Vertical Structure of the Urban Boundary Layer over Marseille Under Sea-Breeze Conditions

Abstract

During the UBL-ESCOMPTE program (June–July 2001), intensive observations were performed in Marseille (France). In particular, a Doppler lidar, located in the north of the city, provided radial velocity measurements on a 6-km radius area in the lowest 3 km of the troposphere. Thus, it is well adapted to document the vertical structure of the atmosphere above complex terrain, notably in Marseille, which is bordered by the Mediterranean sea and framed by numerous massifs. The present study focuses on the last day of the intensive observation period 2 (26 June 2001), which is characterized by a weak synoptic pressure gradient favouring the development of thermal circulations. Under such conditions, a complex stratification of the atmosphere is observed. Three-dimensional numerical simulations, with the Méso-NH atmospheric model including the town energy balance (TEB) urban parameterization, are conducted over south-eastern France. A complete evaluation of the model outputs was already performed at both regional and city scales. Here, the 250-m resolution outputs describing the vertical structure of the atmosphere above the Marseille area are compared to the Doppler lidar data, for which the spatial resolution is comparable. This joint analysis underscores the consistency between the atmospheric boundary layer (ABL) observed by the Doppler lidar and that modelled by Méso-NH. The observations and simulations reveal the presence of a shallow sea breeze (SSB) superimposed on a deep sea breeze (DSB) above Marseille during daytime. Because of the step-like shape of the Marseille coastline, the SSB is organized in two branches of different directions, which converge above the city centre. The analysis of the 250-m wind fields shows evidence of the role of the local topography on the local dynamics. Indeed, the topography tends to reinforce the SSB while it weakens the DSB. The ABL is directly affected by the different sea-breeze circulations, while the urban effects appear to be negligible

North-western Mediterranean sea-breeze circulation in a regional climate system model

“…A large number of data were collected (meteorological data from surface weather stations, radiosondes and constant volume balloons, wind profiles from lidar, radar and sodar profilers, … ; see Cros et al 2004 for more details) and meso-scale numerical simulations performed. They allowed the analysis of the mesoscale transport and dilution by the sea-breeze, of the impact of the topography (Bastin et al 2005a, b;Bastin and Drobinski 2005) and major urban areas (Lemonsu et al 2006) on the sea-breeze circulation. They also allowed the analysis of the contribution of the sea-breeze to the regional transport of humidity (Bastin et al 2005a and pollutants Menut et al 2005) and the evaluation of existing sea-breeze scaling laws with the large body of observations collected during the campaign .…”

“…The vertical variation of the wind direction corresponds to the co-existence of a shallow sea breeze which blows from the surface up to about 400 m below a deep seabreeze which blows above up to about 1 km Lemonsu et al 2006). Similarly to what Banta (1995) showed experimentally, two sea breezes do occur on two different depths and timescales: (1) a localscale temperature contrast close to the shoreline, which has a pattern correlated to the coastline shape, drives a shallow sea breeze blowing perpendicularly to the local coastline; (2) a larger-scale temperature contrast drives a deeper sea breeze blowing from the south (the isotherms have a predominant east-west orientation) that develops later in the day.…”

“…The horizontal extent of the breeze circulation is expected to be even larger in the Southern shore of the Mediterranean region as it scales as the Rossby deformation radius, which is inversely proportional to the Coriolis parameter (Rotunno 1983;Dalu and Pielke 1989;Drobinski and Dubos 2009;Drobinski et al 2011). The inland penetration which controls inland moisture advection and often determines the band where convection is triggered during warm seasons, can be modulated by interaction with mountain slopes (Kusuda and Alpert 1983;Bastin and Drobinski 2005), channeling in local valleys (Bastin et al 2005a, b) or by synoptic wind (Estoque 1962;Arritt 1993), and particularly by Mistral or by major urban areas (Lemonsu et al 2006).…”

Linear breeze scaling: from large‐scale land/sea breezes to mesoscale inland breezes

“…At smaller scales, many studies have shown that landuse practices such as deforestation or changes due to agricultural management as well as urbanization may affect regional climate, ecosystems and water resources (Segal et al, 1988;Stohlgren et al, 1998;Pielke and Avissar, 1990;Lemonsu and Masson, 2002;Lemonsu et al, 2006; LINEAR BREEZE SCALING 1767 Champollion et al, 2009) and, on a global scale, spatially heterogeneous land-use effects may be at least as important in altering the weather as changes in climate patterns associated with greenhouse gases (Pielke et al, 2002;Pielke, 2005). Surface heterogeneities over the continent induce spatial variability in surface heat fluxes that can create inland breezes similar to sea/land breeze systems (Mahfouf et al, 1987;Mahrt et al, 1994;Patton et al, 2005;Courault et al, 2007).…”

Water vapour variability induced by urban/rural surface heterogeneities during convective conditions

“…Paris is a large city of over 2 million inhabitants, located 200 km away from the sea (Atlantic Ocean) and has a quite flat topography (Figure 1). Paris and the surroundings are thus perfectly suited to study urban meteorology (Menut et al, 1999;Lemonsu and Masson, 2002;Troude et al, 2002) since neither the orography nor the sea dominates over the urban processes, in contrast to coastal cities (Ohashi and Kida, 2004;Bastin et al, 2005Bastin et al, , 2007Lemonsu et al, 2006).…”