Analysis of Shear-Relative Asymmetries in Tropical Cyclone Eyewall Slope Using Airborne Doppler Radar Data

Recent studies have analyzed azimuthal mean structure of the slope of the tropical cyclone eyewall. Stern and Nolan (2009) investigated azimuthal mean slope of the Radius of Maximum Wind (RMW) using aircraft Doppler radar velocity data, and Hazelton and Hart (2013) looked at azimuthal mean slope of reflectivity surfaces in the eyewall of 15 TCs, and the relationship between eyewall slope and intensity. That study also showed some significant variance in slope azimuthally around the eyewall in some cases, and speculated that some of this asymmetry was likely due to vertical wind shear. This study further explores azimuthal asymmetries in eyewall slope, and the ways in which they relate to the tilt of the TC vortex due to vertical wind shear. The study looks at the azimuthal variability of several different metrics of TC eyewall slope: the 20 dBZ surface analyzed in Hazelton and Hart (2013), the RMW (analyzed only in an azimuthal mean sense by Stern and Nolan 2009), and the slope of an angular momentum (M) surface that corresponds with the RMW at z = 2 km. The data used for this study are the research-quality Doppler radar composites from the NOAA Hurricane Research Division research flights into TCs. This dataset has been used for other analyses of tropical cyclone wind and precipitation structure (e.g. Rogers et al. 2012, Rogers et al. 2013, Reasor et al. 2013). The analysis uses data from 36 individual flights into 15 different TCs between 1997 and 2010. The radar data are mapped to a shear-relative polar coordinate grid to facilitate analysis of slope asymmetries in four shear-relative quadrants: downshear left, upshear left, downshear right, and upshear right.

Calculation of the mean slope in each quadrant for all 36 cases shows that RMW slope has the strongest shear-relative asymmetry of the slope metrics. The average slope of the two downshear quadrants was 36.5° from vertical, and the average of the two upshear quadrants was 16.3°. This difference is statistically significant (p < 0.01). This result is consistent with a case study by Rogers and Uhlhorn (2008) looking at lower-level RMW in Hurricane Rita. Slope of a momentum surface and dBZ surface also have cases where the eyewall slopes more downshear, but there is more variance than in the RMW slope. However, the M and dBZ surfaces seem to show more of a signal when looking at the change in slope with height, another asymmetry noted by Hazelton and Hart (2013). A cosine function is fitted to the slopes in each quadrant for each case, and this analysis shows that the phase of maximum slope tends to occur most often (for both RMW and M slope) in the downshear region of the TC, particularly downshear left. This result is consistent with Reasor et al. (2013), which found a mean vortex tilt of approximately 10 degrees left of the shear vector. Filtering the cases into high shear and low shear sets illustrates that the tendency for greater slope downshear is magnified for cases with higher shear. In addition, although the dBZ slope shows less shear- relative signal overall (possibly due to rotation of hydrometeors by the TC circulation), the difference between dBZ slope and momentum slope in certain quadrants seems to be an important factor in distinguishing between TCs that are strengthening and TCs that are weakening or steady. Further investigation of this result and the other factors analyzed in the study will help to illustrate the ways in which vertical shear can play a role in changing the structure and intensity of the TC core region.-->