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Subsidence Warming as an Underappreciated Ingredient in Tropical . Part I: Aircraft Observations

BRANDON W. KERNS AND SHUYI S. CHEN Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

(Manuscript received 15 December 2014, in final form 15 July 2015)

ABSTRACT

The development of a compact warm core extending from the mid-upper levels to the lower troposphere and related surface pressure falls leading to (TC genesis) is not well understood. This study documents the evolution of the three-dimensional thermal structure during the early developing stages of Fanapi and Megi using aircraft dropsonde observations from the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign in 2010. Prior to TC genesis, the precursor disturbances were characterized by warm (cool) anomalies above (below) the melting level (;550 hPa) with small surface pressure perturbations. Onion-shaped skew T–logp profiles, which are a known signature of mesoscale subsidence warming induced by organized mesoscale convective systems (MCSs), are ubiquitous throughout the ITOP aircraft missions from the precursor disturbance to the tropical stages. The warming partially erodes the lower-troposphere (850–600 hPa) cool anomalies. This warming results in increased surface pressure falls when superposed with the upper-troposphere warm anomalies associated with the long-lasting MCSs/cloud clusters. Hydrostatic pressure analysis suggests the upper-level warming alone would not result in the initial sea level pressure drop associated with the transformation from a disturbance to a TC. As Fanapi and Megi intensify into strong tropical , aircraft flight-level (700 hPa) and dropsonde data reveal that the warm core extends down to 850–600 hPa and has some characteristics of subsidence warming similar to the eyes of mature TCs.

1. Introduction low vertical of the horizontal wind, enhanced cyclonic circulation and moisture, sufficient Coriolis One of the defining characteristics of a tropical cy- force, and a warm ocean (Gray 1968; McBride and Zehr clone (TC) is its warm core. The development of a deep- 1981; DeMaria et al. 2001). These conditions are often troposphere warm temperature anomaly and low-level associated with the development of numerous long- cyclonic circulation leading to tropical cyclogenesis (TC lasting MCSs, especially in the presence of preexisting genesis) has been the subject of many previous studies. tropical disturbances like easterly waves and monsoon Observational and modeling studies have shown that troughs (Gray 1998). Nearly all TC genesis cases over organized mesoscale convective systems (MCSs) can the western North Pacific are associated with mesoscale lead to the development of a mid-upper-troposphere cloud clusters (with cloud-top temperature , 208 K) (;200–500 hPa) warm anomaly and a midlevel (;500– lasting at least 8 h or longer (e.g., 8-h clusters; Kerns and 600 hPa) vorticity maximum (Chen and Frank 1993; Chen 2013). These long-lasting clusters represent large, Fritsch et al. 1994). However, there is less agreement organized MCSs with extensive regions of stratiform regarding the factors that contribute to the development precipitation (Chen et al. 1996). Observations of these of warming in the lower troposphere (;600–1000 hPa). MCSs have shown enhanced midlevel (;500–600 hPa) Large-scale environment conditions that are condu- vorticity within the stratiform region in midlatitudes cive to TC development include (but are not limited to) during summertime (Brandes 1990) and in the tropics (Bousquet and Chong 2000). Numerical modeling studies have provided some ex- Corresponding author address: Dr. Brandon Kerns, Rosenstiel School of Marine and Atmospheric Science, University of Miami, planations for the development of the midlevel vortex 4600 Rickenbacker Causeway, Miami, FL 33149. and mid-upper-troposphere warming within MCSs. E-mail: [email protected] Chen and Frank (1993) have shown that the mesoscale

DOI: 10.1175/JAS-D-14-0366.1

Ó 2015 American Meteorological Society 4238 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 1. A schematic depiction of a mature midlatitude MCS, following Chen and Frank (1993). The mid-upper-level warm anomaly (black W) and vorticity center (black V), as well as the lower-troposphere adiabatic subsidence warming (red W) are indicated. The skew T–logp diagram in Fig. 13 of Chen and Frank is overlaid with the rear inflow jet, which shows the onion thermodynamic profile in the subsiding region. cyclogenesis is largely due to warming in the saturated thermodynamic structure reflects the collective contri- stratiform region of MCSs enhanced by inertial stability bution from latent heating and evaporative cooling in (Schubert and Hack 1982). Others have shown that the multiple long-lasting MCSs within the large-scale distur- lower-troposphere cooling is related to evaporative bances (Ritchie and Holland 1997; Simpson et al. 1997). cooling of precipitation (Mapes and Houze 1995; Bister The question of how a deep warm core develops and Emanuel 1997; Schumacher et al. 2004). Based on a during TC genesis remains unanswered. Observational numerical modeling experiment, Bister and Emanuel and numerical modeling studies of tropical and mid- (1997) proposed that an intense, long-lived, mesoscale latitude MCSs provide some possible explanations for ‘‘showerhead’’ can produce a low-level cold core prior lower-troposphere warming. Chen and Frank (1993) to TC genesis. They proposed that the low-level cooling described the development of a associated could lead to enhanced air–sea fluxes that can, in turn, with lower-troposphere subsidence warming on the edge eliminate the low-level cool anomaly from below, which of a saturated region where a downward development would favor subsequent deep . However, of a mesovortex with a deep warm core occurred in a there have been few observations during the early TC numerical simulation of mesoscale cyclogenesis. Wake life cycle to evaluate this hypothesis so far. lows were also observed previously in MCSs (Zipser Similar to the temperature anomalies within MCSs, the 1977; Johnson and Nicholls 1983; Johnson and Hamilton upper-troposphere warm and lower-troposphere cool 1988; Johnson et al. 1989). A schematic based on the anomalies have been observed in many tropical distur- model simulation from Chen and Frank (1993) is shown bances (Reed and Recker 1971; Reed et al. 1977; in Fig. 1. In organized MCSs, subsidence warming is McBride 1981; Bessho et al. 2010; Komaromi 2013; often related to mesoscale inflow jets and mesoscale Zawislak and Zipser 2014). To some extent, this observed subsidence regions beneath the melting level, both of

TABLE 1. Summary of the aircraft missions and dropsondes used in this study. Storm intensity is indicated as precursor disturbance (DB), TD, or TS.

Storm/mission Storm intensity Flight level (hPa) First–last dropsonde Sondes Fanapi/F1 DB (15 kt) 350 1916 UTC 12 Sep–0253 UTC 13 Sep 31 Fanapi/F2 DB/TD (20–25 kt) 350 2137 UTC 13 Sep–0240 UTC 14 Sep 21 Fanapi/F3 TD/TS (30–35 kt) 350 2055 UTC 14 Sep–0146 UTC 15 Sep 26 Fanapi/F4 TS (55 kt) 700 2234 UTC 15 Sep–0353 UTC 16 Sep 26 Megi/M1 TD/TS (30–35 kt) 350 1840 UTC 12 Oct–0052 UTC 13 Oct 30 Megi/M2 TS (55 kt) 700 0006–0452 UTC 14 Oct 27 NOVEMBER 2015 K E R N S A N D C H E N 4239

FIG. 2. Storm tracks and dropsonde locations for (a) Fanapi and (b) . The JTWC best track is indicated by the black line with circles for every 6 h. Best track starts at 1200 UTC 14 Sep 2010 for Fanapi and 1800 UTC 12 Oct for Megi. JTWC invest locations are indicated by I every 6 h, starting at 0600 UTC 13 Sep for Fanapi and 1200 UTC 11 Oct for Megi. The location of is indicated by G. which result in characteristic onion-shaped profiles on hydrometeors fall from the saturated anvil cloud into thermodynamic diagrams (Zipser 1977; Smull and Houze subsaturated air, they partially evaporate. Initially, this 1987; Houze 1977; Leary 1984; Johnson and Hamilton evaporative cooling makes the air parcel negatively 1988; Johnson et al. 1989), which are also illustrated in the buoyant. However, in light precipitation, the evapora- inset on the left side of Fig. 1. Rear inflow jets are often tive cooling cannot keep the air parcel saturated and, found on the trailing/rear edges of strong, mature squall- as a result, it warms during the descent because of adi- line systems (Bousquet and Chong 2000; Smull and Houze abatic compression. The adiabatic compression over- 1987). However, such strong squall lines are relatively less comes the evaporative cooling, resulting in net warming common in the tropics. and drying beneath the anvil cloud—the distinctive Subsidence warming can still occur in MCSs without onion-shaped profile on thermodynamic diagrams strong inflow jets. Another commonly observed feature (Zipser 1977; Leary 1980; Jorgensen et al. 1991; Brandes is an unsaturated mesoscale downdraft beneath the and Ziegler 1993; Correia and Arritt 2008). Aircraft data melting layer in the stratiform region of MCSs. As from GATE show that evaporative cooling of rainfall

FIG. 3. Infrared satellite image (color shading) with 8-h cloud cluster tracks for (a) Fanapi and (b) Megi. In (a), the IR image is for 0200 UTC 14 Sep during F2, and the cluster tracks are from 1100 UTC 13 Sep to 0000 UTC 17 Sep. In (b), the IR image is for 0200 UTC 13 Oct during M1, and the cluster tracks are from 1000 UTC 11 Oct to 0400 UTC 15 Oct. White circles are hourly cloud cluster locations, and the circle sizes are proportional to the 208-K IR area. Cloud cluster centroid tracks are drawn in magenta. Magenta dots are the initial locations of the 8-h clusters. 4240 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 4. (a),(c),(e),(g) IR brightness temperature (color shading), flight track, and dropsonde locations (triangles) for the four missions into Typhoon Fanapi. The satellite image valid time is indicated, and the portion of the flight track within a half hour of the satellite image is indicated by the thick black line. (b),(d),(f),(h) GPS dropsonde winds at 850 (blue), 700 (green), and 500 hPa (red) for the four missions into Typhoon Fanapi. Wind barb pennants, barbs, and half barbs represent 50, 10, and 5 kt, respectively. Colored dots indicate calm wind speeds below 2.5 kt. Contours in (b),(d), and (f) are the 900-hPa heights every 5 m and labeled every 10 m; in (h) 900-hPa height contours are every 10 m and labeled every 50 m. and melting in the melting layer induce a nearly uniform Unsaturated mesoscale downdrafts, inflow jets, and area of descent in stratiform areas (Leary and related subsidence warming in MCSs have been well Houze 1979; Leary 1980). Using a numerical model, documented, but their contribution to TC genesis has Brown (1979) determines that this evaporative cooling not been investigated. While subsidence warming is a 2 could induce ;10 cm s 1 of descent in the stratiform rain well-known feature of mature TCs (Malkus 1958; La area, similar to the observation by Gamache and Houze Seur and Hawkins 1963; Halverson et al. 2006), how (1982). In the Bow Echo and Mesoscale Convective early in the TC life cycle these features develop is not 2 Vortex Experiment (BAMEX) descent of 10–50 cm s 1 well known. Stossmeister and Barnes (1992) showed (several meters per second in strong cases) is found in that, in Tropical Storm Isabel (1985), a new circulation the trailing stratiform region (Correia and Arritt 2008). center developed in a relatively warm, dry area beneath Other observational studies have confirmed the pre- the downwind cloud anvil outside of the radius of dominance of mesoscale unsaturated downdrafts in the maximum winds and separate from the strongest con- stratiform rain areas of tropical (Jorgensen et al. 1991) vection. They hypothesized the role of subsidence and midlatitude summer (Brandes 1990; Brandes and warming similar to mesolows. Simpson et al. (1998) find Ziegler 1993; Biggerstaff and Houze 1991) MCSs. that, for Hurricane Daisy (1958) in the Atlantic and NOVEMBER 2015 K E R N S A N D C H E N 4241

FIG.4.(Continued)

Typhoon Oliver (1993) in the Coral Sea, partial eyewalls analysis shows that there is often a relative minimum in developed near the subsidence region adjacent to the relative humidity in the low levels within the large-scale large MCSs near the center. Heymsfield et al. (2006) moist disturbance envelope leading up to TC genesis point out that mesoscale subsidence was present at 700– (Komaromi 2013; Davis and Ahijevych 2013; Zawislak 500 hPa over the exposed circulation center of highly and Zipser 2014), which is consistent with subsidence- sheared Atlantic Tropical Storm Chantal (2001). More induced warming in the lower troposphere. recently, Dolling and Barnes (2012) describe the warm- In this study, we use airborne observations collected core development associated with the transition of in two developing TCs during the Impact of Typhoons Tropical Storm Humberto (2001) from a tropical de- on Ocean in the Pacific (ITOP) field campaign in 2010 pression to a tropical storm. The formation of the warm (D’Asaro et al. 2014) to further examine the evolution of core is due in part to subsidence warming beneath a temperature, sea level pressure (SLP), and low-level trailing anvil at the periphery of an area of persistent circulation. Here, TC genesis is defined as the initial deep convection. The pronounced subsidence warming formation of a tropical depression in the Joint Typhoon 2 in that case (with aircraft-measured descent of 3 m s 1) Warning Center (JTWC) best-track data. The focus will may have been related to a dynamically forced system, be on one underappreciated aspect of organized deep such as a descending inflow jet similar to that shown in convection during TC genesis—the thermodynamic role Fig. 1. These studies suggest that subsidence warming of mesoscale subsidence warming induced by multiple plays a role in the tropical storm stage, but they do not long-lived MCSs. The organization of this paper is as focus on the precursor disturbance and tropical de- follows. Section 2 describes the data and methods of pression stages. Nevertheless, recent dropsonde data analysis. Section 3 gives an overview of two ITOP 4242 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 5. (a),(c) IR brightness temperature (color shading), flight track, and dropsonde locations (triangles) for the two missions into Typhoon Megi. The satellite image valid time is indicated, and the portion of the flight track within a half hour of the satellite image is indicated by the thick black line. (b),(d) Dropsonde winds at 850 (blue), 700 (green), and 500 hPa (red) for the two missions into Typhoon Megi. Wind barb pennants, barbs, and half barbs represent 50, 10, and 5 kt, respectively. Contours in (b) are the 900-hPa heights every 5 m and labeled every 10 m and (d) 900-hPa height contours are every 10 m and labeled every 50 m. storms: Typhoon Fanapi and Typhoon Megi. Section 4 mature typhoons. Several of these missions were con- presents the three-dimensional evolution of winds, ducted at 350 hPa (Table 1), allowing the GPS drop- moisture, and temperature at the early stages of these sondes to sample a deep layer of the troposphere, as two typhoons. These results are summarized in terms of described by Elsberry and Harr (2008), which is gener- the contribution to sea level pressure falls in section 5. ally not available in routine operational missions into Finally, conclusions are presented in section 6. pre-TC disturbances. Typhoon Fanapi was the most thoroughly sampled system, with a series of four daily missions from when it was a pre-TC disturbance to when 2. Data and methods it was an intensifying tropical storm (Table 1, Fig. 2a). The first three missions, referred to as F1, F2, and F3, a. Aircraft data respectively, were square-spiral surveillance missions During the ITOP field campaign from August to Oc- during the pregenesis and early tropical storm stages of tober 2010, U.S. Air Force C-130 aircraft were deployed Fanapi. Dropsondes were generally deployed every 60– into the tropical disturbances prior to TC genesis and 75 nm (110–140 km) in the square-spiral missions. F4 early storm stages of Typhoons Fanapi and Megi from was conducted when Fanapi was a tropical storm. For their base on Guam (Table 1, Fig. 2). Fanapi and Megi F4, an alpha pattern at 700 hPa was conducted. Typhoon were also subsequently sampled as tropical storms and Megi was sampled as a minimal tropical storm (M1) and NOVEMBER 2015 K E R N S A N D C H E N 4243

FIG. 6. Dropsonde analysis for the dropsonde released at 0139 UTC 13 Sep 2010 during F1. (a) Dropsonde location (triangle), coincident IR imagery (color shading), and 900-hPa height contours with low (L) and high (H) pressure as in Fig. 4b. (b) Skew T–logp diagram. Colored solid lines show the temperature (red) and dewpoint (blue) and the reference profile temperature (orange) and dewpoint (magenta). Full (half) barbs are 10 (5) kt. The melting level (ML) and onion feature are indicated by the text and gray shading annotation. The extrapolation of the lapse rate above the onion to the onion layer is indicated by the dashed red line. (c) Relative humidity, (d) temperature anomaly, and (e) moist static energy. In (c)–(e), the individual dropsonde is in blue, the other sondes during the mission are in gray, and the black lines are the reference profile. again the next day (M2) as an intensifying tropical storm observations. The Guam soundings provide upstream with maximum sustained winds of 55 knots (kt; 1 kt 5 environmental data as a context for the disturbed 2 0.51 m s 1)(Table 1, Fig. 2b). For M2, a hybrid square sampled by the dropsondes. The Guam mean spiral–butterfly pattern was conducted from 700 hPa. temperature during August–October 2010 is used as a For the F4 and M2 missions conducted at 700 hPa, the reference profile to help identify the warm and flight-level data provide fine horizontal-resolution data cool anomalies associated with the areas of distur- (1 Hz) within the developing warm core in the lower bed weather. Additionally, 12-hourly soundings were troposphere. The aircraft flight-level temperature (from available from the following stations in the western the Rosemount sensor) and dewpoint data were cor- North Pacific: Yap (PTY: 9.58N, 138.18E), Ponape rected for sensor wetting using the method of Zipser (PTP: 7.08N, 158.28E),Koror(PTR:7.38N, 134.58E), et al. (1981) and Eastin et al. (2002). Chuuk (PTK: 7.48N, 151.88E), and Majuro (PMK: 7.18N, 171.38E). The Guam ITOP mean profile was b. Radiosonde data similar (within 0.58C) to the grand mean profile of all of During ITOP, radiosondes were released from Guam these stations during the ITOP period (not shown). The (PGAC: 13.58N, 144.88E; marked ‘‘G’’ in Fig. 2), every Guam mean profile is considered to be a representative 12 h as part of the routine reference profile for the study region during ITOP. 4244 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG.7.AsinFig. 6, but for the dropsonde released at 0235 UTC 13 Sep 2010 during F1 and without the annotations.

The Guam mean profile mainly reflects undisturbed between consecutive images. The center of the cloud weather conditions that are similar to those in the cluster is defined as the geometric centroid of the 208-K environment surrounding the disturbances sampled area. Long-lived cloud clusters are generally associated during ITOP. Throughout this study, temperature with large areas of stratiform precipitation (Chen et al. anomalies are all relative to the August–October 2010 1996). See Kerns and Chen (2013) for more details re- Guam mean profile. Note that this reference profile is garding the application of the cloud cluster tracking for mainly to facilitate the intercomparison of the various TC genesis. missions, not to identify the distinctive onion-shaped d. Wind analysis profiles. Horizontal pressure level analyses of winds and c. Satellite data and cloud cluster tracking temperature were constructed using the dropsonde Hourly infrared (IR) temperature data are used to data. The aircraft sampled the system over the course trackcloudclustersbasedonthemethodusedin of several hours (Table 1), during which the storm was Williams and Houze (1987) and Chen et al. (1996).The moving. Additionally, the sondes traveled some hori- satellite data are from the IR channel (10.5–11.5 mm) zontal distance between the time they were launched of the geostationary Multifunctional Transport Satel- and the time they hit the sea. Therefore, when using lites (MTSAT). The data are available at 0.058 reso- the sondes to construct a composite wind analysis at lution. Cloud clusters are identified as contiguous the nominal analysis time (0000 UTC), a correction features with IR brightness temperatures below 208 K. was made for the storm motion. The storm motion They are tracked forward and backward in time correction was determined as follows. The JTWC best- whenever they overlap by at least 50% or 5000 km2 track data from 6 h prior and 6 h after the nominal NOVEMBER 2015 K E R N S A N D C H E N 4245

FIG.8.AsinFig. 7, but for the dropsonde released at 0023 UTC 13 Sep 2010 during F1. analysis time (e.g., 1800 and 0600 UTC coordinates for missing. F4 and M2 did not have temperature and wind the nominal analysis time at 0000 UTC) were used to data above 700 hPa. determine the average storm motion during the flight. e. Temperature and geopotential height analysis For the pregenesis stage, the ‘‘invest’’ center locations were used. For F1, invest coordinates were not avail- For the pressure level temperature and height ana- able until 0600 UTC, and the best-track coordinates lyses, a two-dimensional objective analysis was con- from 0600 and 1200 UTC were used for the storm ducted at each pressure level independently using motion correction to the 0000 UTC analysis time. To two-dimensional natural neighbor interpolation with implement the storm motion correction, the drop- Delaunay triangulation (Sibson 1981). In this approach, sonde locations were shifted by the displacement the triangles are constructed using the dropsonde locations, storm would have moved through during the time be- and the points within each triangle are weighted aver- tween the dropsonde time and the nominal time. The ages based on the overlap of the grid cells and the tri- storm motion correction was applied to each level angles. The dropsonde temperature and altitude data independently. were first interpolated to 10-hPa resolution using linear To show the characteristics of the system circulation, interpolation in the logarithm of pressure. For studying layer-mean winds for 840–860, 690–710, and 490– the warm-core development of incipient TCs, in which 510 hPa were calculated. The wind observations within deviations of 18–28C are significant, temperature each layer were weighted by the logarithm of pressure. If anomalies can be easier to interpret than temperature there were no dropsonde data within the 20-hPa thick profiles themselves. As discussed above, the Guam layer, the layer-mean winds were considered to be mean profile is used to calculate the vertical profiles of 4246 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG.9.AsinFig. 7, but for the dropsonde released at 0240 UTC 14 Sep 2010 during F2. The contours in (a) with low (L) and high (H) pressure labels are as in Fig. 4d. temperature anomalies. The estimated error of the altitude. The subscripts ‘‘dropsonde’’ and ‘‘ref’’ refer to temperature sensors on the GPS dropsondes is 0.28C the individual dropsonde and the Guam mean, re- (Hock and Franklin 1999), and the features of interest spectively. The term inside the integral deviate 0.58–2.08C from the reference profile. 2 r 2 r g[ dropsonde(z) ref(z)] f. Hydrostatic surface pressure falls represents the contribution of each altitude’s virtual To quantify the potential contribution of system temperature deviation to the SLP drop relative to the warming to the SLP falls as a function of height, an Guam mean reference sounding. Density was calculated analysis of the contribution of temperature deficits at all from temperature and pressure using the ideal gas law. altitudes was performed, using hydrostatic balance. This Dropsonde data were not available above ;350 hPa, so analysis is similar to that of Dolling and Barnes (2014). the full upper-level contribution to the SLP falls could The hydrostatic SLP difference between two profiles is not be determined. For these calculations, the drop- sonde data were first interpolated to 50-m resolution. DSLP 5 SLP 2 SLP dropsonde ref The interpolation was linear in height. ð TOA 5 2 r 2 r g[ dropsonde(z) ref(z)] dz, 0 3. Overview of Typhoons Fanapi and Megi where TOA is the top of the atmosphere, g is the ac- Typhoon Fanapi formed at 1200 UTC 14 September celeration due to gravity, r is the density, and z is the 2010 about 900 km east of the (Fig. 2a). NOVEMBER 2015 K E R N S A N D C H E N 4247

FIG. 10. As in Fig. 9, but for the dropsonde released at 0109 UTC 14 Sep 2010 during F2.

Fanapi intensified to 105 kt before making landfall in tracked moving generally west-northwestward until . The storm was the most thoroughly sampled 17 September, which was the central dense overcast of storm during ITOP with a series of seven C-130 mis- Fanapi (Fig. 3a). As expected, the aircraft encountered a sions. Several long-lived (e.g., 8 h) clusters could be well-defined circulation center with convection near the tracked as the precursor disturbance moved westward center during F3 and F4 (Figs. 4e–h). (Fig. 3a). This study focuses on the first four missions Typhoon Megi formed 380 km west-southwest of from disturbance to tropical storm (Table 1, Fig. 2). The Guam at 1800 UTC 12 October 2010 (Fig. 2b). The pre- first mission into Fanapi (F1) was 33–40 h prior to cursor invest system and the associated mesoscale cloud tropical depression (TD) formation. The system had clusters passed just south of Guam (Fig. 3b). The long- convection mostly at its periphery (Fig. 4a) and an open range National Weather Service radar from Guam in- wave structure at this time (Fig. 4b). The wave axis and a dicated widespread stratiform rain associated with the low pressure trough were oriented from the west- cloud clusters passing to the south (not shown). According southwest to the east-northeast. The second mission to the JTWC best track, the system was a TD when the (F2) was conducted 9–15 h prior to initiation in the C-130 first reached it around 1800 UTC 12 October (M1); JTWC best track. A nearly closed circulation was pres- however, it was not declared a TD in real-time operational ent at 850 and 700 hPa (Fig. 4d), and the deep convection JTWC advisories for another 12 h. Aircraft data show a was more concentrated near the center (Fig. 4c). Ac- well-defined circulation center with convection organized cording to the best track, the storm attained minimal nearthecenterduringM1(Figs. 5a,b). According to tropical storm (TS) intensity (35 kt) during F3, and it had best track, Megi developed into a tropical storm at an intensity of 55 kt during F4. During F3 and F4, a 0600 UTC 13 October, soon after M1. Not surprisingly, a single large cloud cluster was present, which could be single coherent cloud cluster could be tracked moving to 4248 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 11. Dropsonde analysis of temperature (8C; color bars for each row) at (a)–(c) 350 and (d)–(f) 650 hPa. (a),(d) F1; (b),(e) F2; and (c),(f) F3. Triangles indicate the dropsonde locations adjusted to 0000 UTC. The white areas are outside of the dropsonde data area. The letter L indicates the corresponding low pressure center from Fig. 4. the west-northwest for several days, which is the central structure with a southwest-to-northeast tilt (Fig. 4b). The dense overcast of Megi (Fig. 3b). During M2, it had convection associated with pre-Fanapi was broad reached a best-track intensity of 55 kt. A tight circulation (Fig. 4a) and concentrated in several separated cloud with convection concentrated at the center was found at clusters (Fig. 3a). The wave axis bisected the area of this time (Figs. 5c,d). Note that, during M1, the storm was convection, though there was a ;300-km-wide area of at a similar stage of development as Fanapi was during F3: weaker convection along the wave axis centered near a borderline TD/TS. When the C-130 reached Megi dur- 148N, 1388E(Fig. 4a). The low pressure center, as de- ing M2, it was already a tropical storm with intensity of termined from the dropsonde data, was located between 55 kt, similar to Fanapi during F4. Therefore, the C-130 this clear-air area and the area of convection to the missions considered in this study included a precursor southeast (Figs. 4a,b). Onion profiles were found disturbance (15 kt), a borderline TD (20–25 kt), two throughout the disturbance—in particular, near the in- minimal TSs (35 kt), and two 55-kt TSs. cipient low pressure center. In both cases, there were multiple MCSs (208-K cloud The onion thermodynamic structure of subsidence clusters) that existed simultaneously and evolved in time warming is illustrated by the dropsonde released near (either continuously or discontinuously) over several the circulation center at 0139 UTC during F1 (Fig. 6). days prior to TC genesis (Figs. 3a,b). This mode has been Below the melting level (;550 hPa), there is expected to commonly observed over the western Pacific (e.g., be a competition between evaporative cooling and un- Kerns and Chen 2013). saturated adiabatic subsidence warming. From the melting level down to 800 hPa, the relative humidity decreases from near 100% to below 70%, and temper- 4. Thermodynamic evolution of TC genesis atures are up to 28C cooler than the reference profile (Fig. 6d). This cooling relative to the environment is a. Precursor disturbance stage: Pre-Fanapi related to evaporation as the rain falls from the melting F1 sampled the precursor disturbance of Fanapi layer into the lower-RH air. At 850 hPa, the adiabatic (Figs. 4a,b). The dropsonde winds indicated an open wave subsidence warming becomes dominant, and the profile NOVEMBER 2015 K E R N S A N D C H E N 4249

FIG. 12. As in Fig. 7, but for the dropsonde released at 0041 UTC 15 Sep 2010 during F3. The contours in (a) with low (L) and high pressure (H) labels are as in Fig. 4f. becomes slightly warmer than the reference profile. This the Guam mean MSE profile (black line). Since MSE is is also the level where the driest (i.e., the fattest) part of conserved during adiabatic descent, the fact that the the onion is found (Fig. 6b). There, the relative humidity minimum is located significantly lower in the tropo- is reduced to below 70%, which is significantly drier than sphere is consistent with adiabatic subsidence. the reference profile mean RH of 80%–85% (Fig. 6c). A dropsonde was released at 0235 UTC 13 September While the onion profile was not as intense (i.e., large near the incipient center at the edge of the convective onion) as observed in strong tropical squall-line systems anvil clouds (Fig. 7a). The thermodynamic profile sug- (Zipser 1977) or midlatitude systems (Smull and Houze gests that the low pressure minimum was related to 1987), it was associated with a distinct minimum in rel- warming aloft superposed with subsidence warming in ative humidity (Fig. 6c) and temperatures several de- the lower troposphere (Fig. 7b). The onion layer grees Celsius warmer than without the onion-shaped was located between 900 and 700 hPa. In addition to profile (Figs. 6b,d). Finally, below 900 hPa, the boundary the onion warm anomaly in the lower troposphere, the layer is moister than the onion, with relative humidity upper levels above ;500 hPa were warmer than the around 80% (Fig. 6c). The cooler boundary layer air is reference profile, with a nearly moist adiabatic tem- likely modified by cool air from saturated convective perature profile (Figs. 7b–d), likely because of latent downdrafts (Zipser 1977). heat release in the upper-level anvils. Similar to the The moist static energy (MSE) profile is at a minimum onion profile discussed above, the reduced MSE extends at around 850 hPa, which is the same level as the onion down to ;900 hPa (Fig. 7e). feature (Fig. 6e). Typically, the minimum MSE is found The upper-level warming at the center may not be in the mid troposphere at around 600 hPa, as depicted in enough to induce surface pressure falls (relative to the 4250 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 13. As in Fig. 12, but for the dropsonde released at 2208 UTC 14 Sep 2010 during F3.

Guam mean SLP) if it were to be accompanied with Figs. 6–8). The predominance of anomalously dry and rel- lower-troposphere cool anomalies without the sub- atively warm air at around 800–700 hPa with relatively low sidence warming. For example, a dropsonde released in MSE suggests a history of subsidence below the melting the area of convection to the northeast of the pressure layer. This thermodynamic structure is ubiquitous center at 0024 UTC 13 September (Fig. 8a) shows warm throughout the disturbance in part because heavy rain en- anomalies above 550 hPa but cool anomalies of ;18C compasses only a small fraction of the area of the distur- below (Fig. 8). This profile was nearly saturated from bance. The subsidence warming has a mitigating effect on 900 to 600 hPa (Figs. 8b,c), and it is likely that the lower- the low-level cool anomalies, which would dominate if troposphere cool anomalies of around 18C cooler than evaporative cooling were dominant, such as in heavier the reference profile are due to the dominance of stratiform rain and convective rain. If further lower- evaporative cooling in the heavier rain and convective troposphere subsidence warming were to occur, the cool downdrafts. Unlike in the onion profile cases, the MSE anomaly could be further eroded and/or transformed into a profile does not have as pronounced of a minimum in the warm anomaly. Also, additional anvil warming above lower troposphere, suggesting that subsidence warming 500 hPa could help induce further surface pressure falls. is not a significant factor there (Fig. 8e). b. Tropical depression stage: Fanapi In addition to the individual soundings described above, one common characteristic of the dropsondes relea- The F2 mission into Fanapi was conducted several sed during F1 was a relative minimum in relative humid- hours prior to the initiation of a TD in the JTWC best ity at 700–800 hPa, collocated in the vertical with a track. There was much more convection near the center relative maximum in temperature anomaly (gray curves in during F2 compared with F1 (Figs. 4a,c). The dropsonde NOVEMBER 2015 K E R N S A N D C H E N 4251

FIG. 14. As in Fig. 7, but for the dropsonde released at 0052 UTC 13 Oct 2010 during M1. The contours in (a) with low (L) and high (H) pressure labels are as in Fig. 5b. data indicated a closed or nearly closed circulation reference profile there (Figs. 8d and 9d). This suggests center at 850 and 700 hPa with a wavelike structure at that subsidence warming can erode the lower-troposphere 500 hPa (Fig. 4d). Therefore, F2 is considered to have cool anomalies from above, though the detailed pro- sampled a borderline TD. cesses involved are not fully resolved by the available Similar to F1, the onion thermodynamic structure is data. Note that at the low levels (850 and 950 hPa), cool found near the incipient center during F2 with a distinct anomalies from 21.58 to 21.08C remained similar to F1 minimum in RH at 700–600 hPa (Fig. 9). In general, (Figs. 8d and 9d). dropsondes during F2 had an RH minimum of ;80% Another significant change between F1 and F2 is that between 600 and 800 hPa (gray curves in Fig. 9c). The the upper levels (above 500 hPa) became somewhat minimum MSE corresponds with the onion feature, warmer (Figs. 8d and 9d). However, the largest upper- with a minimum at around 700 hPa consistent with a level warming occurred in the active convection area to history of downward adiabatic transport from the mid- the northeast of the low-level center (Fig. 10)1. The level MSE minimum (Fig. 9e). Note that the MSE was dropsonde released at 0109 UTC shows a nearly satu- significantly higher than near the center in F1 (Fig. 7e), rated profile with a warm, moist adiabatic profile above likely because of an overall increase in midlevel moisture 500 hPa (Figs. 10b,c), suggesting the importance of la- near the incipient center (Wang 2012). A significant tent heat release there. MSE was higher there than at the structural thermodynamic difference between the first and second flights into Fanapi was that the cool anomaly at around 600–700 hPa was significantly reduced through- 1 The dropsonde in Fig. 10 likely experienced sensor wetting out the system, with temperatures warming to near the errors, especially below 500 hPa. 4252 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 15. As in Fig. 14, but for the dropsonde released at 2318 UTC 12 Oct 2010 during M1. center, likely because of upward transport of high-MSE increased by 18–28C(Figs. 11d,e). During F1, the incipient air near the surface (Fig. 10e). The largest surface center was located where the cool anomalies were less pressure falls and low-level circulation center during F2 pronounced. The cool anomalies at the center were 0.58– were located not where upper-level warming was max- 1.08C lower than the reference profile temperature, com- imized, but where the upper-level warming was super- pared with 2.08C cooler nearby to the southwest. Also, the posed with lower-troposphere subsidence warming and warmest air at 350 hPa was to the southwest of the center erosion of the lower-troposphere cool anomaly from (Fig. 11a), where the 650-hPa temperatures were ;18C above (Fig. 9). Also note that there is a clear deep cooler than at the incipient center (Fig. 11d).Bythetimeof cyclonic circulation extending down to 850 hPa as ob- F2, the upper levels were warmer (Fig. 11b), and the cool served by the dropsonde data, which suggests a system- anomalies at 650 hPa were nearly eliminated (e.g., less than mean low-level convergence despite the presence of 0.58C cooler than the Guam mean; Fig. 11e). Both of these subsidence warming indicated by the onion-shaped thermodynamic changes played a role in the initial for- soundings at some locations within the system. mation of the initial low-level closed circulation (Fig. 4d). Spatial maps of analyzed constant pressure level tem- Further intensification to a minimal tropical storm involved peratures show that, between F1 and F2, the warm anom- warming both at 350 and 650 hPa and vertical alignment as alies at 350 hPa increased by 0.58–1.08C(Figs. 11a,b). observed during F3 (Figs. 11c,e). Recent modeling results from Cecelski and Zhang (2013) c. Minimal tropical storm: Fanapi and Megi suggest that upper-level warming is important for TC genesis. However, at the early stage, the center is sensitive According to the JTWC best track, Fanapi became a to the lower-troposphere subsidence warming. The tem- tropical storm during F3, and Megi attained this status peratures below the melting level (;550 hPa) at 650 hPa during the first mission into that storm (M1). Subsidence NOVEMBER 2015 K E R N S A N D C H E N 4253

FIG. 16. Aircraft data for F4, 0215–0330 UTC 16 Sep 2010. (a) Flight track (red), flight-level winds (orange), and infrared satellite image from 0300 UTC 16 Sep 2010. (b) Flight-level temperature (red), dewpoint (blue), pressure (green), and winds (orange). The horizontal dashed red line is the Guam mean temperature. The vertical dashed lines correspond to the B and C markers in (a), which marks the warm, dry air at the center. (c)–(e) Skew T diagrams for the times marked by the green triangles in (b). The dropsonde temperature (red) and dewpoint (blue), as well as the Guam mean temperature (orange) and dewpoint (magenta) are represented.

warming was found below the melting level within the T diagram (Fig. 12b), similar to an onion subsidence developing central dense overcast, especially at the pe- warming feature. The height of this feature would be riphery. The sonde deployed nearest to the center at inconsistent with mesoscale unsaturated downdrafts be- 0042 UTC 15 September during F3 (Fig. 12a) shows low the melting layer and may be indicative of a de- subsaturated conditions both above 500 hPa and below scending inflow jet. Note that most sondes during F3 had a 650 hPa (Figs. 12b,c). Adiabatic subsidence warming subsaturated RH minimum of around 85% and a broad may play a role around 700 hPa, where both the relative MSE minimum between 850 and 650 hPa (gray lines in humidity (Fig. 12c) and MSE are at a minimum Figs. 12c,e), indicating that adiabatic subsidence warming (Fig. 12e). The profile was significantly warmer than the is likely counteracting the evaporative cooling beneath Guam mean above 750 hPa (Fig. 12d), where the tem- the melting layer throughout the system—in particular, in perature profile followed a nearly moist adiabatic profile the outer circulation portion of the storm. A sonde re- (Fig. 12b). This is consistent with recent studies showing leased north of the center but still beneath the central that latent heat release aloft plays a large role in surface dense overcast (Fig. 13a) at 2208 UTC 14 September pressure falls near the center (Zhang and Zhu 2012; shows a narrow onion profile below the melting layer Cecelski and Zhang 2013). However, the subsaturated (Fig. 13b). There is a MSE minimum there (Fig. 13e), conditions above 500 hPa (both with respect to water suggesting unsaturated adiabatic warming at the periph- and ice) would not be consistent with warming solely ery of the developing circulation center. because of latent heat release. Interestingly, at around In M1, the dropsonde released near the center be- 450 hPa there is a minimum in relative humidity of neath the central dense overcast (Fig. 14a) suggests the ;60% (Fig. 12c) and maximum in temperature anomaly superpositioning of warm anomalies above 500 hPa and (Fig. 12d), as well as a fatter appearance on the skew an onionlike feature just below 700 hPa (Fig. 14b). The 4254 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 17. As in Fig. 16, but for M2 during 0320–0500 UTC 14 Oct 2010. The infrared image in (a) is at 0400 UTC. Note that the aircraft made several loops near the center between B and C. sonde was released beneath cloud-top temperatures core in more detail, as will be discussed in the following below 200 K (Fig. 14a). Above ;650 hPa, the profile is section. saturated (with respect to ice) and nearly moist adia- d. Intensifying tropical storms Fanapi and Megi batic. At 700 hPa, the relative humidity is reduced to 82% (Fig. 14c), and temperature anomalies are slightly Fanapi and Megi were both sampled by the C-130 as positive (Fig. 14d). There is also an MSE minimum near intensifying tropical storms with an intensity of 55 kt. In 700 hPa (Fig. 14e). On the periphery of the system, the both missions, the flight track passed through the central onion features were more pronounced: for example, to dense overcast and the circulation center (Figs. 4d and the southwest of the center (Fig. 15). Interestingly, the 5b), providing valuable flight-level data below the sonde released at 2318 UTC 12 October has a double melting layer. The first transect considered here is from onion structure with distinct onion features at 650 and the northwest to the southeast during F4, from 0215 to 800 hPa (Fig. 15b). This may suggest subsidence warm- 0330 UTC 16 September (Fig. 16). The circulation cen- ing due to both descending jet features and mesoscale ter at 700 hPa was located at the eastern periphery of the unsaturated subsidence. Similar to TS Fanapi, the lower- large cloud system (Fig. 16a). As the aircraft flew from troposphere warming due to subsidence may contribute the northwest approaching the center, it encountered to maintaining and intensifying the outer circulation of mostly subsaturated air with the temperature within 18C the storm (if it is widespread), although upper-level of the Guam mean flight-level temperature. The warm warming may play a more important role at the center. core near the circulation center consisted of air at 148– Note that, in F3 and M1, the tight center of the min- 188C, compared with the Guam mean of 9.38C. The imal tropical storms was not adequately resolved by the dewpoints were around 78–98C. This warm air with large dropsonde data. Also, distinct descending inflow jets are dewpoint depressions near the center suggests adiabatic not resolved. The subsequent missions, however, were subsidence warming. Dropsondes released there show conducted at 700 hPa and resolve some features of the dry air and a temperature inversion capping the nearly lower-troposphere component of the developing warm saturated boundary layer at the center (Fig. 16d), which NOVEMBER 2015 K E R N S A N D C H E N 4255

FIG. 18. NRL visible and passive microwave imagery of Tropical Storm Megi during M2. (a) Visible image for 0357 UTC 14 Oct 2011. (b) The 85-GHz channel (color scale on the bottom) and (c) 37-GHz color composite image for 0422 UTC 14 Oct 2011. In (c), pink represents heavy rain and cyan represents moderate rain. The red circle indicates the approximate 700-hPa circulation center as determined from the C-130 flight-level data. The plot area is 88–168N, 1368–1448E. Grid boxes are every 28. is similar to mature TCs. As the aircraft moved south- to the first transect, the warmest air was within the east away from the circulation center, it encountered subsaturated part of the flight leg. The warmest air en- cooler and saturated air. countered was 19.18C with a dewpoint of 2.98C. Drop- The second transect considered here was taken during sondes released within the warm-core region suggest M2 from the northwest to the southeast during 0320– vigorous subsidence reminiscent of soundings taken 0500 UTC 14 October. At this time, convection had in- within mature TC eyes (Figs. 17c,d). In contrast, a creased, and a distinct central dense overcast enveloped dropsonde deployed to the southeast of the warm core the circulation center (Figs. 17a, 18a). The aircraft within the central dense overcast region was saturated made several loops at the center for a period of around and significantly cooler than the reference profile below half an hour (dashed vertical lines in Fig. 17b). Similar 800 hPa (Fig. 17e). 4256 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

FIG. 19. The vertical profiles of the SLP contribution relative to the Guam mean profile. Fanapi missions (a) F1, (b) F2, (c) F3, and (d) F4; and Megi missions (e) M1 and (f) M2. Pressure levels are indicated on the left side. Note different axes scales in (d) and (f).

Passive microwave satellite data from an overpass of SLP be higher or lower than the Guam mean profile. the Advanced Microwave Scanning Radiometer for Pressure fall contributions are negative, to the left of the Earth Observing System (AMSR-E) during M2 suggest vertical dashed line. The net SLP difference is the vertical that the strongest convection was in an arc at the western integral of each level’s contribution (plus unsampled and northern part of the cold cloud area displaced ;50– contributions above flight level). The thermodynamic 100 km from the circulation center (Fig. 18). A large area changes related to the development of the storms are re- of moderate rain extended to the southeast and south of lated to the variations from the left panels to the right the arc of vigorous convection, which is inferred from the panels, which are in order of the degree of storm devel- moderate 85-GHz scattering signal (i.e., green; Fig. 18b) opment. Note that, for each individual flight, the pressure and cyan colors on the 37-GHz color composite (Hawkins falls relative to the immediate storm environment—which and Velden 2011; Kieper and Jiang 2012). The low-level may be cooler or warmer than the Guam mean—are not center and subsaturated warm air encountered by the known. In the early stage of TC genesis (F1), SLP fall C-130 around ;0400 UTC was at the periphery of the contributions are mainly from the upper levels (.4.5 km; moderate stratiform rain area, which may have evolved Figs. 19a,b). These warm anomalies and associated con- from a feature similar to the wake low in Chen and Frank tributions to surface pressure falls are likely caused by the (1993), as depicted in Fig. 1. The cyan spiral band features concentration of latent heat release in the increasingly in Fig. 18c converge inward toward this center. This inertially stable environment (Chen and Frank 1993; structure of the developing center displaced from the Zhang and Zhu 2012). This is a preexisting feature for strongest convection and at the edge of the stratiform rain several days prior to genesis, and it becomes more pro- area is similar to that of Dolling and Barnes (2012). nounced as the system develops (Fig. 19b,c). Below 4.5 km, SLP contributions are mostly positive, which counteracts the SLP falls induced from the upper levels (Fig. 19a). 5. Hydrostatic contribution to the SLP fall However, at ;2km(;800 hPa), there is a relative mini- To quantify the significance of the lower-troposphere mum within the low-level cool anomaly (Fig. 19a). The subsidence warming for SLP falls relative to the Guam lower-troposphere cool anomalies are partially mitigated mean profile, the hydrostatic equation is used as described at around 800 hPa compared with 700 hPa, likely in part in section 2f. The vertical profiles in Fig. 19 show how because of subsidence warming based on the onion-shaped much each vertical level would contribute to making the profiles in Figs. 6 and 7. Nevertheless, the low-level cool NOVEMBER 2015 K E R N S A N D C H E N 4257 anomaly remained a road block for TC genesis in F1. It is in TC genesis: namely, the subsidence warming within unlikely that the observed increased anvil warming above MCSs. The GPS dropsonde measurements from ITOP the melting layer (550 hPa) in F2 and F3 could result in a show that characteristic onion-shaped thermodynamic well-defined TC if the positive pressure tendencies of up profiles are common in the lower troposphere prior to 2 to 10.5 hPa km 1 below 650 hPa were to remain. Indeed, and during TC genesis. This observation helps shed these pressure rise contributions were mostly eliminated in some light on a previously unresolved issue in TC F2 (Fig. 19b) and replaced by pressure falls of up genesis. Prior to TC genesis, lower-troposphere cool 2 to 20.5 hPa km 1 bythetimeofF3(Fig. 19d). Many of the anomalies associated with evaporative cooling within dropsonde thermodynamic profiles for those flights were the stratiform regions in MCSs as shown in Bister and also consistent with subsidence warming (Figs. 9, 13,and Emanuel (1997) counteract the upper-level warm 14), suggesting that subsidence warming may play a role in anomalies described by Chen and Frank (1993), which reducing the lower-troposphere cool anomaly, which inhibit the net hydrostatic surface pressure falls. The makes it more favorable for TC genesis. subsidence warming, especially from 600 to 800 hPa As TC genesis proceeds, the low-level cool anomaly is beneath the melting layer as shown by the dropsonde eroded from above. For the second flight into Fanapi, data in this study, helps erode the lower-troposphere when the system was a borderline TD, most dropsondes cool anomalies. When superimposed with the mid- show significant contributions to falling SLP above 2 km upper-troposphere warming induced by latent heating (;800 hPa), with some counteracting SLP rise contri- and enhanced by inertial stability in long-lived large butions from the remaining near-surface/boundary layer MCSs (Chen and Frank 1993), it can help to reduce the cool anomaly (Fig. 19b). In terms of the contribution to surface pressure, which is critical for TC development. the SLP perturbations, the most significant structural The lowering of the level of minimum MSE throughout thermodynamic difference between the first and second the system, especially near the center in F1, F2, and F3, missions into Fanapi is the transition from positive SLP is consistent with a history of subsidence, while the contributions to neutral/SLP fall contributions between system moistening may partially counteract the drying 2.5 and 4.5 km (Figs. 19a,b). The erosion of the low-level effect. The subsidence warming in the low troposphere cool anomalies allows the net surface pressure pertur- is one of multiple key ingredients of TC genesis, and it bation to be negative, inducing a well-defined low-level would not be expected to cause TC genesis on its own— pressure center (Fig. 4d). This erosion of the low-level in particular, without upper-level warming (Zhang and cool anomaly roadblock from above results in the initial Zhu 2012; Cecelski and Zhang 2013). The vertical net hydrostatic SLP falls associated with the transi- alignment of the mid-upper-level warm anomalies with tion to a borderline TD. By the time of F3, the lower- the low-troposphere warming below the melting layer 2 troposphere contributions of 0.2–0.5 hPa km 1 over is a crucial component of TC genesis for the cases ob- ;6 km can account for several hectopascals in surface served in this study. The results of this study are sum- pressure falls, as opposed to counteracting the upper- marized below and in Fig. 20: level contributions in F1. During M1, the pressure fall d Onion-shaped thermodynamic profiles indicative of contributions were similar to F3. Note that the cool lower-troposphere subsidence warming were ubiqui- anomalies remained near the surface, but they were no tous in pre-Typhoons Fanapi and Megi during ITOP longer sufficient to counteract the effect of the warming from the disturbance to the tropical storm stage. above, resulting in falling SLP. d TC genesis occurred when the subsidence warm For the F4 and M2, SLP fall contributions were anomalies eroded the lower-troposphere cool anom- present in most profiles above ;1km(Figs. 19f,g), with 2 aly and superposed with the preexisting mid-upper- pressure fall contributions of around 3–4 hPa km 1 over troposphere warm anomaly (Fig. 20a) in regions several kilometers in the lower troposphere. These large favorable for TC genesis. pressure fall contributions are related to the eyelike low- d As Fanapi and Megi intensified into strong tropical level structures seen near the centers of these in- storms, there was evidence of vigorous localized sub- tensifying tropical storms (Figs. 17 and 18). Similar to sidence warming similar to the lower-troposphere the results of Dolling and Barnes (2014), the largest structure of mature TC eyes (Fig. 20b). pressure fall contributions were from ;2-km altitude. The flight-level data during the intensifying TS phase of Fanapi and Megi suggest a deepening, intensification, 6. Conclusions and scale contraction of the subsidence warming during The aircraft observations from ITOP have revealed an intensification to a more mature system, similar to that important but previously underappreciated ingredient in Dolling and Barnes (2012), in which the center 4258 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

mean cyclonic circulation and low pressure center ex- tending down to 850–900 hPa (Figs. 4d, 5b), while the subsidence warming is indicated by the onion-shape soundings within the system. The aircraft data available in this study cannot resolve these convective scale pro- cesses, including convective updrafts and downdrafts, which persist and may intensify throughout TC genesis. Similarly, the data presented in this study cannot definitely determine how widespread the mesoscale subsidence warming is. A high-resolution numerical modeling study has been conducted to help clarify the roles of the convective scale features in relation to the lower-troposphere subsidence warming. The re- sults will be described in Part II of this study.

Acknowledgments. We thank Drs. Patrick Harr and

FIG. 20. Schematic of warm-core development in TC genesis. Peter Black for designing the pre-TC flights and col- (left) Vertical cross sections: the dark horizontal shading repre- lecting dropsonde data during ITOP. ITOP data are sents the melting level, and the top of the MCS is at the top of the provided by NCAR/EOL under sponsorship of the troposphere. (right) Plan views with the cross-section location National Science Foundation (http://data.eol.ucar.edu/ marked with the dashed line; dark shading represents heavy con- vective rainfall. For Fanapi and Megi, the spatial scale of the cloudy codiac/projs?ITOP). The aircraft flight-level data were area (lighter gray shading) is ;500 km. (a) A tropical depression obtained through the NOAA Hurricane Research Di- with multiple interacting MCSs with areas of mesoscale subsidence vision of AOML. The hourly satellite IR data were warming (red arrows and W). (b) A tropical storm with more obtained from Kochi University. We also thank Ed pronounced, localized lower-troposphere warming (red W). Ryan for processing satellite IR data and cloud cluster Upper-level warming in the anvil is indicated by the black W. tracking analysis, as well as Paloma Cordero and Ismari Ramos for their contribution to the initial analysis of the develops where there is enhanced lower-troposphere dropsonde data. Infrared satellite data were obtained subsidence in the light stratiform rain area at the from the Kochi University archive. Microwave imagery periphery of the deep convection. One remaining was adopted from the Naval Research Lab tropical cy- question is how the lower-tropospheric warming in- clone website. Discussions with Drs. Falko Judt, Klaus tensifies during the transition from a TD to a TS. A Dolling, Gary M. Barnes, Zhou Wang, and Jon Zawislak possible factor could be the interaction of the multiple have been helpful during the course of this study. MCSs or 8-h clusters (Figs. 3 and 4), similar to that We thank Ajda Savarin and Andrew Smith for copy shown in Kerns and Chen (2013). In addition to the in- editing. Input from Dr. Edward Zipser and two anony- teraction and intensification of the midlevel vortices mous reviewers helped improve the manuscript signifi- (Ritchie and Holland 1997; Simpson et al. 1997), the cantly. This research was supported by the Office of lower-troposphere subsidence warming can potentially Naval Research through Grants N000140810576 and be enhanced if the MCSs are arranged in such a con- N000141010162. figuration that their areas of lower-troposphere me- soscale subsidence reinforce each other (Fig. 20). It is important to note that the lower-troposphere REFERENCES subsidence warming only occurs in the context of large, Bessho, K., T. Nakazawa, S. Nishimura, and K. Kato, 2010: Warm long-lived, organized MCSs, which contain deep con- core structures in organized cloud clusters developing or not vective cores or ‘‘hot towers.’’ These deep convective developing into tropical storms observed by the Advanced Microwave Sounding Unit. Mon. Wea. Rev., 138, 2624–2643, cores can help maintain the system mean convergence doi:10.1175/2010MWR3073.1. and ascent (Riehl and Malkus 1958; Malkus and Riehl Biggerstaff, M. I., and R. A. Houze, 1991: Kinematic and precipi- 1960; Wang 2014) and also provide a source of low-level tation structure of the 10–11 June 1985 squall line. Mon. Wea. relative vorticity (Hendricks et al. 2004). Therefore, the Rev., 119, 3034–3065, doi:10.1175/1520-0493(1991)119,3034: . mesoscale subsidence need not cause the disturbance to KAPSOT 2.0.CO;2. Bister, M., and K. A. Emanuel, 1997: The genesis of Hurricane Guil- dissipate as long as convective cores are maintained lermo: TEXMEX analyses and a modeling study. Mon. Wea. elsewhere within the system. Indeed, the observations Rev., 125, 2662–2682, doi:10.1175/1520-0493(1997)125,2662: obtained in the pre-TCs showed a clear deep system TGOHGT.2.0.CO;2. NOVEMBER 2015 K E R N S A N D C H E N 4259

Bousquet, O., and M. Chong, 2000: The oceanic mesoscale con- ——, 1998: The formation of tropical . Meteor. Atmos. vective system and associated mesovortex observed 12 De- Phys., 67, 37–69, doi:10.1007/BF01277501. cember 1992 during TOGA-COARE. Quart. J. Roy. Meteor. Halverson,J.B.,J.Simpson,G.Heymsfield,H.Pierce,T.Hock, Soc., 126, 189–211, doi:10.1002/qj.49712656210. and L. Ritchie, 2006: Warm core structure of Hurricane Brandes, E. A., 1990: Evolution and structure of the 6–7 May 1985 Erin diagnosed from high altitude dropsondes during mesoscale convective system and associated vortex. Mon. Wea. CAMEX-4. J. Atmos. Sci., 63, 309–324, doi:10.1175/ Rev., 118, 109–127, doi:10.1175/1520-0493(1990)118,0109: JAS3596.1. EASOTM.2.0.CO;2. Hawkins, J., and C. Velden, 2011: Supporting meteorological field ——, and C. L. Ziegler, 1993: Mesoscale downdraft influences on experiment missions and postmission analysis with satellite vertical vorticity in a mature mesoscale convective system. Mon. digital data and products. Bull. Amer. Meteor. Soc., 92, 1009– Wea. Rev., 121, 1337–1353, doi:10.1175/1520-0493(1993)121,1337: 1022, doi:10.1175/2011BAMS3138.1. MDIOVV.2.0.CO;2. Hendricks, E. A., M. T. Montgomery, and C. A. Davis, 2004: The Brown, J. M., 1979: Mesoscale unsaturated downdrafts driven role of ‘‘vortical’’ hot towers in the formation of Tropical by rainfall evaporation: A numerical study. J. Atmos. Diana (1984). J. Atmos. Sci., 61, 1209–1232, Sci., 36, 313–338, doi:10.1175/1520-0469(1979)036,0313: doi:10.1175/1520-0469(2004)061,1209:TROVHT.2.0.CO;2. MUDDBR.2.0.CO;2. Heymsfield, G. M., J. Simpson, J. Halverson, L. Tian, E. Ritchie, Cecelski, S. F., and D.-L. Zhang, 2013: Genesis of Hurricane Julia and J. Molinari, 2006: Structure of highly sheared Tropical (2010) within an African easterly wave: Low-level vortices and Storm Chantal during CAMEX-4. J. Atmos. Sci., 63, 268–287, upper-level warming. J. Atmos. Sci., 70, 3799–3817, doi:10.1175/JAS3602.1. doi:10.1175/JAS-D-13-043.1. Hock, T. F., and J. L. Franklin, 1999: The NCAR GPS dropwind- Chen, S. S., and W. M. Frank, 1993: A numerical study of the sonde. Bull. Amer. Meteor. Soc., 80, 407–420, doi:10.1175/ genesis of extratropical convective . Part I: 1520-0477(1999)080,0407:TNGD.2.0.CO;2. Evolution and dynamics. J. Atmos. Sci., 50, 2401–2426, Houze, R. A., 1977: Structure and dynamics of a tropical squall– doi:10.1175/1520-0469(1993)050,2401:ANSOTG.2.0.CO;2. line system. Mon. Wea. Rev., 105, 1540–1567, doi:10.1175/ ——, R. A. Houze, and B. E. Mapes, 1996: Multiscale variability of 1520-0493(1977)105,1540:SADOAT.2.0.CO;2. deep convection in relation to large-scale circulation in Johnson, R. H., and M. E. Nicholls, 1983: A composite analysis of the TOGA COARE. J. Atmos. Sci., 53, 1380–1409, doi:10.1175/ boundary layer accompanying a tropical squall line. Mon. Wea. 1520-0469(1996)053,1380:MVODCI.2.0.CO;2. Rev., 111, 308–319, doi:10.1175/1520-0493(1983)111,0308: Correia, J., and R. W. Arritt, 2008: Thermodynamic properties of ACAOTB.2.0.CO;2. mesoscale convective systems observed during BAMEX. Mon. ——, and P. J. Hamilton, 1988: The relationship of surface Wea. Rev., 136, 4242–4271, doi:10.1175/2008MWR2284.1. pressure features to the precipitation and airflow structure D’Asaro, E. A., and Coauthors, 2014: Impact of typhoons on the of an intense midlatitude squall line. Mon. Wea. Rev., ocean in the Pacific. Bull. Amer. Meteor. Soc., 95, 1405–1418, 116, 1444–1473, doi:10.1175/1520-0493(1988)116,1444: doi:10.1175/BAMS-D-12-00104.1. TROSPF.2.0.CO;2. Davis, C. A., and D. A. Ahijevych, 2013: Thermodynamic envi- ——, S. Chen, and J. J. Toth, 1989: Circulations associated with a ronments of deep convection in Atlantic tropical disturbances. mature-to-decaying midlatitude mesoscale convective sys- J. Atmos. Sci., 70, 1912–1928, doi:10.1175/JAS-D-12-0278.1. tem. Part I: Surface features—heat bursts and mesolow de- DeMaria, M., J. A. Knaff, and B. H. Connell, 2001: A tropical cy- velopment. Mon. Wea. Rev., 117, 942–959, doi:10.1175/ clone genesis parameter for the tropical Atlantic. Wea. Fore- 1520-0493(1989)117,0942:CAWAMT.2.0.CO;2. casting, 16,219–233,doi:10.1175/1520-0434(2001)016,0219: Jorgensen, D. P., M. A. LeMone, and B. Jong-Dao Jou, 1991: Pre- ATCGPF.2.0.CO;2. cipitation and kinematic structure of an oceanic mesoscale Dolling, K., and G. M. Barnes, 2012: Warm-core formation in convective system. Part I: Convective line structure. Mon. Wea. Tropical Storm Humberto (2001). Mon. Wea. Rev., 140, 1177– Rev., 119, 2608–2637, doi:10.1175/1520-0493(1991)119,2608: 1190, doi:10.1175/MWR-D-11-00183.1. PAKSOA.2.0.CO;2. ——, and ——, 2014: The evolution of Hurricane Humberto (2001). Kerns, B. W., and S. S. Chen, 2013: Cloud clusters and tropical J. Atmos. Sci., 71, 1276–1291, doi:10.1175/JAS-D-13-0164.1. cyclogenesis: Developing and nondeveloping systems and Eastin, M. D., P. G. Black, and W. M. Gray, 2002: Flight-level their large-scale environment. Mon. Wea. Rev., 141, 192–210, thermodynamic instrument wetting errors in hurricanes. Part I: doi:10.1175/MWR-D-11-00239.1. Observations. Mon. Wea. Rev., 130, 825–841, doi:10.1175/ Kieper, M. E., and H. Jiang, 2012: Predicting rapid 1520-0493(2002)130,0825:FLTIWE.2.0.CO;2. intensification using the 37 GHz ring pattern identified from Elsberry, R. L., and P. A. Harr, 2008: Tropical Cyclone Structure passive microwave measurements. Geophys. Res. Lett., 39, (TCS08) field experiment science basis, observational plat- L13804, doi:10.1029/2012GL052115. forms, and strategy. Asia-Pac. J. Atmos. Sci., 44, 209–231. Komaromi, W. A., 2013: An investigation of composite dropsonde Fritsch, J. M., J. D. Murphy, and J. S. Kain, 1994: Warm core vortex profiles for developing and nondeveloping tropical waves amplification over land. J. Atmos. Sci., 51, 1780–1807, during the 2010 PREDICT field campaign. J. Atmos. Sci., 70, doi:10.1175/1520-0469(1994)051,1780:WCVAOL.2.0.CO;2. 542–558, doi:10.1175/JAS-D-12-052.1. Gamache, J. F., and R. A. Houze, 1982: Mesoscale air mo- La Seur, N. E., and H. F. Hawkins, 1963: An analysis of Hurri- tions associated with a tropical squall line. Mon. Wea. cane Cleo (1958) based on data from research re- Rev., 110, 118–135, doi:10.1175/1520-0493(1982)110,0118: connaissance aircraft. Mon. Wea. Rev., 91, 694–709, MAMAWA.2.0.CO;2. doi:10.1175/1520-0493(1963)091,0694:AAOHCB.2.3.CO;2. Gray, W. M., 1968: Global view of the origin of tropical distur- Leary, C. A., 1980: Temperature and humidity profiles in meso- bances and storms. Mon. Wea. Rev., 96, 669–700, doi:10.1175/ scale unsaturated downdrafts. J. Atmos. Sci., 37, 1005–1012, 1520-0493(1968)096,0669:GVOTOO.2.0.CO;2. doi:10.1175/1520-0469(1980)037,1005:TAHPIM.2.0.CO;2. 4260 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 72

——, 1984: Precipitation structure of the cloud clusters in a tropical Sibson, R., 1981: A brief description of natural neighbour inter- easterly wave. Mon. Wea. Rev., 112, 313–325, doi:10.1175/ polation. Interpreting Multivariate Data, V. Barnett, Ed., 1520-0493(1984)112,0313:PSOTCC.2.0.CO;2. Probability and Mathematical Statistics, Vol. 21, John Wiley & ——, and R. A. Houze, 1979: Melting and evaporation of hydro- Sons, 21–36. meteors in precipitation from the anvil clouds of deep trop- Simpson, J., E. A. Ritchie, G. J. Holland, J. Halverson, and ical convection. J. Atmos. Sci., 36, 669–679, doi:10.1175/ S. Stewart, 1997: Mesoscale interactions in tropical cyclone 1520-0469(1979)036,0669:MAEOHI.2.0.CO;2. genesis. Mon. Wea. Rev., 125, 2643–2661, doi:10.1175/ Malkus, J. S., 1958: On the structure and maintenance of the ma- 1520-0493(1997)125,2643:MIITCG.2.0.CO;2. ture hurricane . J. Meteor., 15, 337–349, doi:10.1175/ ——, J. B. Halverson, B. S. Ferrier, W. A. Petersen, R. H. Simpson, 1520-0469(1958)015,0337:OTSAMO.2.0.CO;2. R. Blakeslee, and S. L. Durden, 1998: On the role of ‘‘hot ——, and H. Riehl, 1960: On the dynamics and energy trans- towers’’ in tropical cyclone formation. Meteor. Atmos. Phys., formations in steady-state hurricanes. Tellus, 12A, 1–20, 67, 15–35, doi:10.1007/BF01277500. doi:10.1111/j.2153-3490.1960.tb01279.x. Smull, B. F., and R. A. Houze, 1987: Rear inflow in squall lines Mapes, B. E., and R. A. Houze, 1995: Diabatic divergence profiles with trailing stratiform precipitation. Mon. Wea. Rev., 115, 2869– in western Pacific mesoscale convective systems. J. Atmos. 2889, doi:10.1175/1520-0493(1987)115,2869:RIISLW.2.0.CO;2. Sci., 52, 1807–1828, doi:10.1175/1520-0469(1995)052,1807: Stossmeister, G. J., and G. M. Barnes, 1992: The development DDPIWP.2.0.CO;2. of a second circulation center within Tropical Storm Isabel McBride, J. L., 1981: Observational analysis of tropical cyclone (1985). Mon. Wea. Rev., 120, 685–697, doi:10.1175/ formation. Part I: Basic description of data sets. J. Atmos. 1520-0493(1992)120,0685:TDOASC.2.0.CO;2. Sci., 38, 1117–1131, doi:10.1175/1520-0469(1981)038,1117: Wang, Z., 2012: Thermodynamic aspects of tropical cyclone for- OAOTCF.2.0.CO;2. mation. J. Atmos. Sci., 69, 2433–2451, doi:10.1175/ ——, and R. Zehr, 1981: Observational analysis of tropical cyclone JAS-D-11-0298.1. formation. Part II: Comparison of non-developing versus de- ——, 2014: Characteristics of convective processes and vertical veloping systems. J. Atmos. Sci., 38, 1132–1151, doi:10.1175/ vorticity from the tropical wave to tropical cyclone stage in a 1520-0469(1981)038,1132:OAOTCF.2.0.CO;2. high-resolution numerical model simulation of Tropical Cy- Reed, R. J., and E. E. Recker, 1971: Structure and properties clone Fay (2008). J. Atmos. Sci., 71, 896–915, doi:10.1175/ of synoptic-scale wave disturbances in the equatorial JAS-D-13-0256.1. western Pacific. J. Atmos. Sci., 28, 1117–1133, doi:10.1175/ Williams, M., and R. A. Houze, 1987: Satellite-observed charac- 1520-0469(1971)028,1117:SAPOSS.2.0.CO;2. teristics of winter monsoon cloud clusters. Mon. Wea. ——, D. C. Norquist, and E. E. Recker, 1977: The structure and Rev., 115, 505–519, doi:10.1175/1520-0493(1987)115,0505: properties of African wave disturbances as observed during SOCOWM.2.0.CO;2. phase III of GATE. Mon. Wea. Rev., 105, 317–333, Zawislak, J., and E. J. Zipser, 2014: Analysis of the thermodynamic doi:10.1175/1520-0493(1977)105,0317:TSAPOA.2.0.CO;2. properties of developing and nondeveloping tropical distur- Riehl, H., and J. S. Malkus, 1958: On the heat balance in the bances using a comprehensive dropsonde dataset. Mon. Wea. equatorial trough zone. Geophysica, 6, 503–538. Rev., 142, 1250–1264, doi:10.1175/MWR-D-13-00253.1. Ritchie, E. A., and G. J. Holland, 1997: Scale interactions Zhang, D.-L., and L. Zhu, 2012: Roles of upper-level processes in during the formation of Typhoon Irving. Mon. Wea. Rev., tropical cyclogenesis. Geophys. Res. Lett., 39, L17804, 125, 1377–1396, doi:10.1175/1520-0493(1997)125,1377: doi:10.1029/2012GL053140. SIDTFO.2.0.CO;2. Zipser, E. J., 1977: Mesoscale and convective-scale downdrafts as Schubert, W. H., and J. J. Hack, 1982: Inertial stability and tropical distinct components of squall-line structure. Mon. Wea. Rev., cyclone development. J. Atmos. Sci., 39, 1687–1697, 105, 1568–1589, doi:10.1175/1520-0493(1977)105,1568: doi:10.1175/1520-0469(1982)039,1687:ISATCD.2.0.CO;2. MACDAD.2.0.CO;2. Schumacher, C., R. A. Houze, and I. Kraucunas, 2004: The tropical ——, R. J. Meitín, and M. A. LeMone, 1981: Mesoscale motion dynamical response to latent heating estimates derived from fields associated with a slowly moving GATE convective the TRMM precipitation radar. J. Atmos. Sci., 61, 1341–1358, band. J. Atmos. Sci., 38, 1725–1750, doi:10.1175/ doi:10.1175/1520-0469(2004)061,1341:TTDRTL.2.0.CO;2. 1520-0469(1981)038,1725:MMFAWA.2.0.CO;2.