FAQ's of Summer Weather
In this section we will highlight your questions concerning meteorology, forecasting, and atmospheric properties. Just write to me at Phil.Schumacher@noaa.gov to ask your question and I'll answer you in this forum.
I've heard the term mesocyclone associated with severe thunderstorms. What is this?
A mesocyclone refers to an organized and long lived circulation within a severe thunderstorm that can precede a tornado but not necessarily always does so. The formation of a mesocyclone is dependent on the environmental wind shear or how the wind speed and direction is changing with height. One such favorable wind shear pattern results from southerly winds at the surface becoming southwest and then more westerly while increasing in wind speed with height. This shear pattern enhances the counter-clockwise rotation of a mature thunderstorm and provides a positive feedback whereby the storm increases its rotation. Eventually, but not always, the mesocyclone can become strong enough to generate a smaller tornadic circulation. Strong mesocyclones are easily detectable by the National Weather Service Sioux Falls Doppler Radar by looking at the wind flow within a storm and seeing changing wind directions within the severe thunderstorm. To see a mesocyclone associated with the Colton Tornado from 1996, follow this link. In this picture, looking down on the thunderstorm, blue colors show air flow toward the radar (located to the southeast) and red colors show air flow away from the radar. This is a counter-clockwise rotation associated with the mesocyclone.
What is a bow echo?
A bow echo is a term meteorologists use to describe a line of thunderstorms which has a distinct convex shape, like a backward "C", pointing into the direction of movement. The origin of how the bow echo forms is complex but basically pressure differences within the core of the storm and the environment cause a descending jet of high speed wind to reach into the lower portion of the storm. Since this air is descending and accelerating it acts as a wedge and rapidly lifts air on the leading edge of the line, near the center, to form new thunderstorms while the portion of the line to the north and south lags behind. As this air descends it also causes drying and evaporation of the precipitation resulting in a decrease in radar reflectivity, or notch, just behind the bowing portion of the line. The strongest winds are perpendicular to the bowing segment and are generally comprised of straight line winds oriented in the direction of the bow echo movement. To the north and south at the end of the bow echo there can be two separate vortices called bookend vortices - the one to the north rotating counter-clockwise and the one to the south rotating clockwise. The northern, cyclonic rotating bookend vortex can spawn mesocyclones but in this case it did not. The southern bookend vortex usually is not as strong as the northern one and is short lived. Here is one example of a bow echo that produced damaging straight-line winds in Salix, Iowa on July 17, 1996. This example shows 4 radar reflectivity pictures of the Sioux Falls Doppler Radar showing the evolution of the bow echo from upper left to lower right with a well defined bow echo in the lower right corner during the time of wind damage reports.
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".
