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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

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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).

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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

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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. (NOAA OISSTv2 data are provided by the NOAA/OAR/ ESRL/Physical Sciences Division, Boulder, CO, and are available online at http://www.esrl.noaa.gov/psd/.) steady at 55–60 mm within that same radial distance. There is no accounting for the time it would take a TC Temperatures in the 200- and 250-hPa levels varied only to reach its MPI. about 18C during this initial decay period. Based on b. Evidence of the STJ’s intrusion into the eyewall satellite images and the reanalyses upper-level wind fields, there was one strong outflow channel to the The storm-relative winds derived from the GPS northwest and weaker flow to the southwest. Through- sondes deployed during the two G-IV missions allow us out the period, upper-level divergence did not vary by to estimate when the STJ reached the eyewall of the TC. more than 15%. Other than the passage over cooler water, there were no apparent changes in other envi- ronmental factors that would be strong candidates for causing the initial decay. The ADT shows a reintensification from 1800 UTC 7 August to 0000 UTC 8 August, again showing Felicia approaching its MPI. This was when the area of cold cloud tops over the circulation center expanded considerably. After 1200 UTC 8 August both BT data and the ADT rapidly diverge from MPI, suggesting that this phase of weakening, in contrast to the initial weakening, may not be readily predicted based solely on theoretical thermodynamic controls. Based on MPI theory, Felicia should have been nearly steady at 952 hPa after 1200 UTC 8 August. MPI does not take into account VWS or any internal TC processes such as eyewall replacement cycles or competing rainbands. The theory assumes that the TC has come into ther- FIG. 4. The evolution of Hurricane Felicia’s central pressure according to NHC BT data (black line), CIMSS using the ADT modynamic equilibrium with a maximum inflow (blue line), and Felicia’s thermodynamic limit according to MPI temperature, a minimum outflow temperature, and a theory (red line). Black vertical arrows mark central time of each relative humidity in the environmental inflow of 80%. G-IV circumnavigation.

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FIG. 5. GPS sonde–measured storm-relative wind flow at 200 hPa during the G-IV missions 2 2 2 on (a) 8 and (b) 9 Aug. Full barb is 5 m s 1, half barb is 2.5 m s 1, and black triangle is 25 m s 1. The red circle denotes Felicia’s center of circulation.

Along the circumnavigation on 8 August, 200-hPa winds variable winds, in both magnitude and direction, char- 2 were ;5ms 1, with anticyclonic flow along the acterized the eastern half. However, the winds still northwest–east portions of the ring and cyclonic flow to exhibited a westerly component on the east side of the the southwest and south (Fig. 5a). In contrast, the winds circumnavigation, supporting our conjecture that the on 9 August had a westerly component throughout the STJ’s contribution to adverse VWS had reached 2 circumnavigation, with over 15 m s 1 westerly winds on the eyewall of Felicia before the sampling on 9 August. the west side of the TC (Fig. 5b). Overall, the storm- The storm-relative winds at 250 hPa demonstrated a relative flow at 200 hPa illustrates the intrusion of wavenumber-1 asymmetry and an evolving interaction westerly flow associated with the STJ; the west side of between the STJ and the TC. On 8 August, tangential the circumnavigation had strong west winds while more flow (Fig. 6a) was weak throughout the majority of the

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21 21 FIG. 6. Storm-relative (a) tangential flow (m s ), (b) radial flow (m s ), and (c) ue (K) all at 250 hPa, and 2 (d) VWS from 850 to 225 hPa (m s 1). In (a), the positive tangential flow values are cyclonic. In (b), the positive values are outflow. In (d), VWS is expressed as the difference in the vectors from the bottom to the top level. Each datum describes the deployment position on the circumnavigated ring starting at the southwest quadrant and rotating clockwise. Red (blue) depicts the observations on 8 (9) Aug. circumnavigation save for the southwest quadrant sup- the circumnavigation at 250 hPa indicate the intrusion of porting the contention that the circumnavigation was the STJ was not accompanied by a substantial change in close to the inflection point where flow turns from cy- ue,oru (Fig. 6c). As shown, ue on both 8 and 9 August clonic to anticyclonic. Tangential flow on 9 August be- only varied from 345 to 348 K. The intrusion of the comes more asymmetrical, with stronger cyclonic flow in jet along the northwest quadrant was associated with an the southwest rapidly shifting to anticyclonic flow along increase in ue of only 1–1.5 K. the northwest through northeast portions of the cir- The VWS from 225 to 850 hPa exemplifies the ad- cumnavigation because of the STJ. This pattern is evi- verse impacts of the STJ (Fig. 6d). Here, we have cal- dence that a zone of diffluence existed on the west side culated the shear from two layers (200–250 and 825– of the circumnavigation. The storm-relative radial flow 875 hPa) to avoid any unrepresentative observations at 250 hPa reveals a remarkable intrusion of the STJ from a given level. There were few observations above (Fig. 6b). Specifically, 8 August was characterized by 200 hPa precluding the use of the 175–225-hPa layer. 2 radial inflow from 23to24ms 1 from the southwest Along the 38 latitude circumnavigation, 8 August ex- 2 through northwest parts of the circumnavigation while hibited wind shear of 8–15 m s 1 while9August 2 on 9 August the inflow exceeded 220 m s 1 in the west- demonstrated a doubling of the VWS in the northwest 2 northwest sector. While significant inflow is evident to quadrant to more than 30 m s 1. It is apparent that the the west and northwest, outflow characterizes the op- STJ intruded into the 38 latitude ring along the west– posite side of the TC. north portions while the easternmost extent was shel- 2 Since the amount of moisture at 250 hPa is minimal, tered and remained at less than 15 m s 1 VWS. There 21 analysis of equivalent potential temperature ue is com- waslessthan5ms VWS in the southwest quadrant parable to potential temperature u. The ue values along on 9 August, a location beyond the influence of the STJ.

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by the spatial extent of IR-estimated cloud tops with temperatures less than 2508C collocated within 250 km of the TC center (see Fig. 8). Deep convection covered an area of 38–48 latitude square until 0600– 0700 UTC 9 August, at which point the area ,2508C steadily drops to zero. The arrival time for the STJ at the eyewall is very close in time (62h) to the rapid decrease of the area of the cold tops over Felicia’s core. c. Midlevel dry air is present but remains far from the eyewall The soundings along the circumnavigations revealed

that dry air (low or cool ue) could be found occasionally in the layer from 750 to 500 hPa, with the driest often being near 700 hPa. Storm-relative radial flow in unison 3 25 21 FIG. 7. Divergence ( 10 s ) calculated from the GPS sonde u data for the area encompassed by the G-IV circumnavigations of with e at 700 hPa is therefore analyzed with the primary Felicia on 8 (red) and 9 (blue) Aug. objective to determine if dry air could have been in- gested into the eyewall and be responsible for the rapid The intrusion of the STJ altered the net divergence decay of deep convection there. The storm-relative ra- encompassed by the circumnavigations. Weak di- dial flow demonstrates similar profiles for both 8 and 2 vergence in the upper troposphere on 8 August was 9 August; peak radial outflow of ;5ms 1 in the west- replaced by strong convergence that extended down- northwest sector followed by a gradual decline to very 2 ward to nearly 600 hPa by 9 August (Fig. 7). weak radial inflow from 21to22ms 1 in the Based upon examination of satellite image loops, the northeast–southwest portions of the circumnavigation lower-level circulation was hidden under the high (Fig. 10a). Clockwise from southwest to northeast, both clouds at 0200 UTC 9 August (Fig. 8a). Separation or 8 and 9 August indicated that ue fluctuated between 335 tilting of the centers, based on the patterns of the lower and 342 K (Fig. 10b). The southeast quadrant showed and upper clouds, was starting to be apparent by a marked decrease to 325–330 K during both sampling 0600 UTC 9 August (Fig. 8b). The lower circulation periods. This sector of dry air was collocated with very 2 center, inferred from the cloud patterns, and the upper- weak storm-relative radial inflow from 21to22ms 1. level clouds were clearly separated by 1000 UTC (Fig. 8c) With the assumption that this radial inflow applies to the and that trend continued (1200 UTC 9 August; majority of the midlevel layer and that the radial inflow Fig. 8d). does not increase with decreasing radius, this midlevel

Since both satellite imagery and GPS sonde analyses quadrant of lower ue would have reached Felicia’s RMW support the contention that the STJ had already in- in approximately 2 days, a value far too large to be a truded into Felicia’s core before the end of the G-IV factor in Felicia’s decay when compared to the collapse surveillance on 9 August, the 250-hPa radial inflow of the deep clouds. along the northwest quadrant was linearly interpolated d. SHIPS shear evaluation of Felicia between the two mission periods, resulting in a value 2 from 210 to 212 m s 1 radial inflow at 0000 UTC SHIPS is a statistical–dynamical model that employs 9 August. With the radius of maximum wind (RMW) multiple regression techniques to forecast the intensity 2 evaluated at ;80 km, 211 m s 1 radial inflow in the of TCs (DeMaria et al. 2005). SHIPS forecasts in- northwest quadrant would have reached Felicia’s corporate data from persistence, climatology, and the RMW in approximately 6.5 h. This RMW of 80 km is environment. Atmospheric predictors such as VWS are based upon the NOAA/Hurricane Research Di- retrieved from the NCEP Global Forecast System vision’s 9 August H*WIND analysis, which in- (GFS) and GOES infrared imagery. SHIPS-forecasted corporates the C-130 aircraft data and all other VWS, also known as the SHIPS SHDC variable, is cal- available observations (Powell et al. 1998). The timing culated by first removing the 850-hPa GFS vortex cen- of this estimated intrusion (;0630 UTC 9 August), ter, after which the 850–200-hPa shear magnitude is corresponding with estimates of cold cloud-top area averaged from 0 to 500 km relative to the 850-hPa vortex from IR satellite imagery, is shown in Fig. 9.InFig. 9, center. Since the G-IV datasets on 8 and 9 August were deep convection associated with Felicia is represented assimilated into the SHIPS 1200 UTC GFS model runs,

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FIG.8.GOES-11 10.7-mm satellite images of Hurricane Felicia at (a) 0200, (b) 0600, (c) 1000, and (d) 1200 UTC 9 Aug. Grid lines are every 28 latitude and 28 longitude and the temperature scale is shown below each image. (Images courtesy of Navy Research Laboratory, Monterey, CA.) these prognostications have the added benefit of in the GFS fields would have a tendency to create more in situ measurements in the vicinity of Felicia in order to spread between the G-IV-measured VWS and those better represent the true environmental flow and, evaluated from the GFS model. Somewhat surprisingly, subsequently, VWS. the SHIPS forecast on 6 August demonstrated the Figure 11 presents the SHIPS-forecasted shear after closest trend with regard to the timing of increasing maximum intensity at 0600 UTC 6 August. Also plotted VWS, though it was late by about 24 h; SHIPS VWS are the averaged 850–225-hPa VWS values from the forecasts on 7 and 8 August suggested a reduction in G-IV circumnavigations on 8 and 9 August. While the VWS around Felicia from 1200 UTC 8 August to 0000 SHIPS VWS predictors determine a shear value for an UTC 9 August while both the SHIPS 6 August forecast area out to a 500-km radius relative to the 850-hPa and the trend in G-IV circumnavigations from 8 to vortex center, the storm-relative G-IV computations of 9 August portrayed an increasing trend. We feel that the VWS are solely the average of the values along the 38 SHIPS calculation of VWS is lower most likely because latitude rings. Since the values of VWS are not com- it calculates the value symmetrically around the TC and puted in the same fashion, there are large differences includes regions to the south, away from the STJ where between the two approaches. Additionally, inaccuracies there is far less VWS.

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FIG. 9. Evolution of the spatial extent of cloud-top temperatures less than 2508C (square degrees of latitude) within 250 km of Felicia’s center from 0000 UTC 8 Aug through 1800 UTC 9 Aug.

4. Conclusions The NOAA G-IV circumnavigations have provided insight into the final collapse of Hurricane Felicia (2009). The relative radial flow at 250 hPa increased 2 from 25 to more than 220 m s 1 along the northwest quadrant of the circumnavigation as the range between the TC center and the axis of the subtropical jet stream decreased. The STJ enhanced the vertical shear of the 2 horizontal wind to over 30 m s 1 along the northwest portion of the circumnavigation; the estimated arrival time of this extreme VWS in the eyewall region is well 21 correlated with the dissipation of deep convection lo- FIG. 10. (a) Storm-relative radial flow (m s ) and (b) ue (K) at cated there and new cells developing far downshear. 700 hPa. In (a), the positive values are outflow and negative values Thereafter, only shallow clouds surrounded the vortex are radial inflow. Each datum describes the deployment position on the circumnavigated ring starting at the southwest quadrant and center. Midlevel dry air was present, as could be ex- rotating clockwise. Red (blue) depicts the observations on 8 pected from the hurricane’s location south of the Pacific (9) Aug. subtropical high, but this air did not reach the eyewall region during the rapid dissipation and therefore is not deemed a factor in intensity change. Earlier weakening The physical mechanism of how exactly VWS dis- of the TC can be attributed largely to its passage over rupts the TC is still unknown. Ventilation and sub- cooler water. sequent cooling of the upper-tropospheric warm core Based on this study, we recommend two points. (e.g., Simpson and Riehl 1958; Gray 1968) or tilting of First, VWS should be calculated for at least each the entire warm core column (e.g., DeMaria 1996; quadrant of the TC if at all possible; large annuli or full Frank and Ritchie 2001) has been hypothesized to be circular area means may mask the invasion of strong the cause. Neither argument has had the benefit of winds that result in detrimental VWS. This suggestion confirming observations. To better grasp the physics at follows on with other changes in VWS estimation play, observations in the environment and the warm proposed by Velden and Sears (2014). Second, noting core, garnered by a Global Hawk flying in the lower the difference between the actual intensity and the stratosphere or by judicious use of the now-Doppler- maximum potential intensity is viewed as a useful equipped G-IV, would be highly beneficial. The mis- technique for identifying the influences that contribute sion should include simultaneous sampling of the to TC weakening beyond theoretical thermodynamic eyewall and lower portions of the vortex by one of the considerations. NOAA WP-3D aircraft.

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