Tropical Cyclone Track Sensitivity in Deformation Steering Flow

Abstract

Previous studies have suggested that tropical cyclones (TCs) in deformation steering flows can be associated with large position errors and uncertainty. The goal of this study is to evaluate the sensitivity of position forecasts for three TCs within deformation wind fields [Debby (2012), Joaquin (2015), and Lionrock (2016)] using the ensemble-based sensitivity technique applied to European Centre for Medium-Range Weather Forecasts (ECMWF) ensemble forecasts. In all three cases, the position forecasts are sensitive to uncertainty in the steering wind within 500 km of the 0-h TC position. Subsequently, the TC moves onto either side of the axis of contraction due to the ensemble perturbation steering flow. As a TC moves away from the saddle point, the ensemble members subsequently experience different ensemble-mean steering winds, which act to move the TC away from the ensemble-mean TC position along the axis of dilatation. By contrast, the position forecasts appear to exhibit less sensitivity to the steering wind more than 500 km from the initial TC position, even though the TC may interact with these features later in the forecast. Furthermore, forecasts initialized at later times are characterized by significantly lower position errors and uncertainty once it becomes clear on which side of the axis of contraction the TC will move. These results suggest that TCs in deformation steering flow could be inherently unpredictable and may benefit from densely sampling the near-storm steering flow and TC structure early in their lifetimes.

The Hurricane Harvey (2017) Texas Rainstorm: Synoptic Analysis and Sensitivity to Soil Moisture

“…Following landfall near Rockport, Harvey remained nearly stationary for several days before reemerging in the Gulf of Mexico on 28 August. Harvey's slowed forward motion occurred as it moved into a weak flow region near the saddle point of a deformation steering flow (e.g., Torn et al 2018), characterized by a trough to the north and flanking ridges to the west and east. Heavy rains persisted on the east and northeast side of Harvey in the Texas coastal plain as Harvey's deep tropical moisture was lifted along a frontogenetical coastal front beneath the equatorward entrance region of an upper-level jet.…”

Recent Progress in the Fundamental Understanding of Tropical Cyclone Motion

“…The theoretical reasons behind such large track errors were explored by Torn et al (2018) for the case of Debby (2012), Joaquin (2015), and Lionrock (2016). Using ensemble sensitivity analysis, the authors concluded that the foremost factor in creating large ensemble spread was a steering flow pattern with a saddle point of strong deformation (Fig.…”

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

“…TC track forecasts become more uncertain for moderate amplitudes of deep vertical wind shear when the vortex structure may be difficult to predict (Corbosiero and Molinari 2003;Zhang and Tao 2013;Finocchio et al 2016). Other studies have shown that TC motion could be significantly altered by intense convection near the vortex and the resulting asymmetry of wind and precipitation fields (Dengler and Reeder 1997;Corbosiero and Molinari 2002;Torn and Davis 2012). 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). Uncertainty in the TC position may also translate into differences in the environment with which the vortex interacts and, therefore, may alter TC motion.…”

“…High track forecast uncertainty for Joaquin has been the subject of several recent studies (Nystrom et al 2018;Torn et al 2018;Miller and Zhang 2019;Saunders et al 2019). Using EPS forecasts from the Weather Research and Forecasting (WRF) Model, Nystrom et al (2018) found that the largest contributor to the divergence of Joaquin track forecasts was initial condition errors between 600 and 900 km from the initial TC position.…”