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AUGUST 2015 B U K U N T A N D B A R N E S 1039 The Subtropical Jet Stream Delivers the Coup de Grace^ to Hurricane Felicia (2009) BRANDON P. BUKUNT AND GARY M. BARNES University of Hawai‘i at Manoa, Honolulu, Hawaii (Manuscript received 8 January 2015, in final form 27 April 2015) ABSTRACT The NOAA Gulfstream IV (G-IV) routinely deploys global positioning system dropwindsondes (GPS sondes) to sample the environment around hurricanes that threaten landfall in the United States and neighboring countries. Part of this G-IV synoptic surveillance flight pattern is a circumnavigation 300–350 km from the circulation center of the hurricane. Here, the GPS sondes deployed over two consecutive days around Hurricane Felicia (2009) as it approached Hawaii are examined. The circumnavigations captured only the final stages of decay of the once-category-4 hurricane. Satellite images revealed a rapid collapse of the deep convection in the eyewall region and the appearance of the low-level circulation center over ;8h. Midlevel dry air associated with the Pacific high was present along portions of the circumnavigation but did not reach the eyewall region during the period of rapid dissipation of the deep clouds. In contrast, the sub- tropical jet stream (STJ) enhanced the deep-layer vertical shear of the horizontal wind (VWS; 850–200 hPa) 2 to greater than 30 m s 1 first in the northwest quadrant; ;6 h later the STJ was estimated to reach the eyewall region of the hurricane and was nearly coincident with the dissipation of deep convection in the core of Felicia. Felicia’s demise is an example of the STJ enhancing the VWS and inhibiting intense hurricanes from making landfall in Hawaii. The authors speculate that VWS calculated over quadrants rather than entire annuli around a hurricane may be more appropriate for forecasting intensity change. 1. Introduction (G-IV) for synoptic reconnaissance. Missions similar to these have reduced track forecast errors in the Atlantic Accurate forecasting of the decay of an overwater basin (Burpee et al. 1996), now by as much as 32% tropical cyclone (TC) has obvious benefits to marine (Aberson and Franklin 1999). For Felicia, we wish to interests and coastal communities expecting landfall. explore if the global positioning system dropwindsondes The study of TC decay, however, has taken a distant (GPS sondes) deployed from the G-IV can provide in- second place to intensification. Hurricane Opal (1995) sight into this TC’s rapid filling close to Hawaii. Spe- stands as one of the few TCs where decay has been ex- cifically, we will examine the interaction of the plored prior to extratropical transition or landfall (e.g., subtropical jet stream (STJ) with the upper-level circu- Rodgers et al. 1998; Bosart et al. 2000). Two eastern lation of the TC. Pacific hurricanes, Jimena (1991) and Olivia (1994), are Hawaii is in an enviable location during the hurricane examples of how increasing vertical shear of the hori- season because the tropical upper-tropospheric trough zontal wind (VWS) can impact intensity (Black (TUTT) is fully developed and its axis lies just north of et al. 2002). the islands (Sadler 1975). The TUTT displaces the STJ In August 2009, TC Felicia deepened to category 4 southward over the archipelago. Given the underlying as it approached the Hawaiian Islands. This threat trade wind flow, the result is a belt of strong VWS over prompted the National Oceanic and Atmospheric Ad- the islands (Fig. 1). Strong VWS has been implicated in ministration (NOAA) and the Central Pacific Hurricane the weakening of TCs through several physical mecha- Center (CPHC) to mobilize the NOAA Gulfstream IV nisms (e.g., McBride and Zehr 1981; Zehr 1992; DeMaria 1996; Bender 1997; Frank and Ritchie 2001; Gallina and Velden 2002; Tang and Emanuel 2012; Corresponding author address: G. M. Barnes, Dept. of Atmo- spheric Sciences, University of Hawai‘i at Manoa, 2525 Correa Rd., Dolling and Barnes 2014) and currently serves as a Honolulu, HI 96822. predictor in the Statistical Hurricane Intensity Pre- E-mail: [email protected] diction Scheme (SHIPS; DeMaria et al. 2005). DOI: 10.1175/WAF-D-15-0004.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 09/25/21 11:00 PM UTC 1040 WEATHER AND FORECASTING VOLUME 30 21 21 FIG. 1. Average August 200–850-hPa VWS values (m s ; contours every 4 m s ) for the period 1966–2005. [Adopted from Dettmer-Shea (2008).] 2. Data and methodology (2014). All data are storm relative with TC motion subtracted from Earth-relative winds and the sonde lo- On 8 and 9 August the G-IV sampled the synoptic cation is relative to the TC. During each mission Felicia environment to the west-northwest of the TC, and then (2009) filled by only a few hectopascals, so we assume a performed a circumnavigation of Felicia at a distance steady state during each circumnavigation. approximately 300–350 km from the circulation center Other datasets that we utilized included 1) the Op- (Figs. 2a,b). This distance is partially driven by studies timum Interpolation Sea Surface Temperature, version by Gray (1989, his appendix) and Franklin (1990) that 2 (OISSTv2), fields generated by NOAA for the week revealed TC motion is best correlated with conditions in of 5–12 August; 2) the infrared, visible, and water vapor that radial range. Over 90% of the GPS sondes were images from the Geostationary Operational Environ- successful; the sonde deployment locations including the mental Satellite (GOES); 3) estimates of TC position, failures are depicted in Fig. 2. The period of circum- intensity, and minimum central pressure from the NHC navigation was from 0840:55 to 1059:05 UTC 8 August best-track (BT; Jarvinen et al. 1984)dataset;4)tropical and from 0919:40 to 1137:05 UTC 9 August. cyclone products and estimates of intensity from the The GPS sonde performance is described by Hock Cooperative Institute for Meteorological Satellite and Franklin (1999). The sondes have a 2-Hz sampling Studies (CIMSS) at the University of Wisconsin– rate that translates to vertical resolutions of ;12–14 m at Madison [CIMSS uses the advanced Dvorak tech- 200–300 hPa and ;5–7 m in the lower troposphere. All nique (ADT; Olander and Velden 2007)toestimate sondes were processed with the Atmospheric Sounding intensity]; 5) estimates of maximum potential intensity Processing Environment program (Martin 2007), then (MPI; Emanuel 1986); 6) National Centers for Environ- individually scrutinized to eliminate errors such as sen- mental Prediction–National Center for Atmospheric sor wetting, following Bogner et al. (2000). The GPS Research (NCEP–NCAR) reanalyses with 2.58-latitude sondes are processed using the methodology developed resolution (Kalnay et al. 1996); and 7) forecasted in- by Dolling (2010), and reviewed by Dolling and Barnes tensity from the SHIPS dataset (DeMaria et al. 2005). Unauthenticated | Downloaded 09/25/21 11:00 PM UTC AUGUST 2015 B U K U N T A N D B A R N E S 1041 FIG. 2. NOAA G-IV surveillance missions of Felicia on (a) 8 and (b) 9 Aug 2009 with the numbered chronological deployments of the GPS sondes. The red markers denote unsuccessful sonde deployment locations. The green circle is Felicia’s low-level circulation center as de- termined by NHC BT data. A comprehensive discussion of the datasets and techniques Fig. 4. For the MPI estimate we have used SST 2 1.28C may be found in Bukunt (2014). as the proxy for inflow temperature, following the re- sults of Cione et al. (2000), who determined the average 3. Results difference between sea and air. Our estimate is derived from their data for the annulus from 0.58, the approxi- a. Felicia’s track, initial weakening, and brief mate eyewall location, to 4.08 radius from the center. reintensification Felicia came within 5 hPa of its MPI. During this time Felicia achieved tropical depression status at BT data, the ADT, and MPI exhibited an equivalent 1800 UTC 3 August and became a tropical storm at weakening trend from 0600 UTC 6 August to 0600 UTC 0000 UTC 4 August when it was located at approximately 7 August. This initial decay primarily reflects the reduction 128N, 1228W(Kimberlain et al. 2010). With SSTs from 288 in inflow temperature as Felicia tracked over SSTs that to 298C(Fig. 3) and a synoptic regime characterized by decreased about 28C(Fig. 3; note nearly parallel lines of low VWS, high total precipitable water (.55 mm), and a MPI and actual intensity estimates). moist midlevel troposphere, Felicia rapidly intensified From 0600 UTC 6 August to 0600 UTC 7 August from 0000 UTC 4 August through 0000 UTC 6 August there are no G-IV observations around Felicia, so we while being steered to the west-northwest by a well- have to rely on the NCEP–NCAR reanalyses fields and defined deep-layer ridge. Felicia achieved a minimum CIMSS tropical cyclone products to infer environmental central pressure of 935 hPa at 0000 UTC 6 August 2009. conditions. The CIMSS deep-layer VWS (850–200 hPa; 2 The estimated minimum sea level pressures of Felicia not shown) remained less than 10 m s 1 within 500 km based on BT data, the ADT, and MPI are shown in of the hurricane center. Total precipitable water was Unauthenticated | Downloaded 09/25/21 11:00 PM UTC 1042 WEATHER AND FORECASTING VOLUME 30 FIG. 3. NHC BT data of Hurricane Felicia with center positions every 12 h overlain on the NOAA OISSTv2 weekly averaged satellite-derived SST field (8C; contours every 0.58C). Black dots denote 0000 UTC of the labeled day and the lowest pressure attained by Felicia is noted.

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