1.2-O Effects of offshore wind and sea breeze on the convective boundary layer
development observed by a UHF wind profiler installed near the coast
Jean-Luc Caccia1 and Bernard Campistron2
1
Laboratoire de Sondages Electromagnétiques de l’Environnement Terrestre/CNRS, University of
Toulon, La Garde, France
2
Laboratoire d’Aérologie/Centre de Recherches Atmosphériques/CNRS, Obs. Midi-Pyrénées,
Campistrous, France
1.Introduction
The perturbation of the boundary layer in coastal areas by synoptic winds and land/sea breezes has been and is the
subject of numerous experimental, theoretical and numerical studies (e.g. Bechtold et al, 1991; Simpson, 1994 ; Bastin et
al, 2006). One of the most complex situations occurs in summer daytimes when the ground is strongly heated by the sun,
while the sea surface temperature is still cool. Indeed, in this kind of situation the relatively cool air brought onshore by
the sea-breeze cell perturbs the convective boundary layer (CBL) development. The situation is made still more complex
in presence of offshore wind of synoptic origin. The opposition of those two wind regimes makes it difficult the
prediction of the CBL development and the transport of constituants within the CBL. Our purpose here is to
experimentally investigate their respective capability to act on that developement using wind and turbulence data
obtained by a UHF wind profiler installed near the coast.
2.Experimental and meteorological conditions
The dataset used and discussed here consists of the time evolution of the vertical profiles (from 300 m up to 2000/4000
m, with a 75 m resolution) of the horizontal wind and the kinetic turbulence dissipation rate ε obtained by a UHF wind
profiler (Degréane-Horizon 1238 MHz-radar) installed near the Mediterranean coast in the Marseille area during june
and july 2001 (i.e. during the ESCOMPTE campaign devoted to the study of the pollutant emission and transport, see
Cros et al, 2004). The radar intsallation site, STC, is indicated in Fig.1. Sea-breeze and offshore winds are revealed by
significant south and north meridional component, respectively, whereas the CBL development is revealed by both the ε
values and the height reached by significant ε values.
The here studied onshore wind is the Mistral. It is a north-westerly to northerly low-level, orography-induced, cold-air
out-break over the Gulf of Lions blowing offshore the south-eastern region of France at any season. The climate of this
area is under the influence of the Mistral which brings clear sky. It is frequently observed to extend as far as a few
hundreds of kilometers off shore and is one of the primary cause of storms over the northwestern Mediterranean. In
coastal areas, typical wind speed values at 500 m of altitude are in the 10 - 40 m s-1 range.
The here studied sea-breeze cases are the strongest ones since the period of the ESCOMPTE campaign is at the
beginning of summer. Indeed, at this time the sea surface temperature is still cool (18-20°C in average) while the ground
surface air temperature during daytimes reaches 30-35°C. Typical wind speeds of 5 to 7 m s-1 in the sea-breeze cell result
from such surface temperature contrasts. The breeze blows southerly until 12UTC and progressively turns to a westsouth-west direction at the end of the afternoon because of the Coriolis force.
3.Experimental results
Among the 35 days of available and analyzed data it is found that :
(1) During the 6 days of strong Mistral events, i.e. with offshore winds stronger than 15 m/s at 500 m-height, a
CBL cannot develop significantly. The observations suggest that it is replaced by a lower-level boundary layer
of dynamical origin produced by the surface friction of the Mistral. No evidence of a sea-breeze cell is found.
1
Jean-Luc Caccia, LSEET-LEPI/CNRS, Université du Sud Toulon-Var, bâtF, BP20132, 83957 La Garde Cedex, France
Tel : (+33) 4 94 14 24 15 / Fax : (+33) 4 94 14 24 17 / E-mail : [email protected]
(2) During the 7 days of weak to moderated Mistral events, i.e. with offshore winds weaker than 15 m/s at 500 mheight, the Mistral is lifted up above the sea-breeze cell and a well-marked CBL develops with apparent
characteristics very close to those observed above farther in-land sites in absence of wind. An example of such a
situation is shown in Fig.2 in the Appendix.
(3) during the 9 days of pure sea-breeze regim, i.e. without any synoptic winds, the occurrence of sea-breeze cells
does not prevent the CBL development, but it is less well marked than in the previous case.
4.Conclusions
Those results are very important in the general context of coastal meteorology, but also in the context of
pollution episode forecast, since they reveal that when a Mistral event is forecasted the pollutants are not necessarily
transported over the sea as generally expected. Indeed, results obtained elsewhere suggest that during days corresponding
to the case (2) the pollutant concentration above the Marseille area is the most elevated (Bastin et al, 2006). Moreover,
the atmospheric constituants can be vertically transported in the Mistral/breeze convergence zone up to low free
tropospheric levels. Therefore the CBL is expected to be cleaned from pollutants only if the Mistral event is strong, i.e.
with wind speed greater than 15 m/s at 500 m-height. During the pure sea-breeze regimes the pollutants are transported
over land within the CBL.
Figure 1: Geographical map of the south-eastern France. The dot line shows the Rhône-valley main axis parallely to
which the Mistral blows. The radar installation site is STC (Saint-Chamas).
References
Bechtold P., J.-P. Pinty and P. Mascart, J. Appl. Meteor., 30, 1268-1279, 1991
Bastin S. et al, Mon. Wea. Rev., 134, 1647-1668, 2006
Cros B. et al, Atmos. Res., 69, 241-279, 2004
Simpson J.E., Sea breeze and local wind, Cambridge Univ. Press, New-York, 234pp, 1994
Appendix
Figure 2 : Time-height diagrams of UHF-radar data obtained during the 21-to-23 June 2001 Mistral event above STC.
The vertical dotted lines indicate 0000 UTC. (a) : horizontal wind-vectors. The wind speed values are shown using a
color code, whereas the wind directions are indicated by the superimposed arrows (a northerly wind corresponds to an
arrow toward the bottom). (e) : turbulent dissipation rate values, ε.