DOPPLER SODAR OBSERVATIONS OF A COLD-FRONT DURING THE PYREX EXPERIMENT OROGRAPHIC EFFECTS

During the PYREX experiment, a frontal event was observed by sodar on the lee side of the Pyrenean mountain range where wave-like activity often occurs. Sodar data were combined with ground station measurements in order to assess a complete image of the frontal system in the planetary boundary layer. Its signature differs from the signature observed normally over a flat bare ground without orographic complications. Friction velocity has oscillating values before the frontal passage and does not decrease simultaneously with the horizontal wind speed after the frontal passage. On the sodar facsimile records downward oscillating layers revealing the presence of wave activity have been observed just after the frontal passage. The friction velocity maximum occurs when these layers reach the ground level. On the other hand, sodar and ground station data display the presence of a northerly airflow intrusion on the lee side of the mountain, presumably due to pressure difference between the mountains flanks and the plain on the north of the mountain barrier.