Appearance in Satellite Data

Secondary Lows appear as a cloud vortex within the frontal cloud band. In this study 23 cases of Secondary Lows within an occluded front were investigated.

The formation of a Secondary Low can be seen in a series of satellite images:

  • Initially the cloudiness thickens, and the whole front seems to be undulating.
  • In the developement stage a cloud vortex appears, rotating cyclonically around the secondary low.
  • In the mature stage there are two separate cloud spirals curling around the lows.
  • Visible images are rarely of use, because Secondary Lows within occluded fronts develop mostly over the Northern Atlantic during the winter season, and the time of daylight is short.
  • In infrared images a white to light grey multilayered cloud spiral develops
  • In water vapour images a white to light grey cloud spiral develops around the Secondary Low. Additionally, there is usually a Dark Eye over it, and a Dark Stripe behind the Cold Front and forward of the Occluded Front.

On the 17th of April 2008 at 12.00 UTC there is an occluded front reaching from the Atlantic Ocean to France. The original low is located southwest of Ireland, the secondary has developed over the Bay of Biscay:

 

17 April 2008/12.00 UTC - Meteosat 9 HRVIS IR image
17 April 2008/12.00 UTC - Meteosat 9 WV image

The sequence of satellite images shows the development of a Secondary Low: on 17 April 2008 there is a long Warm Conveyor Belt Occlusion over the northern Atlantic. The original low is seen in the upper left corner of the images as a cloud vortex. The Occlusion point is approximately over the coast of France. The Secondary Low deepens over the Bay of Biscay. At 06.00 UTC there is some thick cloudiness, by 12.00 UTC a small vortex can be seen. The Secondary Low moves slowly eastwards as the vortex becomes more pronounced, while the original low is stationary. In this case the cloud band related to the Occluded Front splits in two parts.

Loop: 01/12.00 - 02/12.00 UTC three-hourly image
17 April 2008/12.00 UTC - Meteosat 9 air mass image
17 April 2008/00.00 UTC - Meteosat 9 IR image