The National Hurricane Center (NHC) recently upgraded Otto to the eighth hurricane of the 2010 season. The NHC has been right-on in regard to Otto transitioning from a subtropical cyclone to a tropical cyclone. It is instructive to compare satellite images from today with those in my previous blog to explain the transition.
Today’s visible satellite image shows a very well defined central dense overcast (CDO) with a curved band to the east and south of the CDO. The upper-level low discussed in my previous blog is no longer evident, and, in fact, now there is at least some degree of upper-level outflow over Otto.
The figure above shows a microwave image from the Special Sensor Microwave Imager/Sounder (SSMIS) onboard one of the Defense Meteorological Satellite Program (DMSP) satellites. It is more like a radar presentation and shows more detail beneath the higher cloud tops than can be seen from the visible and infrared images. Note that an eyewall has formed in Otto. This is an indication that the radius of maximum winds is now much closer to the center than discussed in the previous blog when the cylcone had an exposed low-level center.
The NHC defines a subtropical cyclone as “A non-frontal low pressure system that has characteristics of both tropical and extratropical cyclones. This system is typically an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles or more from the center. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the center, and typically have a less symmetric wind field and distribution of convection.”
Otto is a good example of a transition from subtropical to tropical. The cyclone is no longer involved with an upper-level circulation and the maximum sustained winds are no longer well removed from the center. Today’s satellite imagery also shows a much more summetric distribution of convection.