Monday, 6 January 2014

Update: Cold front and cut-off low turns into a powerful extratropical or mid-latitude cyclone

Images: Eumetsat + Aqua Modis (NASA) (Click on images for larger view.)

The above images illustrates a powerful extratropical or mid-latitude cyclone along the coast of  Namibia and South Africa on the 6 January 2014 at 15h00 SAST.  The unusual spiraling cloud pattern into the intense pressure centre of this mid-latitude cyclone is the result of highly energetic fast moving, warm as well as sub-polar air.  Keep an eye on this system for possible further intensification which can produce damaging winds and possible flooding in areas.

Mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth (outside the tropics) not having tropical characteristics, and are connected with fronts and horizontal gradients in temperature and dew point otherwise known as "baroclinic zones". Mid-latitude cyclones are the everyday phenomena which, along with anticyclones, drive the weather over much of the Earth, producing anything from cloudiness and mild showers to heavy gales and thunderstorms.

The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and cold fronts, where the pressure gradient force is highest. The wind flow around an Extratropical cyclone is clockwise in the southern hemisphere, due to the Coriolis effect (this manner of rotation is generally referred to as cyclonic). Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure.
The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the sea-level pressure is about average. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather maps) a wave-like shape. Due to their appearance on satellite images, extratropical cyclones can also be referred to as frontal waves early in their life cycle.