Eye and Eyewall traits as determined with the NOAA WP-3D Lower Fuselage Radar

Eye and eyewall traits were ascertained for 209 images from 37 tropical cyclones (TCs) using the lower fuselage 5.6 cm radar aboard the two National Oceanic and Atmospheric Administration WP-3Ds. These hand-selected images had discernable eyewalls, regardless of TC characteristics. The TCs used were almost entirely from the Atlantic Basin and were sampled from 1997 to 2012. A pixel analysis was used in order to determine the horizontal area of eye and eyewall features. A scheme for dissecting the features of the eye and eyewall with the goal of maintaining objectivity is presented and detailed examination of situations in which subjectivity is unavoidable is provided.

Eye traits examined included area, maximum diameter, and eye roundness value, a property used in this study that was adapted from circularity. Eyewall traits analyzed included area, completeness, maximum width, maximum reflectivity value and location, number of local reflectivity maxima, and mean rain rate. These variables were compared to TC intensity and motion characteristics from the National Hurricane Center's Best Track Database, and environmental characteristics from the Statistical Hurricane Intensity Prediction Scheme. The environmental characteristics used in this study are many of the properties most often considered influential on a TC, including SST, depth of oceanic warm water (26°C), vertical shear of the horizontal wind magnitude and direction, and mid-tropospheric relative humidity. A schematic incorporating typical eye and eyewall traits is presented as a standard of comparison for quickly referencing anomalous features of individual eyewall in radar images.

Interrelationships between eye and eyewall features were determined based on the correlation coefficient of measured values between the two variables. Relationships considered to be most reliable had a correlation coefficient of 0.2 or higher, corresponding to a 99% confidence interval.

Eyewalls at larger radii have more reflectivity maxima while the eyewall reflectivity field becomes more homogeneous as eye and eyewall areas shrink. Maximum reflectivity and mean rain rate are found to increase as the eyewall becomes wider and more complete. The maximum reflectivity value in the eyewall appears to be independent of eye area and mean rain rate is negatively correlated to eye area. As the TC intensified, the eye area decreased, while the eyewall area increased due to increasing completeness and width. Mean rain rate was also found to be higher for faster moving TCs. Stronger vertical shear of the horizontal wind was found to be associated with more asymmetric, wider eyewalls with higher rain rates. The maximum reflectivity value occurred most often on the downshear side of the eyewall, and to the right of the storm motion, verifying prior research.

There was a surprisingly weak relationship between intensity and eye shape (roundness). Also surprising was that SST was not related to eye or eyewall area and showed a surprisingly weak negative correlated to rain rate. The depth of oceanic warm water, mid-tropospheric relative humidity, and 6h pressure change did not have a detectable relationship with any of the eye or eyewall variables.