Joaquin: Classifying Shear Storms in Cyclone Center

Tropical Storm Joaquin is moving slowly over warm North Atlantic waters this evening.  If atmospheric conditions were ideal, Joaquin would be well on his way to becoming a hurricane.  Instead, he is struggling to develop because the atmospheric winds are creating strong “shear” which displaces the energy source of the storm away from the center.  Watch the animated image below:

Tropical Storm Joaquin during the afternoon of September 29, 2015

Tropical Storm Joaquin during the afternoon of September 29, 2015

This is a classical view of a sheared storm.  First, notice the light gray clouds in the upper center/left of the movie.  These are low-level clouds that are providing a good indication of the near-surface wind circulation.  The gray swirls betray the low-level center of Joaquin which one can find by following the spirals in:

Arrows show approximate low-level circulation

Arrows show approximate low-level circulation

If one were to encounter this storm on Cyclone Center, the magenta dot is the center of circulation that we’re after.  The mass of colder clouds (here shown as yellow, red, and black) are displaced away from the low-level center by strong northwesterly shear.  On Cyclone Center images you can sometimes see this because one side of the cold clouds are “flattened” by the wall of wind, as shown with the magenta line below:

Arrows show direction of wind shear.

Arrows show direction of wind shear.

As you can see, the low-level center is displaced to the northwest of the center of cold cloud.  The distance of this displacement is related to the strength of the storm winds, which is why we ask you this when you are performing a shear classification.  It would not be correct to choose the middle of the cold clouds as the center of the storm – the magenta dot above shows where the middle/upper level circulation center is.  In essence, the storm is being vertically deformed by the strong environmental winds – the more deformed, the weaker the winds.

Now that you are an expert, head on over the Cyclone Center and see if you can find a few shear storm examples of your own!

– Chris Hennon is part of the Cyclone Center Science Team and Associate Professor of Atmospheric Sciences at the University of North Carolina at Asheville

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