A Hypothesis for the Intensification of Tropical Cyclones under Moderate Vertical Wind Shear

Abstract

A major open issue in tropical meteorology is how and why some tropical cyclones intensify under moderate vertical wind shear. This study tackles that issue by diagnosing physical processes of tropical cyclone intensification in a moderately sheared environment using a 20-member ensemble of idealized simulations. Consistent with previous studies, the ensemble shows that the onset of intensification largely depends on the timing of vortex tilt reduction and symmetrization of precipitation. A new contribution of this work is a process-based analysis following a shear-induced midtropospheric vortex with its associated precipitation. This analysis shows that tilt reduction and symmetrization precede intensification because those processes are associated with a substantial increase in near-surface vertical mass fluxes and equivalent potential temperature. A vorticity budget demonstrates that the increased near-surface vertical mass fluxes aid intensification via near-surface stretching of absolute vorticity and free-tropospheric tilting of horizontal vorticity. Importantly, tilt reduction happens because of a vortex merger process—not because of advective vortex alignment—that yields a single closed circulation over a deep layer. Vortex merger only happens after the midtropospheric vortex reaches upshear left, where the flow configuration favors near-surface vortex stretching, deep updrafts, and a substantial reduction of low-entropy fluxes. These results lead to the hypothesis that intensification under moderate shear happens if and when a “restructuring” process is completed, after which a closed circulation favors persistent vorticity spinup and recirculating warm, moist air parcels.

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

“…16b and 16c for the same passes from missions 2 and 4. Aggregate mass flux is calculated to depict its integrated impacts on the profiles of latent heating and vortex stretching (Houze et al 2009;Raymond and López Carrillo 2011;Gjorgjievska and Raymond 2014;Rios-Berrios et al 2018) within the mesoscale environment at and downshear of the low-level circulation. These variables are further partitioned into the moderate and deep convection categories in Fig.…”

“…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.…”

“…While shear typically limits TC intensification, there are some situations where intensification can occur despite the presence of shear, particularly when the shear is of moderate strength (broadly defined as 850-200-hPa shear magnitudes ranging between 5 and 10 m s 21 ). This intensification can occur when there is forcing of strong convection downshear providing a significant projection onto wavenumber-0 (Reasor et al 2009;Nguyen and Molinari 2012), when precipitation and deep convection occur and persist on the upshear side (Jiang 2012;Stevenson et al 2014;Alvey et al 2015;Rogers et al , 2016Susca-Lopata et al 2015;Tao and Zhang 2015;Zawislak et al 2016;Nguyen et al 2017;Munsell et al 2017;Wadler et al 2018;Leighton et al 2018;Rios-Berrios et al 2018), and through downshear reformation (Molinari et al 2004(Molinari et al , 2006Molinari and Vollaro 2010;Nguyen and Molinari 2015;Chen et al 2018). The uncertain impacts of these processes in moderate shear is manifested as a pronounced forecasting challenge (Bhatia and Nolan 2013;Tao and Zhang 2015;Finocchio and Majumdar 2017), and this has been identified as a key challenge by the National Hurricane Center (NTSB 2017).…”

The Evolution and Role of Midtropospheric Cyclonic Vortex in the Formation of Super Typhoon Nepartak (2016)

“…To further analyze the relationship between the convection and the low‐level and midlevel vortices, the Helmhotz decomposition to the horizontal winds is performed in this work following Rios‐Berrios et al (), which is defined as where χ and ψ satisfy ∇ 2 χ = ∇ ⋅ V h and ∇ 2 ψ = ζ . The decomposition is computed in the region centered at 600‐hPa vorticity centroid with the radius less than 2°.…”

“…To further analyze the relationship between the convection and the low-level and midlevel vortices, the Helmhotz decomposition to the horizontal winds is performed in this work following Rios-Berrios et al (2018), which is defined as…”