The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part II: Vortex Tilt

Abstract

We investigate a class of tropical cyclones (TCs) that undergo rapid intensification (RI) in moderate vertical wind shear through analysis of a series of idealized model simulations. Two key findings derived from observational analysis are that the average 200–850-hPa shear value is 7.5 m s−1 and that the TCs displayed coherent cloud structures, deemed tilt-modulated convective asymmetries (TCA), which feature pulses of deep convection with periods of between 4 and 8 h. Additionally, all of the TCs are embedded in an environment that is characterized by shear associated with anticyclones, a factor that limits depth of the strongest environmental winds in the vertical. The idealized TC develops in the presence of relatively shallow environmental wind shear of an anticyclone. An analysis of the TC tilt in the vertical demonstrates that the source of the observed 4–8-h periodicity of the TCAs can be explained by smaller-scale nutations of the tilt on the longer, slower upshear precession. When the environmental wind shear occurs over a deeper layer similar to that of a trough, the TC does not develop. The TCAs are characterized as collections of updrafts that are buoyant throughout the depth of the TC since they rise into a cold anomaly caused by the tilting vortex. At 90 h into the simulation, RI occurs, and the tilt nutations (and hence the TCAs) cease to occur.

Simulation of Rapid Intensification of Super Typhoon Lekima (2019). Part I: Evolution Characteristics of Asymmetric Convection Under Upper-Level Vertical Wind Shear

“…Notice that the TC is located to the southern flank of the upper-level anticyclone. This pattern agrees well with the findings by Ryglicki et al (2018a) and Ryglicki et al (2018b). The authors pointed out that, the upper-level anticyclone generally creates moderate upper-level VWS, and TCs embedded in such environment likely have a potential to undergo RI.…”

“…With times, the most active convection moves anticlockwise, and approaches to east flank of the storm (Figure 5). This evolution feature bears many similarities as previous studies (Ryglicki et al, 2018a;Ryglicki et al, 2018b;Li et al, 2020), indicating that an inward-spiraling inner cloud band moves to upshear flank to form the eyewall cloud. This process agrees with Moon and Nolan (2010), in which they proposed a hypothesis that inner rainbands are simply convective clouds advected by the rapidly rotating TC wind, and then likely deformed into spiral shapes.…”

“…TC vortices tilt the most under lowerlevel VWS and are unable to process upshear and realign, thus fail to intensify. Some recent observational and numerical studies (Leighton et al, 2018;Ryglicki et al, 2018a;Ryglicki et al, 2018b;Li et al, 2020) found that TCs can experience an RI under moderate VWS (e.g., 5-10 m s −1 ). For this atypical class of TCs underwent RI, it is hypothesized that they are closely associated with the tilt-modulated convective asymmetries (TCA).…”

Precipitation Processes and Vortex Alignment during the Intensification of a Weak Tropical Cyclone in Moderate Vertical Shear

“…Although the intensification mechanism involving localized, rotating convection is fundamentally three-dimensional and asymmetric, the evolution of axisymmetric tangential wind can still be usefully viewed in terms of the conventional axisymmetric view extended to include eddy fluxes of heat and momentum and unbalanced boundary layer processes (e.g., Smith et al 2009). The development of a deep, vertically aligned vortex, either through the reduction of tilt or the development of a vortex above (below) an existing low-level (midlevel) circulation center, facilitates symmetry and is thus a critical step in the intensification process (Dunkerton et al 2009;Wang et al 2012;Munsell et al 2017;Rios-Berrios et al 2018;Ryglicki et al 2018;Miyamoto and Nolan 2018;Chen et al 2018;Alvey et al 2020). This consideration is relevant for TCs at all stages in their early life cycle: from predepression up through tropical storm strength.…”

Track Uncertainty in High-Resolution HWRF Ensemble Forecasts of Hurricane Joaquin

“…In addition, deep vertical wind shear is capable of tilting the TC vortex, and this tilt has small-amplitude implications for TC motion (Flatau et al 1994). Previous studies have shown that the vertical profile of the environmental wind (e.g., helicity) is a determining factor in the TC vortex response to vertical wind shear (Onderlinde and Nolan 2016;Ryglicki et al 2018), and the resulting TC vortex structure controls the atmospheric layer responsible for steering the TC. Further, TC positions used to initialize model forecasts are imprecise, especially for weaker TCs without aircraft or land-based observations (e.g., Torn and Snyder 2012;Landsea and Franklin 2013).…”