SATELLITE
INTERPRETATION
DISCUSSION

NOAA/NESDIS  Regional and Mesoscale Meteorology Team Cooperative Institute for Research in the Atmosphere (CIRA) Colorado State University   Fort Collins, Colorado

October 17, 2000

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A local study  by J. Knaff, J. Kossin, M. DeMaria and V. Larson which is being readied for submission for publication discusses a subclass of tropical cyclones that has the following persistent (3 hours) characteristics as viewed by satellite imagery.

• A persistent symmetric ring of deep convection surrounding a larger than normal or normal sized eye.
• A lack of spiral banding
• Intensities of ~85% of the maximum potential intensity with respect to SST (an empirical relationship).  Note: this results in a very strong and long lived hurricane.

Examining the environments associated with doughnuts, reveals that they exist when the environment is characterized by:

• Weak environmental wind shear from an easterly or east southeasterly direction (i.e. under the influence of a ridge throughout the troposphere)
• No upper level trough interactions
• SST conditions that are constant or decreasing with time and that are less than 28.6C and are greater than 25.5C

The sequence of events that lead to doughnut hurricanes is as follows:

• A very intense hurricane over warm waters (SST>28.5) undergoes what can be best described as an internal mixing event.  Mixing events come in the form of eyewall replacement cycles (4 of 6) or eye disintegration (2 of 6)
• This mixing event is followed by the formation of a large eye.

Figure 2:  Storm relative IR loop of hurricane Howard 1998 transitioning to a doughnut hurricane.  In this transition a small eye is replaced by a larger eye in an eyewall replacement cycle (see Mayfield, cited 2000).  See Table 1 for more details of the doughnut period of Howard.

Interesting supporting evidence for mixing comes from the 4-hour average radial wind profiles of the two storms (Luis 1995, Dora 1999) where aircraft reconnaissance was available, shown in Fig. 3.  The vorticity profile of these storms is nearly uniform from the radius of maximum wind to the center of the storm, suggesting that a strong mixing event has recently occurred in these storms and that the resulting flows are stable to barotropic instability (i. e. the formation of vortex Rossby waves) (see Schubert et. al. 1999, and Kossin and Eastin 2000).
 

Figure 3:  Average radial wind and vorticity profiles coming from doughnut hurricanes  Luis and Dora (above).

This is a good example of when a sequence images can offer much more than a picture.  In this case information about the environment, storm structure and even past evolution can be inferred.

Acknowledgments:

I would like to acknowledge the contributions by my co-authors on this work.  This presentation is based upon a soon-to-be-submitted publication by J. Knaff., J. Kossin, M. DeMaria, and V. Larson.

References:

Kossin, J. P., and M. D. Eastin, 2000: Two distinct regimes in the kinematic and thermodynamic structure of the hurricane eye and eyewall. J.  Atmos. Sci., in press.

Mayfield, M., cited 2000:  Preliminary report Hurricane Howard 20-30 August 1998. [Available on-line from
http://www.nhc.noaa.gov/1998howard.html]

Schubert, W. H., M. T. Montgomery, R. K. Taft, T. A. Guinn, S. R. Fulton, J. P. Kossin, and J. P.  Edwards, 1999: Polygonal eyewalls, asymmetric eye contraction, and potential vorticity mixing in hurricanes. J. Atmos. Sci., 56, 1197--1223.

DeMaria, M. and J. Kaplan 1999: An updated statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic and eastern North Pacific basins. Wea. Forecasting, 14, 326-337.

DeMaria, M. and J. Kaplan 1994: A statistical hurricane intensity prediction scheme (SHIPS) for the Atlantic basin. Wea. Forecasting, 9, 209-220.