A Closer Look at Rotational Clouds

#1908 "Cirrious Stories" Pixels Link

To better understand clouds, one just needs their right hand, your imagination and some time.

Wind shear is required to make the fastest swirls and we know that those curls must be translating in the atmospheric frame of reference. This also means they must be moving with respect to the earth. If you watch the cloud move, one can determine the direction of the wind shear in just a few minutes. Remember that moisture is required to trace the movement of the wind over time. The same movement of air occurs with or without tracer moisture. Clouds are just the integral of the motion of that moisture over time.

Now follow the cloud edge in the same direction that the clouds are moving. If you can see the entire curl, you will reach the cusp which is the end of that line. Using your right hand, align your fingers in the same way that the cloud edge curls. If your thumb point upward then that is a vorticity maximum in meteorologist jargon. If your thumb is point down then that is a vorticity minimum swirl. It is very instructive to know which when you apply the deformation zone conceptual model.  It does not matter which of the cloud edges you follow to reach the cusp at the end of the curl – both cloud edges must curl the same way.

If the moisture used as a tracer for the rotation is originally located on the outer fringe of the rotation, the point of inflection of the two cloud arcs  will be bordered by clear skies. I called this abrupt point of inflection an "outward cusp". This pattern is by far the most common and occurs frequently on the pole-ward side of the jet stream.

If the centre of rotation is actually within the moisture,  the point of inflection of the two cloud arcs  will be bordered by cloudy skies. I called this point of inflection an inward cusp.  Some drawings are worth thousands of words and will help to make this clearer.

These cloud patterns look different only because of where the moisture is being drawn from. If you mentally switch the white cloud to the blue clear sky in either of the patterns, you will achieve the other pattern. Similarly the mirror reflection of a cyclonic rotation is an anticyclonic rotation and that is how I quickly made all of my instructional graphics. Swirls really are that simple whether you call them rotation or vorticity centres.

Looking back to the early 1980's when I first started playing these mental meteorological games (mainly on midnight shifts), this all seems so simple now. At the time, it was anything but clear - the solution was very cloudy in fact ... and there was a lot of push back.

Without the support of the COMET Program and my friends in Boulder, Colorado, this research might never have extended beyond my own meteorological and artistic applications. This research is available through the COMET program around the world for free through enjoyable, entertaining and interactive on-line training sessions. http://www.comet.ucar.edu/

The Satellite Palette Flash Screen circa 2005

For me it was a fusion of science and art and I branded these sessions as the “Satellite Palette”. The colours on my artistic palette were replaced by the conceptual models that could be creatively applied to any weather situation to better understand and solve the forecast problem of the day.

In the next post we will revisit the all important line in the sky. The deformation zone was described in terms of a decrease of the wind in the very first post "Cloud Shapes and Lines in the Atmosphere".
The deformation zone can also be diagnosed in terms of the four swirls that shape that line.

Warmest regards,
Phil the Forecaster