Shear-Relative Asymmetries in Tropical Cyclone Eyewall Slope

Abstract

ecent studies have analyzed the azimuthal mean slope of the tropical cyclone (TC) eyewall. This study looks at the shear-relative azimuthal variability of different metrics of eyewall slope: the 20-dBZ surface, the radius of maximum wind (RMW), and an angular momentum (M) surface passing through the RMW. The data used are Doppler radar composites from the NOAA Hurricane Research Division (HRD). This study examines 34 TCs, with intensities ranging from 3 to 75 m s?1 and shear magnitudes ranging from 0 to 10 m s?1. Calculation of the mean slope in each quadrant for all cases shows that RMW slope has the strongest asymmetry, with downshear slope larger than upshear in 62% of cases. Slopes of momentum surfaces and dBZ surfaces are also greater downshear in some cases (65% for M and 47% for dBZ), but there is more variance than in the RMW slope. The azimuthal phase of maximum slope occurs most often downshear, particularly downshear left, consistent with the depiction of a mean vortex tilt approximately 10° left of shear. Filtering the cases into high and low shear illustrates that the tendency for greater slope downshear is magnified for high-shear cases. In addition, although the dBZ slope shows less shear-relative signal overall, the difference between the dBZ slope and momentum slope is an important factor in distinguishing between strengthening and weakening or steady TCs. Intensifying TCs tend to have dBZ surfaces that are more upright than M surfaces. Further investigation of these results will help to illustrate the ways in which vertical shear can play a role in altering the structure of the TC core region.