Interaction of a cold front with a sea‐breeze front Observations

On 9 May 1989, during the field experiment FRONTEX, a synoptic‐scale cold front was observed which moved from the North Sea to Northern Germany and interacted with a sea‐breeze front. The modification of the cold front is documented by satellite images and measurements over the sea, at the coast and further inland. The synoptic‐scale front was characterized by weak frontal gradients over the sea. It was aligned approximately parallel to the coast as was approximately the wind ahead of it. While the synoptic‐scale front approached the coast during the forenoon hours, a strong temperature contrast developed between sea and land due to solar heating of the land surface. This led to the formation of a sea‐breeze front associated with a stronger temperature gradient than the synoptic‐scale front. At about noon, when the synoptic‐scale front almost reached the coast, the sea‐breeze front began to move inland. The onshore wind behind the sea‐breeze front and ahead of the synoptic‐scale front was so large that the wind field at the synoptic‐scale front changed from confluence to difluence. This process was supported by a shallow inversion ahead of the synoptic‐scale front which confined the vertical depth of the sea‐breeze. The former sea‐breeze front overtook the main frontal characteristics, continued its inland propagation and was the only frontal event observed over the land. As a result of the interaction, the synoptic‐scale front was significantly intensified in the boundary layer.