COASTAL CONVERGENCE - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS
by FMI
In vertical cross-sections the convergence caused by differential friction between sea and land is normally restricted to the lowest kilometre of the atmosphere. Depending on the general stability of the atmosphere low-level convergence may induce shallow or deep vertical motion. Wind blowing at a large angle to the coast causes only minor vertical motions, whereas wind blowing at a small angle induces deeper vertical motion.
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12 October 1999/00.00 UTC
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12 October 1999/00 UTC - Low-level divergence (positive values), convergence (negative values) and streamlines in
Northern Europe. The red line shows the orientation of the cross section (from west to east)
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October 1999/00 UTC - Cross section from Sweden to Baltic States. Red values indicate divergence, blue values
convergence
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The cross-section across the Baltic Sea shows the effect of acceleration and deceleration of the wind by differential friction. Divergence is induced over the east coast of Sweden, whereas convergence is present over the coast of Estonia. The vertical extent of the convergence as well as the vertical motion in this case is only a few hundred metres. The corresponding IR image shows the Swedish coast to be cloud-free, while some lumpy cloudiness can be seen on the other side of the Baltic Sea. Whether or not these clouds are caused by the Coastal Convergence is not easy to conclude. The clouds did not remain on coast, but moved ahead of a fast-moving trough. The associated convective clouds can already be seen over southern Sweden.
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03 October 1999/12.00 UTC - Meteosat IR image
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03 October 1999/12 UTC - Low-level divergence (positive values), convergence (negative values) and streamlines in
western Europe. The red line shows the orientation of the cross section (from west to east).
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03 October 1999/12 UTC - Cross section from Great Britain to Germany. Red values indicate divergence, blue values
convergence. Dashed magenta lines indicate ascending motion, solid magenta lines descending motion.
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This example shows a southwesterly flow meeting the Dutch coast at a small angle. The remnants of a Cold Front are seen in the right edge of the satellite image and cross section. The Coastal Convergence cloud is well developed with a few embedded intensive convective centers. The horizontal distribution of divergence and convergence does not suggest much convergence along the coast, but the cross section reveals that indeed there is a maximum of convergence on the Dutch coast, around longitude 8E. The convergence extends up to 850 hPa, the associated maximum ascending motion being around -0.6 Pa/s.
