suppression of the urban heat island over mountainous terrain in winter, 29 th International Technical Meeting on Air Pollution Modelling and its Application, September 24, Aveiro – Portugal http://www2.dao.ua.pt/itm/29th/presentations.htm

[4] Masson, V. Grimmond, C. S. B., Oke T. R, 2002, . of the Town Energy Balance (TEB) with direct measurements from dry districts in two cities, Journal of applied meteorology, 41, 1011-1036