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 o ften occurs. Sodar data were combined with ground station measurements in order to assess a complete image of the frontal system in the plan etary boundary layer. Its signature differs from the signature observe d normally over a flat bare ground without orographic complications. F riction 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 oscillat ing layers revealing the presence of wave activity have been observed just after the frontal passage. The friction velocity maximum occurs w hen these layers reach the ground level. On the other hand, sodar and ground station data display the presence of a northerly airflow intrus ion on the lee side of the mountain, presumably due to pressure differ ence between the mountains flanks and the plain on the north of the mo untain barrier.