Dynamical and Physical Processes Leading to Tropical Cyclone Intensification under Upper-Level Trough Forcing

Abstract

The rapid intensification of Tropical Cyclone (TC) Dora (2007, southwest Indian Ocean) under upper-level trough forcing is investigated. TC–trough interaction is simulated using a limited-area operational numerical weather prediction model. The interaction between the storm and the trough involves a coupled evolution of vertical wind shear and binary vortex interaction in the horizontal and vertical dimensions. The three-dimensional potential vorticity structure associated with the trough undergoes strong deformation as it approaches the storm. Potential vorticity (PV) is advected toward the tropical cyclone core over a thick layer from 200 to 500 hPa while the TC upper-level flow turns cyclonic from the continuous import of angular momentum. It is found that vortex intensification first occurs inside the eyewall and results from PV superposition in the thick aforementioned layer. The main pathway to further storm intensification is associated with secondary eyewall formation triggered by external forcing. Eddy angular momentum convergence and eddy PV fluxes are responsible for spinning up an outer eyewall over the entire troposphere, while spindown is observed within the primary eyewall. The 8-km-resolution model is able to reproduce the main features of the eyewall replacement cycle observed for TC Dora. The outer eyewall intensifies further through mean vertical advection under dynamically forced upward motion. The processes are illustrated and quantified using various diagnostics.

A Statistical Analysis of the Relationship between Upper-Tropospheric Cold Low and Tropical Cyclone Track and Intensity Change over the Western North Pacific

“…Previous studies also indicated that the increased convergence of eddy angular momentum resulting from the interaction of TCs with upper-level disturbances may facilitate TC intensification (e.g., Molinari and Vollaro 1989;Davidson and Kumar 1990;DeMaria et al 1993;Hanley et al 2001;Hanley 2002). Cyclonic potential vorticity (PV) advection toward the TC core during such events has also been demonstrated to play a role in TC intensification (e.g., Molinari et al 1995Molinari et al , 1998Montgomery and Farrell 1993;Leroux et al 2013). …”

Upper-tropospheric environment–tropical cyclone interactions over the western North Pacific: A statistical study

“…Their results showed that 78% of superposition and 61% of distant interaction cases deepened. Although these results confirm that interaction favors TC intensification in a statistical sense, to identify whether or not a particular trough interaction is conducive to TC intensification is still not straightforward (Hanley et al, 2001;Leroux et al, 2013). Besides, these two studies ignored the role of upper-level inertial stability, emphasized in a number of other studies (e.g., Holland and Merrill, 1984;Rappin et al, 2011).…”

“…17), so that their warm core structure will not be destroyed by persistently strong VWS. Besides, interaction should not last more than around two days, playing its role as a triggering effect for initiating wind-induced surface heat exchange (Emanuel, 1986) or the eyewall replacement cycle (Willoughby et al, 1982;Leroux et al, 2013), as long-lasting interaction will eventually bring persistent VWS, which is destructive to a TC's warm core. Finally, interaction should appear at low latitudes so that environmental inertial stability is relatively weak and thus the TC's axisymmetric secondary circulation is more sensitive to the upper-level asymmetric forcing.…”

“…A large number of interaction cases have been reported in the literature, such as Hurricanes (or Tropical Storms) Dorothy (1966) (Erickson, 1967), Elena (1985) (Molinari and Vollaro, 1989), Danny (1985) (Molinari et al, 1998), Florence (1988) (Rodgers et al, 1991;Shi et al, 1997), Opal (1995) (Rodgers et al, 1998;Bosart et al, 2000), Bertha (1996) (Hanley, 2002) and Gabrielle (2001) (Molinari et al, 2006), over the Atlantic basin; Typhoons (or Tropical Storms) Flo (1990) (Wu and Cheng, 1999), Gene (1990) (Wu and Cheng, 1999), Winnie (1997) (Li et al, 2006), Olga (1999) (Yu and Kwon, 2005), Prapiroon (2000) (Yu and Kwon, 2005) and Haima (2004) (Qian et al, 2011), over the western North Pacific basin; as well as Tropical Cyclone Dora (2007) (Leroux et al, 2013) over the southwest Indian Ocean. These cases also show that intensification induced by upper-level environments can be found in all stages of TCs, such as tropical depression formation , tropical depression (Bosart and Bartlo, 1991) or tropical storm (Shi et al, 1997;Molinari et al, 1998) to hurricane transition, rapid intensification to a category 5 hurricane or supertyphoon (Titley and Elsberry, 2000), and tropical storm (Qian et al, 2011) or typhoon (Li et al, 2006) to extratropical cyclone transition.…”

On the Sensitivity of Tropical Cyclone Intensification under Upper-Level Trough Forcing

“…Although the focus of the research community is mostly toward the dynamics of the storm's inner core within ;200 km of the storm center, the review presented at the Eighth WMO International Workshop on Tropical Cyclones (IWTC-8; Leroux et al 2014) indicates that continued study of external influences on TC intensity change is warranted because the environment hostility controls both the development and predictability of TCs (Zhang and Tao 2013;Tao and Zhang 2014). The knowledge gaps or suitable research routes regarding the problem of TC intensity change under external influences are summarized in a comprehensive sketch (Leroux et al 2014, see their Fig.…”