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

Abstract

This study is part of the efforts undertaken to resolve the ''bad trough/good trough'' issue for tropical cyclone (TC) intensity changes and to improve the prediction of these challenging events. Sensitivity experiments are run at 8-km resolution with vortex bogusing to extend the previous analysis of a real case of TC-trough interaction (Dora in 2007). The initial position and intensity of the TC are modified, leaving the trough unchanged to describe a realistic environment. Simulations are designed to analyze the sensitivity of TC prediction to both the variety of TC-trough configurations and the current uncertainty in model analysis of TC intensity and position.Results show that TC intensification under upper-level forcing is greater for stronger vortices. The timing and geometry of the interaction between the two cyclonic potential vorticity anomalies associated with the cutoff low and the TC also play a major role in storm intensification. The intensification rate increases when the TC (initially located 128 northwest of the trough) is displaced 18 closer. By allowing a gradual deformation and equatorward tilting of the trough, both scenarios foster an extended ''inflow channel'' of cyclonic vorticity at midlevels toward the vortex inner core. Conversely, unfavorable interaction is found for vortices displaced 38 or 48 east or northeast. Variations in environmental forcing relative to the reference simulation illustrate that the relationship between intensity change and the 850-200-hPa wind shear is not systematic and that the 200-hPa divergence, 335-350-K mean potential vorticity, or 200-hPa relative eddy momentum fluxes may be better predictors of TC intensification during TC-trough interactions.

On the Dynamics of Tropical Cyclone and Trough Interactions

“…Vollaro (1989, 1990) propose that positive EFC triggers an inward-propagating maximum in y r, which produces a balanced vortex response and thereby is associated with an inward-propagating region of vertical velocity w. This inward-propagating region of positive w would then trigger a secondary eyewall formation, associated with a temporary weakening of the TC but followed by a stronger final intensity due to the convergence of angular momentum associated with the eddies. Leroux et al (2013) found a similar evolution, with trough interaction triggering secondary eyewall formation for TC Dora in the Indian Ocean. In a follow-on study, Leroux et al (2016) found an optimal position of the TC relative to the trough in order for favorable interaction to occur.…”

Characteristics and Impacts of Extratropical Rossby Wave Breaking during the Atlantic Hurricane Season

“…How RWB affects TC development involves competing physical processes (Leroux et al 2016) and is often obscured by transient features. Studying a large number of cases is thus very demanding and likely undermined by the subjectivity in analyses.…”

A Global Statistical–Dynamical Tropical Cyclone Wind Radii Forecast Scheme

“…Intensification (i.e., positive PER) promotes the slight shrinking of TCs in all basins. Similarly, positive (Northern Hemisphere) REFC, as it promotes vortex intensification when other factors are held constant or favorable (e.g., Molinari andVollaro 1989, 1990;DeMaria et al 1993;Leroux et al 2016, and additional references therein), also promotes a reduction in storm sizes. Positive Z850 (Northern Hemisphere) appears to be related to TC growth in the SHEM and WPAC where TCs often form in monsoon trough environments (equatorial westerlies converging with poleward easterlies).…”