The Unexpected Rapid Intensification of Tropical Cyclones in Moderate Vertical Wind Shear. Part I: Overview and Observations

Abstract

A satellite-based investigation is performed of a class of tropical cyclones (TCs) that unexpectedly undergo rapid intensification (RI) in moderate vertical wind shear between 5 and 10 m s−1 calculated as 200–850-hPa shear. This study makes use of both infrared (IR; 11 μm) and water vapor (WV; 6.5 μm) geostationary satellite data, the Statistical Hurricane Prediction Intensity System (SHIPS), and model reanalyses to highlight commonalities of the six TCs. The commonalities serve as predictive guides for forecasters and common features that can be used to constrain and verify idealized modeling studies. Each of the TCs exhibits a convective cloud structure that is identified as a tilt-modulated convective asymmetry (TCA). These TCAs share similar shapes, upshear-relative positions, and IR cloud-top temperatures (below −70°C). They pulse over the core of the TC with a periodicity of between 4 and 8 h. Using WV satellite imagery, two additional features identified are asymmetric warming/drying upshear of the TC relative to downshear, as well as radially thin arc-shaped clouds on the upshear side. The WV brightness temperatures of these arcs are between −40° and −60°C. All of the TCs are sheared by upper-level anticyclones, which limits the strongest environmental winds to near the tropopause.

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

“…As Ryglicki and Hart (2015) and Nguyen et al (2014) summarized, there exist myriad ways to calculate the center of a TC, and the notion of a single ''center'' throughout the TC typically does not exist. Misdiagnosis of the center can result in inconsistencies when measuring tilt (Ryglicki and Hart 2015) or when decomposing 2D quantities such as vorticity or winds (Ryglicki and Hodyss 2016). of multiple tilt calculations is that since a key component of this evolution is related to the tilt, we are able to demonstrate that our findings are robust, regardless of center-finding method.…”

“…of multiple tilt calculations is that since a key component of this evolution is related to the tilt, we are able to demonstrate that our findings are robust, regardless of center-finding method. Determining statisticaldynamical relationships among the different centerfinding methods is beyond the scope of this study [see Ryglicki and Hart (2015) for more details].…”

“…From there, an 800 km 3 800 km subdomain is extracted and used for calculation of the following center-finding methods. The reason for this is that during early stages of the TC, when the storm is weak, the pressure minimum is not always a reliable first guess of the center location at upper levels (Ryglicki and Hart 2015).…”

“…Through a series of satellite and model analyses, Ryglicki et al (2018; hereafter Part I) presented a class of tropical cyclones (TCs) that underwent rapid intensification (RI; 30 kt or ;15 m s 21 increase in 24 h) in an environment featuring moderate vertical wind shear (5-10 m s 21 ; Rios-Berrios and Torn 2017), contrary to climatological expectations of vertical wind shear and RI (i.e., Gray 1968;Zeng et al 2008;Kaplan et al 2010;Wang et al 2015). Members of this TC class include 1997 eastern Pacific (EPAC) Guillermo, 2008EPAC Hernan, 2008EPAC Norbert, 2012EPAC Fabio, 2015 EPAC Hilda, and 2015 northern Atlantic (NATL) Joaquin.…”

A Tropical Cyclone Rapid Intensification Prediction Aid for the Joint Typhoon Warning Center’s Areas of Responsibility

“…Nonetheless, there is still plenty of room for improvement. Forecast busts are often a function of storm structure or unique environmental features (e.g., the cases discussed in Ryglicki et al 2018) that are currently difficult to capture with existing NWP models and certainly statistical aids. Future work will involve studying false alarm and missed forecast cases for common features that could aid forecasters and algorithm developers and expanding RIPA capabilities to include other thresholds and other TC basins.…”