Downloaded 10/05/21 12:12 PM UTC 1340 MONTHLY WEATHER REVIEW VOLUME 145

APRIL 2017 K O M A R O M I A N D D O Y L E 1339

Tropical Cyclone Outﬂow and Warm Core Structure as Revealed by HS3 Dropsonde Data

WILLIAM A. KOMAROMI AND JAMES D. DOYLE Naval Research Laboratory, Monterey, California

(Manuscript received 5 May 2016, in ﬁnal form 29 November 2016)

ABSTRACT

Dropsonde data collected during the NASA Hurricane and Severe Storm Sentinel (HS3) field campaign from 16 research missions spanning 6 tropical cyclones (TCs) are investigated, with an emphasis on TC outflow and the warm core. The Global Hawk (GH) AV-6 aircraft provided a unique opportunity to investigate the outflow characteristics due to a combination of 181-h flight durations and the ability to release dropsondes from high altitudes above 100 hPa. Intensifying TCs are found to be associated with stronger upper-level divergence and radial outflow relative to nonintensifying TCs in the sample, regardless of 2 currentintensity.Alayerof2–4ms 1 inflow 20–50 hPa deep is also observed 50–100 hPa above the maximum outflow layer, which appears to be associated with lower-stratospheric descent above the eye. The potential temperature of the outflow is found to be more strongly correlated with the equivalent potential temperature of the boundary layer inflow than to the present storm intensity, consistent with the outflow temperature having a stronger relationship with potential intensity than actual intensity. Finally, the outflow originates from a region of low inertial stability that extends above the cyclone from 300 to 150 hPa and from 50- to 200-km radius. The unique nature of this dataset allows the height and structure of the warm core also to be investigated. The magnitude of the warm core was found to be positively correlated with TC intensity, while the height of the warm core was weakly positively correlated with intensity. Finally, neither the height nor magnitude of the warm core exhibits any meaningful relationship with intensity change.

1. Introduction sampled the outflow up to 195 hPa (Titley and Elsberry 2000). Only just recently with the NASA Genesis and Until recently, a single ER-2 flight over Hurricane Rapid Intensification Processes (GRIP) field experi- Erin (2001) provided the only direct dropsonde obser- ment of 20101 (Braun et al. 2013) and the subsequent vations through the full depth of the tropical cyclone NASA Hurricane and Severe Storm Sentinel (HS3; (TC) outflow layer (Halverson et al. 2006). Conven- Braun et al. 2016) investigation from 2012 to 2014 have tional aircraft observations of TCs, such as by the U.S. routine observations of the TC upper levels been ob- Air Force C-130s and the NOAA P-3s, tend to be lim- tained, and only for HS3 were dropsondes deployed. By ited to the middle to lower levels of the cyclone with a capitalizing on the ability of the unmanned NASA Global typical flight level of 700 hPa (Aberson et al. 2006). Hawk (GH) to fly at a cruising altitude above 100 hPa and Synoptic observations provided by the NOAA G-IV are also overfly the TC core (Braun et al. 2013),HS3madeit of higher altitude, up to about 150 hPa (Rogers et al. possible to not only sample the entire depth of the outflow 2002), but still frequently fail to capture the top of the layer, but also to sample the region in which the radial outflow layer. The tropical cyclone (TC) outflow layer is outflow originates over the deepest convection, hereafter challenging to observe due to the fact that it can extend referred to as the outflow roots. Several studies have al- from 300 hPa to as high as 100 hPa, above where most ready examined data from GRIP, focusing on both gen- research aircraft operate. The G-IV also generally esis and rapid intensification (e.g., Davis and Ahijevych avoids overflying the TC core. Finally, special NASA 2013; Zawislak and Zipser 2014; Helms and Hart 2015; DC-8 dropsondes into Super Typhoon Flo (1990)

Corresponding author e-mail: William A. Komaromi, will.komaromi@ 1 Note that the Global Hawk did not release any dropsondes nrlmry.navy.mil during GRIP.

DOI: 10.1175/MWR-D-16-0172.1 For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/ PUBSReuseLicenses). Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1340 MONTHLY WEATHER REVIEW VOLUME 145

Rogers et al. 2015). In this study, we focus on the full the vertical depth of the outflow layer during the in- quality-controlled HS3 dropsonde dataset. tensification of Super Typhoon Flo (1990). Their results The primary goal of HS3 was to observe hurricane show the equatorward outflow jet maximum to occur at a formation and intensity change, motivated by hypothe- higher level than the poleward outflow, but with slightly ses related to the complex interaction between the large- stronger maximum velocities in the northern outflow. scale environment and hurricane internal dynamics Here, we revisit some of these previous results given the (Braun et al. 2016). HS3 operated research flights during superior spatial density and high vertical resolution of the 5-week campaigns in the August–September time frame HS3 dataset. during the 3-yr period of 2012–14. Two GH aircraft were To understand the relationship between outflow, di- available for HS3, designated ‘‘air vehicle one’’ (AV-1) vergence, and the upper-level anticyclone above the TC, and ‘‘air vehicle six’’ (AV-6). AV-6 was equipped with it is beneficial to first examine the vorticity equation in an NCAR–NOAA miniature dropsonde system known pressure coordinates (e.g., Holton 2004): as the Airborne Vertical Atmospheric Profiling System D ›u ›y ›v ›y ›v ›u (AVAPS) II, which was designed for remote operation, (z 1 f ) 52(z 1 f ) 1 2 2 , and could launch up to 88 dropsondes in a single flight Dt ›x ›y ›x ›p ›y ›p while tracking up to eight dropsondes in the air simul- (1) taneously. There were 25 research flights conducted by HS3 from 2012 through 2014, resulting in 1506 quality- where z is the relative vorticity on a constant pressure controlled dropsondes in the final dataset (Wick 2015). surface; f is the Coriolis parameter; u and y are the zonal In addition to the ability to operate at very high alti- and meridional components of wind, respectively; p is tudes, the GH also had the unique capability to fly very pressure; and v [ Dp/Dt is the pressure change following long missions of ;24-h duration. This capability allowed the motion. The two terms on the right-hand side of the aircraft to perform multiple transects over the core Eq. (1) are known as the stretching and tilting terms, while also sampling the environment around the TC, respectively. TC outflow originates above the deep con- making it possible to render a more complete depiction vective core of the cyclone as air parcels ascend parallel of the three-dimensional structure of the cyclone and its to lines of constant angular momentum, which are steeply environment from the dropsonde data alone. slanted in the eyewall but tilt to become horizontal at While there are a wide number of potential applica- upper levels (Emanuel 1986). In this region, convergence tions of the HS3 dataset, it is clear that one of the most will result in an increase in z while divergence will result unique aspects of this dataset is the high spatial resolution in a decrease in z for a given air parcel, as a result of the and coverage of in situ observations above the outflow stretching term assuming all else being equal and at and outflow-roots region of the TC. Because of the lim- constant latitude. Divergent flow in the outflow roots ited nature of in situ measurements of TC outflow, pre- region will contribute to anticyclonic z, which in turn vious studies (e.g., Sadler 1976, 1978; Merrill 1988a,b; intensifies and builds the upper-level anticyclone. The Merrill and Velden 1996) used a combination of satellite- radius at which the outflow will expand against the am- based atmospheric motion vectors (AMVs), commercial bient environment is theoretically equal to the Rossby aircraft [or NASA DC8 dropsondes from the Tropical radius of deformation (Rd; Rappin et al. 2011), although Cyclone Motion (TCM-90) field experiment in the case of in the real atmosphere it is often difficult to compute Rd 2 Merrill and Velden (1996)], and rawinsondes of limited accurately. spatial density to generate analyses of the outflow layer. It has been argued that TCs primarily interact with Merrill (1988a) found stronger radial outflow, but weaker environmental flow at the upper levels where the inertial anticyclonic flow to be associated with the intensifying stability I is weaker (Holland and Merrill 1984): cyclones, with greater environmental vertical wind shear 2V near or over the TC center for nonintensifying cases. I2 5 f 1 t (f 1 z), (2) Anticyclonic flow associated with intensifying TCs also r tended to be more directly centered on the TC itself, z 5 › › whereas for nonintensifying TCs the anticyclone was where (1/r)[ (rVt)/ r] and Vt is the tangential com- displaced to the downshear right (Merrill 1988a). A ponent of wind (Alaka 1961). Greater I at the low to number of studies (Sadler 1976, 1978; Merrill and Velden 1996) also found an association between TCs with multiple outflow channels and intensification. Additionally, 2 The fact that Rd is inversely proportional to f, which varies with Merrill and Velden (1996) found an increase in the height latitude, also complicates matters, favoring a smaller (larger) ra- of the level of strongest outflow as well as an increase in dius anticyclone poleward (equatorward) of the TC.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1341 midlevels resists radial inflow or outflow, especially for Tropical Storm (TS) Humberto (2001). However, the stronger storms (Schubert and Hack 1982; Shapiro and occurrence of a warm core at such low levels does not Willoughby 1982; Hack and Schubert 1986). In one of a appear to be typical for mature TCs. Durden (2013) ex- very few studies to compute I using dropsonde data in the amined dropsondes and surface-based soundings from outflow region, Molinari and Vollaro (2014) found radial various NASA field campaigns with data up to ;250 hPa, outflow to be maximized at the outer edge of the in- and found that the level of the strongest warm core varies ertially unstable region, and hypothesized that the length between 760 and 250 hPa, with the higher warm cores of time that the inertially unstable region persists may be more often associated with stronger TCs but with a large related to the nature or even the length of the period of degree of variability. Durden (2013) also noted that TC intensification. A few recent modeling studies also Danielle (1998) and Georges (1998) had prominent examine the relationship between changes in environ- double warm cores, while Earl (2010) also featured a mental or vortex inertial stability and changes in TC double warm core early on. However, the lower warm structure and intensity. Rappin et al. (2011) argue that a core for Earl is only evident when comparing against local reduction in inertial stability north of the TC associated environmental dropsondes as a reference profile, and is with the insertion of a westerly zonal jet minimizes the no longer apparent when using the Dunion (2011) moist energy expenditure of forced subsidence by enhancing tropical sounding as a reference profile. the outflow along a single narrow path, allowing radia- While observations of the warm core remain limited, tional cooling to lessen the work of subsidence and recently there have been numerous modeling studies thereby intensifying the TC. Rozoff et al. (2012) compute focused on the warm core. Stern and Nolan (2012) found I up to a height of 13 km out to a radius of 100 km the primary warm core to be in the 4–8-km-altitude from high-resolution Weather Research and Forecasting range using idealized WRF Model simulations, in con- (WRF) Model simulations, and demonstrate a reduction trast to the more conventionally accepted 10 km or of I in the TC core and moat region along with an increase greater found in earlier observational studies. They also in I outside of the primary eyewall during secondary note a secondary, weaker warm core at 13–14 km in eyewall formation. However, they do not include the some simulations. Using idealized Nonhydrostatic Ico- outflow region in their analyses. sahedral Atmospheric Model (NICAM) simulations, As is the case with outflow, the TC warm core has Ohno and Satoh (2015) found initial warm core devel- traditionally been infrequently documented by in situ opment from 8 to 9 km. However, a secondary warm observations because of its combination of high altitude core developed around 16 km at the top of the tropo- and collocation with the core of the cyclone, making it sphere, penetrating into the lower stratosphere, which difficult for conventional research aircraft to reach. eventually became the primary warm core as the system However, documenting the magnitude and height of the intensified to ,900 hPa. It has been argued for some warm core may be fundamental to fully understanding time now that dry descent in the eye of the TC con- the balanced structure of the TC vortex. Early studies tributes to intensification (e.g., Malkus and Riehl 1960). such as La Seur and Hawkins (1963), Hawkins and This idea has since been supported by ER-2 observa- Rubsam (1968), and Hawkins and Imbembo (1979) all tions of Hurricane Bonnie in 1998 (Heymsfield et al. find maximum warm anomalies of 98–168C close to 2001) and a number of recent modeling studies (Chen 250 hPa. Hawkins and Imbembo (1976) also found a and Zhang 2013; Miller et al. 2015; Ohno and Satoh lower secondary maximum in Hurricane Inez (1966) 2015). In fact, these recent modeling studies suggest that from 650 to 600 hPa. However, these flights all inferred eye decent is likely to be associated with horizontal the height and strength of the warm core based upon convergence above the outflow layer and may originate interpolation of flight-level data from three to five in the stratosphere. While the lower stratosphere is at different vertical levels, so uncertainty was large. the limits of what the GH AVAPS system is capable of Halverson et al. (2006) found a warm core maximum of sampling, the HS3 data will be examined with these 118C at 500 hPa in Hurricane Erin (2001) using direct hypotheses in mind. dropsonde measurements. They attributed finding such a low-altitude warm core to the fact that the storm 2. Methodology was steadily weakening during the observational period. Examining a combination of dropsondes, aircraft flight- Twenty-five research flights were conducted from level data and WP-3D-based radar data, Dolling and 2012 to 2014 during HS3, 16 of which are included in this Barnes (2012) argue that there is an early warm-core- study. This subset includes all but three TC missions of like feature with a maximum temperature anomaly of at least tropical depression (TD) status and one mission 78C as low as 800 hPa during the development of into Gabrielle (2013) prior to genesis. The three TC

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1342 MONTHLY WEATHER REVIEW VOLUME 145

TABLE 1. Summary of cases examined in this study and their significance is computed using a two-sample Welch’s t respective dates of occurrence. For classification, TD 5 tropical test (Welch 1947). The Welch’s test has the advantage 5 5 depression, TS tropical storm, and H hurricane. Classification over the Student’s t test of being able to test hypotheses and storm intensity refers to the HURDAT2 intensity at the temporal median of the dropsonde release pattern, while trend for different-sized samples and for samples with unequal refers to the total change in intensity from the time of release of the variances. first dropsonde to the time of release of the final dropsonde. Most systems flown during HS3 were well sampled in all four quadrants using either butterfly or lawnmower Date (first System Intensity Trend dropsonde) name Classification (kt) (kt) patterns [for descriptions and figures of these patterns, see Helms and Hart (2013)]. Therefore, it is possible to 11 Sep 2012 Nadine TD 30 0 14 Sep 2012 Nadine H 70 110 generate a reasonable three-dimensional representation 19 Sep 2012 Nadine TS 50 0 of the TC, the outflow, and the near-storm environment 22 Sep 2012 Nadine TS 45 15 from dropsondes alone. These dropsondes were re- 26 Sep 2012 Nadine TS 50 15 leased over a 12–18-h period, and occasionally even , 29 Aug 2013 Gabrielle Pregenesis 25 0 longer, for numerous missions throughout HS3. Storm 4 Sep 2013 Gabrielle TD 30 0 7 Sep 2013 Gabrielle Disturbance 25 0 motion can be substantial over these time periods, es- 16 Sep 2013 Humberto TS 35 25 pecially for fast-moving systems. As such, results will be 26 Aug 2014 Cristobal H 70 15 presented in a storm-relative coordinate system com- 29 Aug 2014 Cristobal H 70 210 puted relative to 2-min interval center fixes provided by 2 2 Sep 2014 Dolly TS 40 5 the NOAA/Hurricane Research Division (HRD; avail- 12 Sep 2014 Edouard TS 35 15 14 Sep 2014 Edouard H 80 115 able online at http://www.aoml.noaa.gov/hrd/data_sub/ 16 Sep 2014 Edouard H 85 220 hurr.html). When the HRD centers are not available, 18 Sep 2014 Edouard H 65 210 the TC center is calculated using a linear interpolation of NHC’s 3- (when available) and 6-hourly center fixes to the nearest minute of each dropsonde release. The missions that are not included are the 7 September 2012 dropsondes’ onboard GPS is utilized to account for drift flight into Hurricane Leslie during an aircraft ferry and data are averaged vertically over 5-hPa increments flight in which only part of the environment around the to reduce noise. Finally, data are interpolated onto a TC was sampled; the mission on 20 August 2013 that Cartesian grid with 25-km horizontal grid spacing observed the remnants of Erin, in which case there was using a natural-neighbor interpolation method (Sibson an early failure of the dropsonde system; and an 1981), which can be thought of as a compromise between attempted mission over Hurricane Ingrid on 9 Sep- linear and cubic interpolation. Analyses will be pre- tember 2013 that ended prematurely because of cold sented on constant pressure surfaces, where the x and y fuel temperatures (Braun et al. 2016). The 16 missions coordinates refer to the distance in kilometers from the analyzed in the present study span the evolution of six center of the TC. Radius–pressure (r–p) azimuthal- TCs, as well as cyclone intensification sampled six mean sections, as well as three-dimensional (x–y–p) times, while five TCs of steady intensity, and five cases plots will also be shown. For the radius–pressure plots, of weakening are captured, per the National Hurricane the horizontally interpolated data are averaged in Center’s (NHC) second-generation NHC ‘‘best track’’ 25-km-radius bins and plotted at the midpoint of each hurricane database (HURDAT2; Landsea and Franklin bin (e.g., data in the 0–25-km bin are plotted at 12.5 km). 2013). Throughout the text, the storm intensity will refer The top of the outflow layer is typically capped by the to the HURDAT2 intensity at the temporal median of high static stability at the tropopause level. Here, we are the dropsonde release pattern, while the intensity trend interested in assessing the vertical structure of the out- refers to the total change in intensity from the time of flow relative to the tropopause. As such, static stability is release of the first dropsonde to the time of release of quantified in terms of the lapse rate g: the final dropsonde. A summary of missions included in this study appears in Table 1. Six missions, includ- dT g 52 , (3) ing one over Nadine, two over Cristobal, and three over dz Edouard, sampled TCs at hurricane intensity, while 10 missions sampled systems of TD or TS strength. where T is the temperature in degrees Celsius and z is While the dataset is admittedly small from a statisti- the altitude in kilometers. It has also been established cal perspective, to our knowledge it is the most com- that inertial stability is relevant to various aspects of TC plete dataset of in situ observations above the TC core structure, including the behavior of the outflow. In cal- and in the outflow region available to date. Statistical culating I [Eq. (2)], note that it is trivial to decompose

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1343 the wind into radial (Vr) and tangential (Vt)components positive Vr originating within 500 km of the TC, the once the dropsondes are adjusted to storm-relative co- upper-level anticyclone will be any contiguous region ordinates. However, caution must be used in interpreting of negative Vt within 1000 km of the TC that exists the results, as winds, particularly Vr, can be quite sensitive over at least two quadrants, and the environmental to small errors in storm center location [implied by flow will be computed as the sum of the mean wind Willoughby (1992) and Marks et al. (1992), quantified by computed in each of four quadrants surrounding the Ryglicki and Hodyss (2016)]. Also note that Vr and Vt in TC separately. For Nadine, cyclonic tangential flow this study are computed with storm motion removed. becomes weaker and more spatially confined with Inertial instability or I2 , 0 corresponds to an imbalance decreasing pressure above 200 hPa. Consistent with of pressure gradient and total centrifugal forces, and is southwesterly environmental flow, a positive radial 2 possible in regions of negative absolute vorticity in the wind maximum of 25–30 m s 1 was observed at Northern Hemisphere or when the absolute angular 200 hPa in the northeast quadrant of the storm, with 2 momentum decreases outward, although meridional and net inflow of up to 20 m s 1 in the southwest quadrant radial mixing generally prevent this from occurring (Fig. 1a). This structure to the radial wind field is (Knox 1997). Of significance to the present study, qualitatively consistent with the aforementioned Molinari and Vollaro (2014) found inertial instability in structure at 150 and 130 hPa as well (Figs. 1b,c), but the outflow region of from roughly 10.5 to 12 km and from with winds weakening with height, while the second- 350- to 450-km radius in an azimuthal mean of G-IV ary circulation appears to collapse completely by dropsondes for Hurricane Ivan (2004). 100 hPa (Fig. 1d). Overall, the upper-level anticyclone Motivated by the Davis and Ahijevych (2012) tech- contracts with height, with the minimum Vt of less 2 nique for estimating vertical wind shear from a set of than 230 m s 1 at 200 and 150 hPa occurring 500 km or dropsondes alone, an approximate estimate of the en- more from the center both beyond and close to, re- vironmental flow is obtained by averaging dropsonde spectively, the edge of the dropsondes, while the winds in each of the four quadrants around the minimum Vt appears to fall well within the dropsonde cyclone, then averaging those four wind vectors. Av- pattern at 130 hPa, closer to 400-km radius. Un- eraging of the four quadrants will largely eliminate the fortunately most of the northern dropsondes did not wavenumber-0 vortex component to the wind. The begin reporting until below 100 hPa,3 but data from advantage of this technique is that, as long as all four 120–110 hPa (not shown) suggest that the anticyclone quadrants are sampled at fairly comparable radii, the continues to contract above 130 hPa. Whether or not results are relatively robust despite an unequal number the contraction of the radius of minimum Vt is related of dropsondes in each quadrant. to the decrease in magnitude and area of positive Vt with height is yet to be determined. Additional cases will be examined in order to determine the variability 3. Results in the rate of contraction of the upper-level anticyclone with height among the HS3 dataset. a. Upper-level anticyclone and outflow For other hurricanes sampled during HS3, including First, the upper-level structure of Hurricane Nadine 80-kt Hurricane Edouard on 14 September 2014 as sampled on 14 September 2012 is examined. Nadine (Figs. 2a–d) and 70-kt Hurricane Cristobal on 26 August 21 was a 60-kt (1 kt 5 0.5144 m s )TSatthetimeof 2014 (Figs. 3a,b), the region of Vt 5 0, or the interface release of the first dropsonde, and strengthened into a between cyclonic and anticyclonic flow, clearly contracts 70-kt hurricane by the temporal midpoint of the with height. However, there are not enough data at large dropsonde deployment (Table 1). At the time, Nadine radii to convincingly determine whether or not the 21 was under a region of relatively strong (15–20 m s ) minimum in Vt also contracts with height for either southwesterly upper-level environmental flow. This Cristobal or Edouard. In the case of Edouard, outflow is strong environmental flow overpowered the weak maximized along a single outflow channel to the north- upper-level cyclonic circulation, resulting in pro- west that is strongest at 200 hPa and decreasing but of nounced asymmetries, including net anticyclonic tan- the same direction there above. Similar to Nadine, the gential flow northwest of the center of the TC and strongest outflow occurs downwind of the environmen- inflow southwest of the storm center (Figs. 1a–d). In the tal flow at that level. In the case of Cristobal, there are composite horizontal cross sections, by convention the flow associated with the TC itself will be defined as the contiguous region of positive Vt closest to the origin 3 Note that an open circle without a wind barb indicates the lo- (0, 0), the outflow will be any contiguous region of cation of a dropsonde but with no or missing data at that level.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1344 MONTHLY WEATHER REVIEW VOLUME 145

21 FIG. 1. Tangential wind (shaded, 5 m s increments) and radial wind (positive, contoured; negative, dashed; 2 5ms 1 increments) for 70-kt Hurricane Nadine on 14 Sep 2012. Analyses are at (a) 200, (b) 150, (c) 130, and (d) 100 hPa in storm-relative coordinates (km from center in the x and y directions). Gray dots indicate locations of dropsondes. two partially connected outflow channels: one to the (not shown). However, why this difference in vertical northeast and one to the south. Within the sampled re- structure just below the tropopause is observed in the gion, the northeast channel is maximized at 180 hPa dropsondes remains elusive at this time. Note that while (Fig. 3a) while the one to the south is strongest at Cristobal (2014) was an intensifying TC at the time of 130 hPa (Fig. 3c). The strongest outflow from 200 to development of dual outflow channels, there were in- 150 hPa exists almost due downwind of the southwest- sufficient samples to relate the development of multiple erly environmental flow, while the outflow maximum at outflow channels to intensification. 130 hPa is actually upwind. Consistent with the results of Although the tropopause does not appear to be Merrill and Velden (1996), the jet maximum of the meaningfully higher to the south compared to the southern outflow channel occurs at higher altitude than north of Cristobal, there does appear to be a slight that of the northern channel, with stronger radial ve- upward bulge of the tropopause, as indicated by the locities in the northern outflow channel. It was initially overall lower tropopause pressures directly above the hypothesized that the higher outflow to the south would TC (Fig. 3d). To systematically investigate whether or be associated with a higher tropopause. The data reveal not there is an upward bulge in the tropopause across that the cold-point tropopause is indeed slightly colder the broader HS3 sample, the difference between the on average to the south (Fig. 3c). However, and some- mean 0–100-km radius and the 400–600-km-radius what unexpectedly, tropopause pressures are roughly tropopause pressure results is calculated and plotted equal north and south of the TC (Fig. 3d). Examination versus storm intensity (Fig. 4a). The results appear to of individual dropsondes reveals the cold point to be be mixed, as there are eight cases where the tropo- slightly ‘‘sharper’’ south of the TC but of roughly the pause is elevated to lower pressures above the TC, same level as to the north, explaining the discrepancy while in the other eight cases the tropopause actually

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1345

FIG.2.AsinFig. 1, but for 80-kt Hurricane Edouard on 14 Sep 2014. dips downward to higher pressures above the cyclone. compare two intensifying TCs to two weakening TCs, However, there appears to be a greater occurrence of all of similar intensities. Note that the uncertainty in having an elevated tropopause above the TC for the divergence calculation is somewhat greater for stronger storms, for which the sample is unfortunately Cristobal on 29 August because of a more rapid for- quite small. Five out of six TCs of 65 kt or greater inward motion versus the other TCs in the sample. For tensity are associated with an elevated tropopause above the Nadine, Cristobal, and the earlier Edouard ana- the TC, with the greatest pressure difference of 212 hPa lyses, it is evident that the region of strongest di- for 85-kt Hurricane Edouard, the strongest storm in the vergence is downstream of the environmental flow and, sample. Overall, the tropopause is also found to be perhaps, slightly left of downstream for Edouard and somewhat warmer (colder) above the TC core for most Cristobal. Since the upper-level flow vector is roughly stronger (weaker) storms in the sample, but with a large consistent with the shear vector for each of these three degree of variability (Fig. 4b). cases (not shown), this observation is consistent with It is also critical to examine the divergence field in having the strongest convection developing downshear order to gain a more complete understanding of the left. Also apparent is the fact that the two intensifying upper-level anticyclone, since divergence can generate systems, Nadine on 14 September 2012 and Edouard on negative z via the stretching term [Eq. (1), which may 14 September 2014, are both associated with much ultimately act to build or strengthen a preexisting stronger upper-level divergence than either of the two upper-level anticyclone. Upper-level divergence at weakening TCs, despite the fact that these systems are 180 hPa is examined for four individual cases: 70-kt all of similar intensities. HurricaneNadineon14September2012(Fig. 5a), To further investigate the relationship between the 80-kt Hurricane Edouard on 14 September 2014 strength of upper-level divergence and both present (Fig. 5b), 70-kt Hurricane Cristobal on 29 August 2014 intensity and intensity change, data are binned for each (Fig. 5c), and 85-kt Hurricane Edouard on 16 September of four categories: hurricanes (Fig. 6a), tropical storms 2014 (Fig. 5d). These cases are chosen as they allow us to and tropical depressions (Fig. 6b), intensifying TCs

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1346 MONTHLY WEATHER REVIEW VOLUME 145

21 FIG. 3. Tangential wind (shaded, 5 m s increments) and radial wind (positive, contoured; negative, dashed; 2 5ms 1 increments) for 70-kt Hurricane Cristobal on 26 Aug 2014. Analyses are at (a) 180 and (b) 130 hPa in storm- relative coordinates (km from center in the x and y directions). Gray dots indicate locations of dropsondes. Also included are the cold-point tropopause (c) temperature (8C) and (d) pressure (hPa; data interpolated to 5-hPa increments). Magenta text in (c) and (d) indicates values where the cold point is at the top of the sounding and the actual tropopause may be colder and of lower pressure than indicated.

(Fig. 6c), and nonintensifying and weakening TCs displacement of the divergence implies that the con- (Fig. 6d). For each category, 300–100-hPa-layer mean vection is also significantly displaced from the center of upper-level divergence is computed for each case in- circulation, and is consistent with strong westerly or dividually; the results are then averaged over all cases southwesterly shear in a large number of non- in each bin. Note that there are only 6 cases in both the intensifying cases sampled during HS3. hurricanes and intensifying samples (but not the same b. Azimuthally averaged cross sections set of six cases) while there are 10 in both the weak TC and nonintensifying samples. Clearly, hurricanes are Azimuthally averaged cross sections in radius– associated, on average, with stronger upper-level di- pressure coordinates are examined to better elucidate vergence over a larger area close to the circulation the mean structure of the sampled TCs. We begin ex- center (at the origin) than TDs and TSs. The associa- amining the primary circulation by computing compos- tion between upper-level divergence and intensity ites of Vt for hurricanes (Fig. 7a), TDs and TSs (Fig. 7b), change also appears to be robust, as intensifying sys- intensifying TCs (Fig. 7c), and nonintensifying and tems in the HS3 dataset are overall associated with weakening TCs (Fig. 7d). Recall that data are first in- stronger upper-level divergence than weakening or terpolated onto a storm-relative Cartesian grid on steady systems, regardless of current intensity. Addi- pressure levels, then averaged in 25-km-radius bins and tionally, stronger divergence tends to be vertically plotted at the midpoint of each bin. The number of collocated with the center of circulation for in- dropsondes in each bin is indicated in the figures. Data tensifying systems, while it is displaced well to the are only plotted out to 500-km radius, as the HS3 flight northeast in the nonintensifying composite. This patterns became quite asymmetric about the TC at

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1347

FIG. 4. Scatterplot of azimuthal-mean quantities for all cases with a current intensity $30 kt (see Table 1). Included are (a) the cold-point tropopause pressure difference (hPa) between the mean from r 5 0 to 100 km and the mean from r 5 400 to 600 km, with negative values indicating a higher tropopause over the TC center, and (b) the cold-point tropopause temperature difference (8C) between the mean from r 5 0 to 100 km and the mean from r 5 400 to 600 km, with negative values indicating a colder tropopause over the TC center.

larger radii. The mean Vt at the radius of maximum wind Vr. Four representative cases are chosen to dem- winds (RMW) is significantly stronger for hurricanes onstrate both the degree of variability within the HS3 than for TDs and TSs, as expected. More notably, the sample as well as some commonalities across differing RMW is much larger for TDs and TSs than for hurri- scenarios. These cases include steady-state 50-kt TS canes, likely because of the fact that both early de- Nadine on 19 September 2012, intensifying 70-kt Hur- veloping TCs and TCs late in their life cycle undergoing ricane Cristobal on 26 August 2014, intensifying 80-kt extratropical transition tend to be associated with Hurricane Edouard on 14 September 2014, and weak- broader wind fields than mature TCs. While there are ening 85-kt Hurricane Edouard on 16 September 2014. comparatively fewer dropsondes in the two lowest-radii The secondary circulation is evident in all four repre- bins than there are at larger radii in the TD and TS sentative cases, with the strongest inflow at the low composite, a complete lack of inner-core data cannot be levels maximized from 1000 to 950 hPa and the strongest to blame for the unusually large RMW. Negative Vt at outflow between 300 and 100 hPa. However, the mag- upper levels is also notably stronger in the hurricane nitude of the upper-level outflow is notably weaker, sample, associated with a stronger upper-level anticy- particularly at small radii for Cristobal and at all radii for clone. This observation could be due to either the Nadine when compared against the two Edouard cases. anticyclone being stronger or being more vertically For all four cases, vertical gradients in Vr are quite collocated with the TC in the hurricane composite, or sharp, sharper than gradients in Vt in the composites both. When comparing the intensifying to the non- (Fig. 7) or even when comparing individual cases (not 21 intensifying composite Vt, it is apparent that intensifying shown). A small inflow layer of 22to24ms and 20– systems tend to be associated with a smaller RMW, 50 hPa deep is also observed 50–100 hPa above the stronger cyclonic Vt at the RMW, and a deeper vortex. region of maximum outflow in all four cases. It is However, in terms of negative Vt, the strength of the hypothesized that this inflow is associated with dry- upper-level anticyclone appears to be a function of the adiabatic descent above the eye in the lower stratosphere, strength of the vortex itself and not a function of in- resulting in a small reverse secondary circulation in which tensity change. Intensifying cyclones include weaker the primary outflow layer also serves as the ‘‘out’’ com- systems as well as some stronger TCs, so both cyclones ponent of an ‘‘in–down–out’’ circulation pattern. If this and anticyclones are weaker than in the hurricane were the case, one might expect to observe a lowering of composite. Weaker but intensifying TCs are not neces- the tropopause above the eye. However, this result is not sarily in an environment that precludes the development inconsistent with the earlier finding of the tropopause of a stronger anticyclone, but the storm needs time to being elevated above the core of stronger TCs (Fig. 4a), as intensify long enough to realize that anticyclone. it is possible for there to be a net increase in height of the Next, the structure of the secondary circulation is tropopause above the cyclone core along with a simulta- examined by computing the azimuthally averaged radial neous highly localized decrease in the height of the

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1348 MONTHLY WEATHER REVIEW VOLUME 145

21 FIG. 5. Streamline analysis, divergence (s ), and wind barbs (kt) at dropsonde locations at 180 hPa for (a) 70-kt Hurricane Nadine on 14 Sep 2012, (b) 80-kt Hurricane Edouard on 14 Sep 2014, (c) 70-kt Hurricane Cristobal on 29 Aug 2014, and (d) 85-kt Hurricane Edouard on 16 Sep 2014.

tropopause directly above the eye. Warming from adia- Composites of Vr, inertial stability, and G in radius– batic descent above the eye is also consistent with the pressure coordinates for all intensifying (Figs. 8a,b) and earlier finding of having a slightly warmer tropopause nonintensifying and weakening (Figs. 8c,d) TCs are also directly above the TC for stronger storms (Fig. 4b). examined. The level of maximum Vr is fairly consistent Multiple inflow layers are also observed below the between intensifying (Fig. 8a) and nonintensifying 2 outflow channel, generally 22ms 1 or weaker. We (Fig. 8c) TCs, with a peak between 200 and 150 hPa in originally speculated that the rightward deflection of ra- the means. Among the full set of individual cases, there dial outflow by Coriolis would produce a minimum in Vt does not appear to be a meaningful relationship between at the same level as the maximum Vr. Somewhat un- the level of strongest radially averaged positive Vr expectedly, for most cases the level of maximum Vr is (computed as the level at which the strongest radially found to occur 40–80 hPa below the level of minimum Vt. averaged positive Vr from 100 to 500 km occurs) and the The level of maximum Vr is still below the minimum Vt current intensity, regardless of whether this value is even when plotted on theta surfaces (not shown), so this computed as a function of pressure (Fig. 9a)oru result is not simply a function of theta surfaces sloping (Fig. 9c). However, there is an overall slight tendency for downward with increasing radius. It is unclear whether intensifying TCs to have a higher level of peak azi- this discrepancy is due to having a preexisting upper-level muthally averaged outflow compared to weakening TCs, anticyclone prior to TC genesis and/or intensification of a but with very large variability (Fig. 9b). Upper-level synoptic-scale anticyclone that exists above the level of outflow is overall much stronger in the intensifying than maximum outflow, a decrease in height of the level of the nonintensifying composite at virtually all radii: 2 maximum radial outflow with time resulting in a ‘‘rem- 5–7 m s 1 at the maximum for intensifying cases versus 2 nant’’ diabatically generated remnant upper-level anti- 2–4 m s 1 for nonintensifying and weakening cases, cyclone as the outflow descends, or some other process. which would suggest via the continuity equation stronger

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1349

FIG. 6. Composites of 300–100-hPa layer-average divergence in storm-relative coordinates for all cases of (a) hurricane intensity, (b) TS or TD intensity, or for cases that are (c) intensifying or (d) nonintensifying or weakening. The number of cases in each sample n is indicated.

4 vertical motion and thereby a stronger secondary cir- value of this radially averaged Vr. While one might expect culation (Figs. 8a,c). This result is consistent with the the ue of the inflow to be directly proportional to storm earlier finding that most intensifying systems are associ- intensity, the notable differences between these two fig- ated with stronger upper-level divergence than non- ures suggest that this is not always the case. This may have 2 intensifying systems. A layer of inflow 21to22ms 1 is further implications for the TC potential intensity also evident above the outflow in both composites from (Emanuel 1986). 125 to 100 hPa, although subtle differences in the exact The intensifying composite features a sharp layer of height of this feature between various cases cause it to greater static stability at lower levels in the inner core 2 appear to be weaker in the mean than what was observed in a region of ;28–38Ckm 1 lapse rates from 850 to in the individual cases (Fig. 10). Interestingly, this inflow 900 hPa (Fig. 8b). In contrast, the nonintensifying com- is slightly stronger in the nonintensifying composite at posite is associated with a deeper but less well-defined 2 radii $200 km, but is approximately equal between the layer of greater static stability of ;38–48Ckm 1 from two samples within 200 km of the center. 950 to 800 hPa (Fig. 8d). As will be shown in section 3c, A meaningful relationship between the u of the subtle differences between the two stability profiles at low maximum 100–500-km radially averaged Vr and the radii are likely a consequence of differences in the vertical present intensity was not observed (Fig. 9c). However, a extent of the warm core. Note that the top of the outflow notable positive relationship between the u of the out-

flow layer and the ue of the boundary layer inflow, which 4 is a proxy for low-level moisture and SSTs, does occur While ue of the inflow continues to increase with decreasing (Fig. 9d). These values are obtained by first computing radius and should better match the actual values of outflow u, inflow becomes increasingly noisy inside of 100-km radius in the in- azimuthal-mean Vr in radius–u (radius ue) coordinates, dividual cases, with distinct local maxima that occur on ue levels averaging Vr from 100- to 500-km radius, and finding the that differ by as much as 5 K for a single case. Because of large u(ue) level corresponding to the maximum (minimum) uncertainty in the 0–100-km Vr, 100–500-km Vr is depicted instead.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1350 MONTHLY WEATHER REVIEW VOLUME 145

21 21 FIG. 7. Tangential wind (m s ; 2.5 m s increments) as a function of radius (km) and pressure (hPa), computed as an azimuthal mean of the interpolated data in 25-km radius bins, composited for all cases of (a) hurricane intensity, (b) TS or TD intensity, or cases that are (c) intensifying or (d) nonintensifying or weakening. The number of dropsondes in each bin is labeled in magenta. layer (Figs. 8a,c) is constrained in the vertical by the very unstable (I , 0) region in either composite. However, high static stability at the tropopause, indicated by a lapse consistent with Molinari and Vollaro (2014), Vr continues rate that approaches zero. Greater static stability above to increase with radius just below the tropopause as I the outflow is consistent with a sharper vertical gradient in continues to decrease with radius. Unfortunately, the data Vr above rather than below the peak outflow. For the in- coverage at radii $500 km here does not allow us to com- tensifying (nonintensifying) composite, the radial outflow pare the radius of minimum I to the radius of maximum Vr. isfoundtooriginateradiallyjust inside (outside) of the To better conceptualize the mean vertical structure 2 2 region of higher inertial stability values of 1 3 10 4 s 1, and variability of the secondary circulation, the 100– but the bulk of the outflow is primarily outside of this region. 500-km mean Vr is plotted as a function of height with the This region, previously referred to as the outflow roots, is 61s region shaded, comparing hurricanes to TDs and found from approximately 50- to 200-km radius between TSs (Fig. 11a) and intensifying to nonintensifying and 250 and 150 hPa. For larger (smaller) systems, the outflow weakening TCs (Fig. 11b). The statistical significance of roots will be of slightly larger (smaller) radius. In the the difference between the two samples is computed at 21 composites, the radius at which Vr exceeds 1 m s is the 5% level using a two-sample Welch’s t test in 5-hPa greater in the nonintensifying than the intensifying increments. While the mean outflow from 250 to 150 hPa 2 composite (Figs. 8a,c), consistent with a larger mean is 1.5–2 m s 1 stronger for hurricanes than for TDs and RMW (Figs. 7a,c). With higher values of I at lower levels TSs (Fig. 11a) with an overall tendency for stronger TCs or closer to the core, the recirculating wind is too strong for to be associated with stronger outflow (Fig. 11c), the any appreciable Vr to develop, as described by Schubert difference is not statistically significant as a result of the and Hack (1982), Shapiro and Willoughby (1982),and large standard deviation of Vr in this region. The strength Hack and Schubert (1986). We do not find an inertially of the outflow also exhibits a positive relationship with

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1351

21 21 21 FIG. 8. (a),(c) Radial wind (m s ), and (b),(d) lapse rate (8Ckm , shaded) and inertial stability (s , contoured) as a function of radius (km) and pressure (hPa), computed as an azimuthal mean of the interpolated data in 25-km radius bins for all (a),(b) intensifying (see Table 1) and (c),(d) nonintensifying or weakening TCs. The number of dropsondes in each bin is labeled in magenta. the intensity change, but once again with large variability within the dataset, we will first examine the warm core in (Fig. 11d). Nonetheless, intensifying TCs are associated binned radius–pressure azimuthal-mean cross sections 21 with 2–3 m s stronger Vr than nonintensifying TCs in for a subset of six HS3 flights (Fig. 12). These cases in- the 250–150-hPa layer, which is statistically significant clude 50-kt TS Nadine on 19 September 2012, 45-kt TS in the ;160–210-hPa layer (Fig. 11b). One notable outlier Nadine on 22 September 2012, 35-kt TS Humberto on in the scatter of individual cases is 85-kt Hurricane 16 September 2013, 70-kt Hurricane Cristobal on Edouard on 16 September 2014, which exhibited rela- 26 August 2014, 80-kt Hurricane Edouard on 14 Sep- tively strong radial outflow despite having weakened by tember 2014, and 85-kt Hurricane Edouard on 16 Sep- 20 kt throughout the dropsonde release period. There- tember 2014. The potential temperature anomaly is fore, it is clear that present intensity also remains a factor, computed with respect to the mean u profile of the in- but these data suggest that perhaps the intensity change is terpolated data of radius 500 # r # 1500 km in each the more significant of the two. mission.5 For both Nadine flights (Figs. 12a,b), both Edouard missions (Figs. 12e,f), and Cristobal (Fig. 12d), c. Height and structure of the warm core the maximum u anomaly occurs very near 300 hPa. For During HS3, the Global Hawk often made multiple Humberto, there are unfortunately no dropsondes transects over the TC core while releasing dropsondes within a radius of 25 km from the center of the TC. from the lower stratosphere. This sampling allowed for unprecedented sampling of the warm core through the entire depth of the cyclone. To first gain an un- 5 Occasionally during HS3, a few dropsondes were deployed well derstanding of both the commonalities and the vari- outside of the target TC for a variety of reasons, such as to test the ability in the magnitude and structure of the warm core dropsonde system early in the flight.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1352 MONTHLY WEATHER REVIEW VOLUME 145

FIG. 9. Pressure (hPa) of the maximum azimuthal-mean radial wind averaged from 100- to 500-km radius for each case, plotted as a function of (a) intensity (kt) and (b) intensity change. Also included are the mean potential temperature (u, K) of the level of strongest azimuthally averaged outﬂow plotted as a function of (c) intensity (kt), and (d) the mean equivalent potential temperature (ue, K) of the level of strongest 100–500-km radius-mean azimuthally averaged radial inﬂow.

However, the natural-neighbor interpolation allows for issues in the sampling of the environment around estimation of the local temperature maximum based Nadine. On 22 September, Nadine existed in a region of upon the horizontal temperature gradient (Fig. 12c), and significant northwest (NW)-to-southeast (SE) baro- the storm had a broad circulation pattern with a large clinicity, with colder air to the NW where many of the RMW, so a lack of data near r 5 0 km should not be a ‘‘reference’’ dropsondes were deployed. While there is problem. This analysis suggests a distinct double-warm- still evidence of a weak low-level warm core, the low- core structure, with one maximum around 200 hPa and a level warm anomaly associated with Nadine likely re- second maximum around 800 hPa, as well as negative sults from baroclinicity and the asymmetric sampling of u anomalies between these two maxima. However, fur- the environment. Finally, the warm core of Edouard has ther analysis suggests that this pseudo-upper-level warm become stronger at virtually all levels between 14 and core is actually a stratospheric intrusion, and the actual 16 September 2014, with the strength of the warm core upper-level vortex was associated with the observed increasing from 10 to 15 K, primarily as a result of in- cold core. As a result of this hybrid structure, Humberto tensification. The level of the peak warm anomaly re- is not representative of a typical, fully tropical TC. mains unchanged. Warming on 16 September at larger Nadine on 22 September 2012 also features two differ- radii (300–500 km) is likely an effect of the TC also ent maxima in u, with the upper-level warm core the moving northward into an environment with a cooler stronger of the two with a 7-K anomaly versus a 5-K reference profile, which also contributes to some addi- anomaly associated with the lower maximum. Nadine’s tional ‘‘apparent’’ warming of the core. However, this is upper-level warm core is also deeper in the vertical than believed to be a smaller factor than the warming due to the lower warm anomaly. However, there were potential intensification.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1353

21 21 FIG. 10. Radial wind (m s ;1ms increments) for four individual cases as a function of radius (km) and pressure (hPa), computed as an azimuthal mean of the interpolated data in 25-km radius bins. Cases include (a) 50-kt TS Nadine on 19 Sep 2012, (b) 70-kt Hurricane Cristobal on 26 Aug 2014, (c) 80-kt Hurricane Edouard on 14 Sep 2014, and (d) 85-kt Hurricane Edouard on 16 Sep 2014. The number of dropsondes in each bin is labeled in magenta.

Upon examination of the entire HS3 sample of TCs examined. These data originate from a combination of with a present intensity of $30 kt, there is a very evi- 18 dropsondes across HS3 within 25 km of the center of dent positive relationship between the maximum circulation and the interpolated values. Differences warm core temperature anomaly (regardless of height) between the hurricane category and the TD and TS and the intensity (Fig. 13a), consistent with the litera- category are statistically signiﬁcant through a deep layer ture (e.g., Durden 2013). The pressure at which the from just above 600 hPa through slightly above 200 hPa maximum temperature anomaly occurs does not ap- (Fig. 14a). The hurricane warm core was, on average, pear to be strongly correlated with intensity (Fig. 13c). approximately 4–5 K warmer than the mean for TDs and However, while having a warm core above 500 hPa TSs in the HS3 sample. The greatest variability in the is equally likely as having one below 500 hPa for strength of the warm core for weaker systems occurs at TCs # 45 kt, all but one of the TCs in the sample .45 kt and just below the level of the greatest warm anomaly, have a warm core between 350 and 250 hPa. Neither with a secondary peak in variability at lower levels from the magnitude (Fig. 13b) nor the pressure (Fig. 13d) 950 to 800 hPa and less variability at middle levels. The of the warm core shows any meaningful correlation larger standard deviation observed at low levels appears with the intensity change. No evidence was found to be associated with some TDs and TSs having a sec- for a relationship between the presence of a single ondary low-level warm core, while others do not have versus a double warm core and storm intensity or this feature at all. For hurricanes, the standard deviation intensity change. is fairly uniform throughout the column but greatest Finally, the 0–25-km radial-mean temperature anom- from approximately 850 to 550 hPa. In this region, the aly and standard deviation as a function of pressure is larger variance appears to be a function of how far

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1354 MONTHLY WEATHER REVIEW VOLUME 145

FIG. 11. Vertical profile of azimuthal-mean radial wind as a function of pressure (hPa) averaged from 100- to 500-km radius comparing (a) hurricanes (blue) to TDs and TSs (red), and (b) intensifying (green) to nonintensifying and weakening (red) TCs. The solid line denotes the mean, shading indicates the 61 standard deviation region, and small circles denote values in which the difference between the two subsets of data is significant at the 5% level using atwo-sampleWelch’st test. Also included are scatterplots of mean radial wind in the 250–150-hPa layer averaged from 100- to 500-km radius for each case, plotted as a function of (c) intensity (kt) and (d) intensity change (kt). downward the primary warm core extends below the strength, height, and structure of the TC outflow, and maximum. While there is much greater variability in the how both the commonalities and the variability within height and strength of the warm core for nonintensifying the sample relate to either the current TC intensity or and weakening TCs than there is for strengthening TCs, the intensity change. The height and structure of the the differences in the means are not statistically signifi- warm core and its relationship to intensity and intensity cant at any level (Fig. 14b). Note, however, that there change are also examined. is a greater downward extension of the primary warm Overall, hurricanes in the HS3 sample tend to be as- core in the nonintensifying than in the intensifying sociated with stronger upper-level divergence than TDs composite. This structure would result in the greater and TSs. Perhaps more intriguingly, stronger upper-level static stability observed in the nonintensifying compos- divergence is found for intensifying rather than non- ite near 800 hPa in section 3b. intensifying and weakening systems regardless of current intensity. The divergence also tends to be more concen- trated close to the center of circulation for intensifying 4. Discussion and conclusions rather than weakening TCs. Similarly, azimuthal-mean In this study, storm-relative constant pressure, radius–pressure cross sections reveal stronger radial wind azimuthal-mean radius height, and three-dimensional in the outflow region for intensifying than nonintensifying analyses of 16 HS3 missions investigating six different TCs, with statistical significance, throughout the 250– TCs are examined. This unique dataset has made it 150-hPa layer. This finding is consistent with the results of possible to develop unprecedented analyses of both TC Merrill (1988a). A weaker but nonetheless positive re- outflow and the warm core structure entirely from lationship between the strength of the outflow and pres- dropsonde data. The primary focus of this study is on the ent storm intensity was also observed. The strongest

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1355

FIG. 12. Potential temperature (K, contoured) and anomalies (K, shaded) with respect to the interpolated analysis of radius 500 # r # 1500 km, as a function of radius (km) and pressure (hPa), computed as an azimuthal mean of the interpolated data in 25-km radius bins. Cases include (a) 50-kt TS Nadine on 19 Sep 2012, (b) 45-kt TS Nadine on 22 Sep 2012, (c) 35-kt TS Humberto on 16 Sep 2013, (d) 70-kt Hurricane Cristobal on 26 Aug 2014, (e) 80-kt Hurricane Edouard on 14 Sep 2014, and (f) 85-kt Hurricane Edouard on 16 Sep 2014. The number of dropsondes in each bin is labeled in magenta. outflow is generally found to occur from 50 to 100 hPa higher levels. Additionally, while the u of the outflow below the tropopause, with virtually all the outflow oc- does not appear to be strongly tied to the intensity, the curring within 150 hPa of the tropopause. The pressure at u of the outflow exhibits a positive relationship with the which the outflow maximum occurs is also found to be mean ue of the level of strongest low-level inflow. This loosely correlated with intensity change, where in- result is somewhat unexpected, as the low-level ue is ex- tensifying systems often have their strongest outflow at pected to be roughly proportional to TC intensity.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1356 MONTHLY WEATHER REVIEW VOLUME 145

FIG. 13. Scatterplots of (a),(b) the warm core temperature anomaly (8C) and (c),(d) the pressure at which the maximum temperature anomaly occurs (hPa) vs (a),(c) the current storm intensity (kt) and (b),(d) the 12-h intensity change (kt). The warm core is computed as the difference between the radial average of the horizontally interpolated data between 0 and 25 km and radius 500 # r # 1500 km.

Both Vr and Vt are associated with sharp vertical isentropic outflow rising to lower pressure levels with gradients in the outflow layer, although the gradient is increasing radius, and deflection to the right due to found to be sharper for Vr than Vt, especially above the Coriolis, although this effect does not appear to be suf- level of maximum Vr. This result is believed to be a ficient to fully explain the discrepancy. Finally, the consequence of the greater static stability at the tropo- strength of the upper-level anticyclone does not appear pause above the outflow region. The level of minimum to be a discriminating factor between intensifying and Vt associated with the upper-level anticyclone is also nonintensifying TCs. found to be notably (;40–80 hPa) above the level of In this study, an attempt has been made to quantify a maximum Vr, suggesting some disconnect between the range of pressure levels and a range of radii from which height of the upper-level anticyclone at large radius and the outflow originates. This outflow roots region was divergence near the outflow roots at small radius. This also quantified in terms of static and inertial stability. Of finding could possibly be due to the presence of a pre- course, the interaction between the radial and tangential existing upper-level anticyclone above the primary flow is a complex issue. Positive Vr tends to be associated outflow layer collocated with the T; a lowering of the with divergence above the TC, which in turn generates level of maximum radial outflow with time, which negative vorticity. On the other hand, stronger swirling leaves a remnant diabatically induced upper-level anti- winds associated with greater inertial stability should cyclone above; or some combination of the two. Since delay the outward acceleration of air parcels until they u surfaces slope upward with increasing radius at the reach sufficient distance from the primary circulation. upper levels away from the TC for most cases (Fig. 12), This effect is why regions of reduced inertial stability in part of this phenomenon could also be the result of the environment of the TC tend to promote enhanced

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1357

FIG. 14. Potential temperature (K) anomalies (solid lines) and 61 standard deviation region (shaded) of the inner-most 0–25-km radius bin as a function of pressure (hPa), comparing (a) hurricanes (blue) to TDs and TSs (red), and (b) intensifying (green) to nonintensifying and weakening (red) TCs. Small circles denote values in which the difference between the two subsets of data is significant at the 5% level using a two-sample Welch’s t test. outflow in the direction of the inertial stability mini- intensifying, if not rapidly intensifying, TCs, and not mum (e.g., Rappin et al. 2011). Therefore, it is neces- steady-state or weakening TCs. sary to quantify the inertial stability to adequately It is unclear from this dataset whether multiple out- describe the secondary circulation as a whole. Consis- flow channels are correlated with TC intensification. tent with Molinari and Vollaro (2014),greaterVr tends The only case of dual outflow channels developing for an to occur at higher levels (but below the tropopause) intensifying TC during HS3 occurred with Hurricane and larger radii where values of I are lower. From these Cristobal (2014); however, the first flight into Cristobal data, the outflow roots region in which the azimuthal- occurred near the end of the intensification phase. Ad- 21 mean Vr exceeds 1 m s occurs for I values close to or ditionally, six other cases of intensification in the dataset 2 2 slightly above 1 3 10 4 s 1, which exists in a region occurred in the presence of only a single outflow chan- from 300 to 150 hPa and from 50- to 200-km radius in nel. The northern outflow channel for Cristobal is at its the HS3 mean. strongest at 180 hPa, while the southern outflow channel One or more inflow layers above and below the outflow peaks near 130 hPa. This discrepancy was originally channel are also observed in several cases, with peak hypothesized to be associated with a downward-sloping 21 negative Vr of 22to24ms over a shallow layer 20– tropopause with latitude, but while the tropopause cools 50 hPa deep. The highest observed inflow occurs 50– from north to south in the environment surrounding 100 hPa above the level of strongest outflow. This inflow Cristobal, the tropopause is not correspondingly higher is believed to be associated with a ‘‘reverse’’ secondary south of the TC. However, the tropopause was found to circulation pattern, associated with dry-adiabatic descent be systematically higher above the TC for five out of six above the eye in the lower stratosphere. This upper-level TCs of 65-kt intensity or stronger, with a tropopause inflow layer is also evident in both intensifying and non- pressure 12 hPa lower than the surrounding environ- intensifying composites from 125 to 100 hPa, although ment above 85-kt Hurricane Edouard on 16 September subtle differences in the exact height of this feature be- 2014, the strongest TC in the sample. tween various cases cause it to appear to be weaker in the Finally, stronger TCs are found to be associated mean than what was observed in the individual cases. with stronger warm cores, with a maximum u anomaly Interestingly, this inflow is slightly stronger in the non- of .14 K at 260 hPa for 85-kt Hurricane Edouard intensifying composite at radii $200 km, but is approxi- and a warm anomaly of at least 6.5 K for all TCs mately equal between the two samples within 100 km of of $65 kt. The height of the primary warm core was the center. This finding is generally inconsistent with the found to occur between 250 and 350 hPa for more than idea that upper-tropospheric (Heymsfield et al. 2001)or half of the cases, but was found as low as 850–700 hPa lower-stratospheric (Ohno and Satoh 2015; Miller et al. for a few weaker systems. Stronger TCs were more 2015) descent radially inward of deep convective bursts consistently associated with higher warm cores than along the eyewall should be more characteristic of weaker TCs, in agreement with Durden (2013) and

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC 1358 MONTHLY WEATHER REVIEW VOLUME 145

Ohno and Satoh (2015), although several TDs and TSs REFERENCES also had warm cores at or above 300 hPa. The fact that Aberson, S. D., M. L. Black, R. A. Black, R. W. Burpee, J. J. Cione, the warm core occurred at 350 hPa or above for all C. W. Landsea, and F. D. Marks Jr., 2006: Thirty years systems of 70-kt intensity or above is somewhat in of tropical cyclone research with the NOAA P-3 aircraft. disagreement with the midlevel warm core suggested Bull. Amer. Meteor. Soc., 87, 1039–1055, doi:10.1175/ by Stern and Nolan (2012).WhileHalverson et al. BAMS-87-8-1039. Alaka, M. A., 1961: The occurrence of anomalous winds and (2006) suggest that the TC weakening phase may be their significance. Mon. Wea. Rev., 89, 482–494, doi:10.1175/ associated with a lowering of the warm core, we do not 1520-0493(1961)089,0482:TOOAWA.2.0.CO;2. find a relationship between the height (or strength) of Braun, S. A., and Coauthors, 2013: NASA’s Genesis and Rapid the warm core and the intensity change. Several TCs Intensification Processes (GRIP) field experiment. Bull. Amer. exhibited double-warm-core structures with a large Meteor. Soc., 94, 345–363, doi:10.1175/BAMS-D-11-00232.1. degree of variability. TS Nadine on 22 September ——, P. A. Newman, and G. M. Heymsfield, 2016: NASA’s Hurricane and Severe Storm Sentinel (HS3) investigation. 2012 featured a weaker secondary warm core around Bull. Amer. Meteor. Soc., 97, 2085–2102, doi:10.1175/ 850–900 hPa. TS Humberto was associated with a cold- BAMS-D-15-00186.1. core-over-warm-core hybrid structure, with the pri- Chen, H., and D.-L. Zhang, 2013: On the rapid intensification of mary warm core around 800 hPa. An upper-level Hurricane Wilma (2005). Part II: Convective bursts and the warm anomaly at 200 hPa of comparable magnitude upper-level warm core. J. Atmos. Sci., 70, 146–162, doi:10.1175/ JAS-D-12-062.1. to the low-level warm core was also observed for Davis, C. A., and D. A. Ahijevych, 2012: Mesoscale structural Humberto. However, this feature appears to be asso- evolution of three tropical weather systems observed during ciated with a lowering of the stratosphere and not a PREDICT. J. Atmos. Sci., 69, 1284–1305, doi:10.1175/ tropospheric warm core. Finally, the warm core as- JAS-D-11-0225.1. sociated with Edouard strengthened at virtually all ——, and ——, 2013: Thermodynamic environments of deep con- levels between 14 September 2014 and 16 September vection in Atlantic tropical disturbances. J. Atmos. Sci., 70, 1912–1928, doi:10.1175/JAS-D-12-0278.1. 2016 as the TC first strengthened from 80 to 105 kt, Dolling, K., and G. M. Barnes, 2012: Warm-core formation in then weakened to 85 kt. The level of the warm core Tropical Storm Humberto (2001). Mon. Wea. Rev., 140, 1177– remained steady close to 300 hPa and the overall 1190, doi:10.1175/MWR-D-11-00183.1. structure did not change during this period. Dunion, J. P., 2011: Rewriting the climatology of the tropical North Overall, these results suggest that there is significant Atlantic and Caribbean Sea atmosphere. J. Climate, 24, 893– 908, doi:10.1175/2010JCLI3496.1. variability as a function of height as well as significant Durden, S. L., 2013: Observed tropical cyclone eye thermal case-to-case variability in terms of the structure of the anomaly profiles extending above 300 hPa. Mon. Wea. Rev., tropical cyclone outflow layer and the height and mag- 141, 4256–4268, doi:10.1175/MWR-D-13-00021.1. nitude of the warm core. High-altitude dropsonde ob- Emanuel, K. A., 1986: An air–sea interaction theory for tropical cy- servations from HS3 made it possible to observe and clones. Part I: Steady-state maintenance. J. Atmos. Sci., 43,585– , . document these features in greater detail and over a 605, doi:10.1175/1520-0469(1986)043 0585:AASITF 2.0.CO;2. Hack, J. J., and W. H. Schubert, 1986: Nonlinear response of greater number of cases than ever before. As always, the atmospheric vortices to heating by organized cumulus results presented in this paper should be reevaluated as convection. J. Atmos. Sci., 43, 1559–1573, doi:10.1175/ additional in situ data in the upper levels of the TC be- 1520-0469(1986)043,1559:NROAVT.2.0.CO;2. come available. Halverson, J. B., J. Simpson, G. Heymsfield, H. Pierce, T. Hock, and L. Ritchie, 2006: Warm core structure of Hurricane Erin diagnosed from high altitude dropsondes during CAMEX-4. Acknowledgments. We thank NASA and the HS3 J. Atmos. Sci., 63, 309–324, doi:10.1175/JAS3596.1. leadership, Scott A. Braun and Paul A. Newman, as well Hawkins, H. F., and D. T. Rubsam, 1968: Hurricane Hilda, 1964. II: as the entire HS3 team. We would also like to thank Structure and budgets of the hurricane on October 1, 1964. Mon. Greg Tripoli, Dan Stern, Dave Ryglicki, Jon Moskaitis, Wea. Rev., 96, 617–636, doi:10.1175/1520-0493(1968)096,0617: . and two anonymous reviewers for their helpful com- HH 2.0.CO;2. ——, and S. M. Imbembo, 1976: The structure of a small, intense ments on this work. This research was supported by hurricane—Inez 1966. Mon. Wea. Rev., 104, 418–442, the NRL’s Karle fellowship program, the Chief of doi:10.1175/1520-0493(1976)104,0418:TSOASI.2.0.CO;2. Naval Research through the NRL Base Program, PE Helms, C. N., and R. E. Hart, 2013: A polygon-based line integral 0601153N, and the Office of Naval Research as part of method for calculating vorticity, divergence, and deformation the Tropical Cyclone Intensity Departmental Research from nonuniform observations. J. Appl. Meteor. Climatol., 52, Initiative (TCI-DRI). We acknowledge computational 1511–1521, doi:10.1175/JAMC-D-12-0248.1. ——, and ——, 2015: The evolution of dropsonde-derived kine- support from a grant of High Performance Computing matic and thermodynamic structures in developing and non- (HPC) time from the Navy Defense Resource Center developing Atlantic tropical convective systems. Mon. Wea. (DSRC) at Stennis, Mississippi. Rev., 143, 3109–3135, doi:10.1175/MWR-D-14-00242.1.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC APRIL 2017 K O M A R O M I A N D D O Y L E 1359

Heymsfield, G. M., J. B. Halverson, J. Simpson, L. Tian, and T. P. Wea. Rev., 130, 180–187, doi:10.1175/1520-0493(2002)130,0180: Bui, 2001: ER-2 Doppler radar investigations of the eyewall APUTWA.2.0.CO;2. of Hurricane Bonnie during the Convection and Moisture ——, P. D. Reasor, and J. A. Zhang, 2015: Multiscale structure Experiment-3. J. Appl. Meteor., 40, 1310–1330, doi:10.1175/ and evolution of Hurricane Earl (2010) during rapid in- 1520-0450(2001)040,1310:EDRIOT.2.0.CO;2. tensification. Mon. Wea. Rev., 143, 536–562, doi:10.1175/ Holland, G. J., and R. T. Merrill, 1984: On the dynamics of tropical MWR-D-14-00175.1. cyclone structural changes. Quart. J. Roy. Meteor. Soc., 110, Rozoff, C. M., D. S. Nolan, J. P. Kossin, F. Zhang, and J. Fang, 723–745, doi:10.1002/qj.49711046510. 2012: The roles of an expanding wind field and inertial stability Holton, J. R., 2004: An Introduction to Dynamic Meteorology. in tropical cyclone secondary eyewall formation. J. Atmos. 4th ed. Elsevier Academic, 535 pp. Sci., 69, 2621–2643, doi:10.1175/JAS-D-11-0326.1. Knox, J. A., 1997: Generalized nonlinear balance criteria and in- Ryglicki, D. R., and D. Hodyss, 2016: A deeper analysis of center- ertial stability. J. Atmos. Sci., 54, 967–985, doi:10.1175/ finding techniques for tropical cyclones in mesoscale models. 1520-0469(1997)054,0967:GNBCAI.2.0.CO;2. Part I: Low wavenumber analysis. J. Appl. Meteor. Climatol., Landsea, C. W., and J. L. Franklin, 2013: Atlantic hurricane 55, 531–559, doi:10.1175/JAMC-D-15-0125.1. database uncertainty and presentation of a new database Sadler, J. C., 1976: A role of the tropical upper tropospheric format. Mon. Wea. Rev., 141, 3576–3592, doi:10.1175/ trough in early season typhoon development. Mon. Wea. MWR-D-12-00254.1. Rev., 104, 1266–1278, doi:10.1175/1520-0493(1976)104,1266: LaSeur,N.E.,andH.F.Hawkins,1963:AnanalysisofHurricaneCleo AROTTU.2.0.CO;2. (1958) based on data from research reconnaissance aircraft. Mon. ——, 1978: Mid-season typhoon development and intensity Wea. Rev., 91, 694–709, doi:10.1175/1520-0493(1963)091,0694: changes and the tropical upper tropospheric trough. Mon. Wea. AAOHCB.2.3.CO;2. Rev., 106, 1137–1152, doi:10.1175/1520-0493(1978)106,1137: Malkus, J. S., and H. Riehl, 1960: On the dynamics and energy MSTDAI.2.0.CO;2. transformations in steady-state hurricanes. Tellus, 12, 1–20, Schubert, W. H., and J. J. Hack, 1982: Inertial stability and tropical doi:10.3402/tellusa.v12i1.9351. cyclone development. J. Atmos. Sci., 39, 1687–1697, doi:10.1175/ Marks, F. D., Jr., R. A. Houze Jr., and J. F. Gamache, 1992: Dual- 1520-0469(1982)039,1687:ISATCD.2.0.CO;2. aircraft investigation of the inner core of Hurricane Norbert. Shapiro, L. J., and H. E. Willoughby, 1982: The response of balanced Part I: Kinematic structure. J. Atmos. Sci., 49, 919–942, hurricanes to local sources of heat and momentum. J. Atmos. doi:10.1175/1520-0469(1992)049,0919:DAIOTI.2.0.CO;2. Sci., 39, 378–394, doi:10.1175/1520-0469(1982)039,0378: Merrill, R. T., 1988a: Environmental influences on hurricane in- TROBHT.2.0.CO;2. tensification. J. Atmos. Sci., 45, 1678–1687, doi:10.1175/ Sibson, R., 1981: A brief description of natural neighbor in- 1520-0469(1988)045,1678:EIOHI.2.0.CO;2. terpolation. Interpreting Multivariate Data, V. Barnett, Ed., ——, 1988b: Characteristics of the upper-tropospheric environ- John Wiley and Sons, 21–36. mental flow around hurricanes. J. Atmos. Sci., 45, 1665–1677, Stern, D. P., and D. S. Nolan, 2012: On the height of the warm core doi:10.1175/1520-0469(1988)045,1665:COTUTE.2.0.CO;2. in tropical cyclones. J. Atmos. Sci., 69, 1657–1680, doi:10.1175/ ——, and C. S. Velden, 1996: A three-dimensional analysis of JAS-D-11-010.1. the outflow layer of Supertyphoon Flo (1990). Mon. Wea. Titley, D. W., and R. L. Elsberry, 2000: Large intensity changes in Rev., 124, 47–63, doi:10.1175/1520-0493(1996)124,0047: tropical cyclones: A case study of Supertyphoon Flo during ATDAOT.2.0.CO;2. TCM-90. Mon. Wea. Rev., 128, 3556–3573, doi:10.1175/ Miller, W., H. Chen, and D. L. Zhang, 2015: On the in- 1520-0493(2000)128,3556:LICITC.2.0.CO;2. tensification of Hurricane Wilma (2005). Part III: Effects Welch, B. L., 1947: The generalization of ‘‘student’s’’ problem of latent heat of fusion. J. Atmos. Sci., 72, 3829–3849, when several different population variances are involved. doi:10.1175/JAS-D-14-0386.1. Biometrika, 34, 28–35. Molinari, J., and D. Vollaro, 2014: Symmetric instability in the Wick, G., 2015: Hurricane and Severe Storm Sentinel (HS3) outflow layer of a major hurricane. J. Atmos. Sci., 71, 3739– Global Hawk AVAPS dropsonde system. NASA Global 3746, doi:10.1175/JAS-D-14-0117.1. Hydrology Resource Center DAAC, Huntsville, AL, accessed Ohno, T., and M. Satoh, 2015: On the warm core of a tropical cy- 10 August 2016, doi:10.5067/HS3/AVAPS/DROPSONDE/ clone formed near the tropopause. J. Atmos. Sci., 72, 551–571, DATA201. doi:10.1175/JAS-D-14-0078.1. Willoughby, H. E., 1992: Linear motion of a shallow-water Rappin, E. D., M. C. Morgan, and G. J. Tripoli, 2011: The impact barotropic vortex as an initial-value problem. J. Atmos. of outflow environment on tropical cyclone intensification Sci., 49, 2015–2031, doi:10.1175/1520-0469(1992)049,2015: and structure. J. Atmos. Sci., 68, 177–194, doi:10.1175/ LMOASW.2.0.CO;2. 2009JAS2970.1. Zawislak, J., and E. J. Zipser, 2014: A multisatellite investigation of Rogers, R., S. Aberson, J. Kaplan, and S. Goldenberg, 2002: A pro- the convective properties of developing and nondeveloping nounced upper-tropospheric warm anomaly encountered by the tropical disturbances. Mon. Wea. Rev., 142, 4624–4645, NOAA G-IV aircraft in the vicinity of deep convection. Mon. doi:10.1175/MWR-D-14-00028.1.

Unauthenticated | Downloaded 10/05/21 12:12 PM UTC