Using Airborne Radar Reflectivity Data to Analyze Mean Structure and Asymmetries of TC Eyewall Slope

Understanding and predicting the evolution of the inner-core structure of a tropical cyclone (TC) continues to be a major research focus in the field of tropical meteorology. For example, much recent work has been devoted to better understanding and predicting the formation and development of the TC eye (e.g. Vigh et al. 2012) and the process of secondary eyewall formation and eyewall replacement cycles (e.g. Kossin and Sitkowski 2009). In this study, we examine the slopes of the eyewall of TCs, using radar reflectivity data from 124 flight legs from NOAA-P3 flights by the Hurricane Research Division into 15 Atlantic TCs from 2004-2011. The 20 dBZ contour is used as a proxy for the interface between the eye and the eyewall updraft, and the slope is calculated azimuthally every five degrees around the eyewall. The slopes from each flight leg are averaged into 6-hour increments around the “Best Track” times from the National Hurricane Center in order to assure statistical independence and allow for an easy comparison with storm intensity. We do find a statistically significant relationship between both the azimuthal mean slope and pressure (r = 0.33) and slope and wind (r = -0.41). In addition, several individual TCs show much higher correlation between slope and intensity. These results are somewhat different from the findings of Stern and Nolan (2009), who found no relationship between the slope of the radius of maximum wind (RMW) and storm intensity. We also find a relatively strong correlation between slope and eye size at 3 km (r = 0.62), but size shows little correlation with intensity. Using vertical shear data from the SHIPS archive, we compare the eyewall slopes along the shear vector upshear and downshear, and this comparison shows a tendency for the eyewall to tilt downshear, by an average of approximately 10 degrees. Another significant asymmetry is a systematic tendency for the upper part of the eyewall to slope more sharply than the lower part, by an average of approximately 10 degrees. Analysis of case studies shows that the processes responsible for changes in TC core structure and intensity also affect eyewall slope, such as vertical shear and eyewall replacement cycles. Eyewall replacement cycles lead to relatively significant short-term changes in core structure that seem to “wash out” the slope-intensity relationship, and shear can lead to significant differences in slope across the eyewall. These results indicate that eyewall slope is an important measure of TC inner-core structure, and may prove useful for future study of the processes that drive changes in the TC core, as well as evaluation of model simulations of TC structure and intensity.