An annular tropical cyclone is a tropical cyclone that features a normal to large, symmetric eye surrounded by a thick and uniform ring of intense convection, often having a relative lack of discrete rainbands, and bearing a symmetric appearance in general. As a result, the appearance of an annular tropical cyclone can be referred to as akin to a tire or doughnut. Annular characteristics can be attained as tropical cyclones intensify; however, outside the processes that drive the transition from asymmetric systems to annular systems and the abnormal resistance to negative environmental factors found in storms with annular features, annular tropical cyclones behave similarly to asymmetric storms. Most research related to annular tropical cyclones is limited to satellite imagery and aircraft reconnaissance as the conditions thought to give rise to annular characteristics normally occur over open water, well removed from landmasses where surface observations are possible. Characteristics and identification The annular hurricane was first defined as a subset of tropical cyclones by John Knaff of Colorado State University and James Kossin of the University of Wisconsin–Madison in 2002 by use of infrared satellite imagery, which serves as the visual means of ascertaining annular characteristics within tropical cyclone. Knaff and Kossin defined an annular tropical cyclone as a tropical cyclone that maintains either an average or larger-than-average eye surrounded by deep convection containing the storm's inner core and a lack of convection occurring outside the central dense overcast for at least three hours. As a result, annular storms lack the rainbands characteristic of typical tropical cyclone. These features lend the storm an axisymmetric appearance common to annular tropical cyclones. However, this definition is only applicable while a storm maintains these characteristics—when and while a storm does not feature annular characteristics, the tropical cyclone is considered asymmetric. In addition to the primary defining characteristics, the diurnal pulsation of the cirrus cloud canopy associated with outflow is subdued once storms become annular. Some annular tropical cyclones may also display a "pinwheel eye", a feature in which conditions in the storm causes its eye to take the appearance of a spoked wheel. An algorithm for identification of annular tropical cyclones in real-time by objective criteria—the Annular Hurricane Index—has been developed, and shows some power, but was not yet operational, as of 2008. However, the National Hurricane Center (NHC) later started used the Annular Hurricane Index for the purpose of assessing annular tropical cyclones. The