Impact of the Rho ˆne and Durance valleys on sea-breeze circulation in the Marseille area Sophie Bastin a, * , Philippe Drobinski a , Alain Dabas b , Patricia Delville c , Oliver Reitebuch d , Christian Werner d a Institut Pierre Simon Laplace/Service d’Ae ´ronomie, Paris, France b Centre National de Recherches Me ´te ´orologiques, Me ´te ´o-France, Toulouse, France c Institut National des Sciences de l’Univers-Division Technique, Meudon, France d Institut fu ¨r Physik der Atmospha ¨re, DLR, Oberpfae ´nhofen, Germany Received 1 July 2003; received in revised form 1 February 2004; accepted 1 April 2004 Abstract Sea-breeze dynamics in the Marseille area, in the south of France, is investigated in the framework of the ESCOMPTE experiment conducted during summer 2001 in order to evaluate the role of thermal circulations on pollutant transport and ventilation. Under particular attention in this paper is the sea-breeze channelling by the broad Rho ˆne valley and the narrow Durance valley, both oriented nearly-north–south, i.e., perpendicular to the coastline, and its possible impact on the sea- breeze penetration, intensity and depth, which are key information for air pollution issues. One situation of slight synoptic pressure gradient leading to a northerly flow in the Rho ˆ ne valley (25 June 2001) and one situation of a weak onshore prevailing synoptic wind (26 June 2001) are compared. The impact of the Rho ˆne and Durance valleys on the sea-breeze dynamics on these two typical days is generalized to the whole ESCOMPTE observing period. The present study shows by combining simple scaling analysis with wind data from meteorological surface stations and Doppler lidars that (i) the Durance valley always affects the sea breeze by accelerating the flow. A consequence is that the Durance valley contributes to weaken the temperature gradient along the valley and thus the sea-breeze circulation. In some cases, the acceleration of the channelled flow in the Durance valley suppresses the sea-breeze flow by temperature gradient inhibition; (ii) the Rho ˆne valley does not generally affect the sea breeze 0169-8095/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2004.04.014 * Corresponding author. Laboratoire de Me ´te ´orologie Dynamique, Institut Pierre Simon Laplace, Ecole Polytechnique, 91128 Palaiseau Ce ´dex, France. Tel.: +33 1 69 33 31 98; fax: +33 1 69 33 30 05. E-mail address: sophie.bastin@aero.jussieu.fr (S. Bastin). Atmospheric Research 74 (2005) 303 – 328 www.elsevier.com/locate/atmos