Temperature and wind velocity oscillations along a gentle slope during sea-breeze events.

The flow structure at Vallon d'Ol on a gentle slope in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment.

The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60 to 90~min oscillation period) and several days (about 120 to 180~min oscillation period) during the whole observing period.

Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.

Comparatively to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published, probably because of temperature inversion breakup that inhibits upslope winds. The present paper shows that cold air advection by sea-breeze generates a mesoscale horizontal temperature gradient that forces the baroclinicity of the atmosphere which then allows low-frequency fluctuations, similarly to katabatic flow. An expression of the oscillation period is derived which accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period are compared to the measurements and a very good agreement is found.

The statistical analysis of the wind flow at Vallon d'Ol shows a dominant northeasterly-easterly flow pattern for nighttime oscillations and a dominant southwesterly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results which show that the air drains off the mountain along the maximum slope direction which in the studied case is oriented southwest/northeast.