Thursday, April 5, 2012

Cold Air Funnels

From Ruminater David Darrow
Sedgwick Co. Kansas recently had a couple cold air funnel events happen near the city of Wichita.  Both events, this last Wednesday and the previous Thursday, were created by cold air masses over the city.  The air mass tends to cause a little spin in the atmosphere and cold air brings it down.  Sometimes rain can help form them as well.  They are usually high based, weak, and rarely touch down.  If they do, they may cause weak EF0 damage (65-85 mph winds).  For a little longer discussion on cold air funnels, look below. 
From Ruminater David Darrow
Looking to the SW from Kechi, Kansas.  Funnel is very faint in the middle of the photo.

I took this from the Sioux Falls, SD National Weather Service.

What is a cold air funnel?

There is much debate about what causes a "cold air funnel" and scientists are still researching this phenomenon, however, the term "cold air funnel" is one of the terms used by meteorologists to identify funnel clouds that are not produced by a mesocyclone and/or are not in an environment typically associated with severe thunderstorms. Basically all funnel clouds are the same. The over-all weather pattern in which they form may be different but the mechanism for producing the funnel, namely, increasing cyclonic (or counter-clockwise) rotation, is the same and involves the vertical stretching of vorticity.

Vorticity can basically be thought of as the amount of "spin" inherent in the atmosphere produced by wind shear. As cyclonic vorticity (or rotation) is stretched vertically the area of rotation decreases so the speed of rotation must increase to compensate. This is analogous to an ice skater pulling in their arms and legs and increasing their rotation.

Based upon observations of cold air funnel events during the past few years we have noticed that they seem to generally prefer an environment when there is large scale cyclonic (counter-clockwise) rotation associated with a large, slow moving, closed low in the upper atmosphere. Since these lows have been circulating in the atmosphere for a long time (compared to the time scale of severe thunderstorms) they stir the air and generate smaller scale rotation providing a "vorticity-rich" environment. These lows also have a cold pool of air associated with them resulting in unstable conditions for weak thunderstorms to form. This is why the term cold air funnel got its origin.

However the fact that there is cold air aloft does not govern how these funnels form, it only provides the instability for thunderstorms (convection) to form. Based on our observations one explanation of how cold air funnels form may be that in the growth stage of these weak thunderstorms the updraft stretches pre-existing smaller scale vortices within the vorticity-rich airmass causing an increase in rotation and the formation of a funnel. This process is similar to that shown by research meteorologists in the formation of landspouts which typically form on the High Plains. Cold air funnels generally are weak, short lived, and usually do not touch down. One of the reasons that they do not touch down and, by definition become a tornado, may be that there is little rainfall associated with these storms to produce a downdraft in order to generate a low level circulation. Recent research suggests that the interaction of descending rainfall near the surface and the thunderstorm updraft may be very important in generating a low level circulation which links with the upper level mesocyclone circulation to form the tornado in typical severe thunderstorms. These processes are not present with "cold air funnels".

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