Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 6 2 2006 10.5194/acpd-6-2809-2006 http://www.atmos-chem-phys-discuss.net/6/2809/2006/ http://www.atmos-chem-phys-discuss.net/6/2809/2006/acpd-6-2809-2006.html http://www.atmos-chem-phys-discuss.net/6/2809/2006/acpd-6-2809-2006.pdf 2809 2852 2006-04-11 Mesoscale circulations over complex terrain in the Valencia coastal region, Spain, Part 1: simulation of diurnal circulation regimes G. Pérez-Landa P. Ciais M. J. Sanz B. Gioli F. Miglietta J. L. Palau G. Gangoiti M. M. Millán Fundaci&oacute;n CEAM. Parque Tecnol&oacute;gico, c/ Charles R. Darwin 14, 46980 Paterna (Valencia), Spain Laboratoire des Sciences du Climat et de l’Environnement, UMR Commissariat &agrave; l’Energie Atomique/CNRS 1572, Gif-sur-Yvette, France IBIMET-CNR, Instituto di Biometeorologia, Consiglio Nazionale delle Ricerche, Firenze, Italy Escuela T&eacute;cnica Superior de Ingenieros Industriales de Bilbao, Universidad del Pa&iacute;s Vasco/Euskal Herriko Unibertsitatea, Bilbao, Spain We collected ground-based and aircraft data on meteorological parameters and CO<sub>2</sub> fluxes and concentrations during a 2-week intensive campaign over the Valencia basin, as part of a process study to understand how mesoscale circulations over complex terrain develop and affect the atmospheric transport acting on surface CO<sub>2</sub> fluxes. In this paper, we interpret the meteorological data during a selected case, with the help of a very high resolution mesoscale model to understand the diurnal cycle of mesoscale flow regimes, characterized by night-time katabatic drainage, morning sea-breeze development and subsequent coupling with mountain up-slopes, and evening flow-veering under larger-scale influences. At each step, a careful statistical analysis of the model performances is carried out. Despite the inherent complexity of the processes interacting with each other, and large model uncertainties for soil moisture boundary conditions and turbulence parameterizations, we show that it is possible to simulate faithfully the flow regimes, especially the inland progression and organization of the sea breeze. This provides confidence with respect to the future applicability of mesoscale models to establish a missing link between surface sources of CO<sub>2</sub> and atmospheric concentration signals over complex terrain.