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1 Subsidence Warming in the Tropical Cyclogenesis of Cindy (2017): CPEX

2 observations and Coupled Modeling

∗ 3 Edoardo Mazza and Shuyi S. Chen

4 Department of Atmospheric Sciences, University of Washington, Seattle Manuscript

∗ 5 Corresponding author: [email protected]

1 ABSTRACT

6 The formation of tropical cyclones (TC) in unfavorable large-scale environments remains a chal-

7 lenge for TC forecasting. Tropical Storm (TS) Cindy (2017) formed at 1800 UTC 20 June in

8 the Gulf of Mexico despite strong vertical wind shear, low mid-tropospheric relative humidity,

9 and poorly organized convection. A key to TC genesis is the initial development of a warm

10 core within an emergent cyclonic vortex, a process which occurs on small spatial scales and is

11 often difficult to observe. TS Cindy was observed during the Convective Processes Experiment

12 (CPEX) field campaign in 2017 by the NASA DC-8 aircraft, equipped with a Doppler wind lidar,

13 Manuscript precipitation radar, and GPS dropsondes. This study combines CPEX observations and a cloud-

14 resolving, fully-coupled atmosphere-wave-ocean numerical simulation to investigate the formation

15 of TS Cindy. Prior to TC genesis, a shallow cyclonic circulation was embedded in a deep layer

16 of west-southwesterly flow associated with an upper-level trough. Within the disturbance, a warm

17 and dry anomaly was observed by dropsondes near the center of the cyclonic circulation, with

18 a maximum at about the 2.5 km level. The temperature perturbation reaches 5°C along with a

19 dew point temperature depression of 8°C in the coupled model simulation. Backward trajectory

20 analysis shows that subsidence is primarily associated with a thermally indirect circulation along

21 the western flank of the storm. Air parcels descend more than 1000 m towards the lower tropo-

22 sphere while warming up by 9-12°C. The subsidence-induced virtual temperature perturbation in

23 the 1.5-3.5 km layer accounts for 50 % of the sea-level pressure depression. Subsidence warming

24 therefore played a key role in the genesis of TS Cindy.

2 25 1. Introduction

26 The genesis of tropical cyclones is a multiscale process that involves the transformation of a

27 precursor disturbance into a warm core, low-pressure system with a closed surface circulation.

28 Precursor disturbances can be tropical waves in the tropical basins (Frank and Roundy 2006), long-

29 lasting mesoscale convective systems (MCSs) or cloud clusters (Kerns and Chen 2013), monsoon

30 lows, Central-American gyres (Papin et al. 2017) or have an extra-tropical origin (Davis and Bosart

31 2003). TC genesis is facilitated when a set of large-scale conditions are met, such as low vertical

32 wind shear, a moist mid-troposphere, a vorticity-rich low troposphere, a deep and warm ocean

33 Manuscript mixed layer (Gray 1968; McBride and Zehr 1981) and a large thermodynamic disequilibrium

34 between the tropopause and the sea surface (McTaggart-Cowan et al. 2015). TC genesis involves

35 two fundamental processes: the amplification and organization of cyclonic vorticity, and the

36 formation of a warm core vortex. Pre-existent, low-level cyclonic vorticity can be amplified and

37 axisymmetrized by vortical hot towers (Hendricks et al. 2004; Montgomery et al. 2006). Mid-

38 tropospheric vortices can instead result from diabatic heating in the stratiform region of long-lived

39 MCSs (Chen and Frank 1993) or from evaporative cooling in the precipitating region of MCSs

40 (Bister and Emanuel 1997). The formation of the warm core is supported by diabatic heating in

41 the convective and stratiform cloud region (Chen and Frank 1993; Dolling and Barnes 2012a). An

42 observational study by Kerns and Chen (2015) showed that subsidence warming associated with

43 MCSs can contribute directly to development of warm core vortex in TC genesis.

44 About 60 % of all TC genesis events in the North Atlantic from 1948 to 2010 involved a varying

45 degree of baroclinicity (McTaggart-Cowan et al. 2013). In the Western Caribbean Basin and

46 Gulf of Mexico, TC genesis often occurs in unfavorable environments, with upper-tropospheric

47 disturbances enhancing the vertical wind shear and promoting mid-tropospheric dry air intrusions

3 48 (Gray 1968). Bracken and Bosart (2000) found that a pronounced upper-tropospheric trough-ridge

49 pattern is often associated with TC genesis events in the Bahamas and Gulf of Mexico. Strong

50 vertical wind shear is considered to be unfavorable for TC genesis as observed in the Atlantic and

51 the west Pacific (McBride and Zehr 1981; Kerns and Chen 2013), while weak-to-moderate westerly

52 shear can instead assist TC genesis (Bracken and Bosart 2000; Nolan and McGauley 2012; Reasor

53 and Montgomery 2001). Wind shear induces significant structural changes in TCs, which can

54 hinder their development or intensification: idealized experiments (Jones 1995; Frank and Ritchie

55 1999, 2001) and observational studies (Rodgers et al. 1994; Black et al. 2002; Chen et al. 2006;

56 Corbosiero and MolinariManuscript 2002; Reasor et al. 2013) indicate that the structure of sheared TCs is

57 highly asymmetric, as the deepest convection focuses in the downshear quadrants. Another factor

58 that greatly affects the formation of TCs is mid-tropospheric humidity (Malkus 1958; Gray 1975;

59 McBride and Zehr 1981). TC genesis is favored by high relative humidity in the mid-levels, whereas

60 intrusions of dry air can delay or suppress TC development (e.g. Dunion and Velden 2004; Wang

61 2012; Kerns and Chen 2013). Ventilation of dry and cool air into the developing TC disrupts its

62 thermodynamic structure and suppresses convection by reducing the updrafts buoyancy (Simpson

63 and Riehl 1958; Shelton and Molinari 2009; Riemer and Montgomery 2011; Tang and Emanuel

64 2012; Ge et al. 2013). Importantly, how TCs develop in relatively unfavorable environmental

65 conditions over the Gulf of Mexico remains an active area of research.

66 Subsidence-driven temperature anomalies associated with oceanic convective systems have been

67 documented since the GATE field campaign (Houze 1977; Zipser 1977). Simpson et al. (1997)

68 argue that subsidence is the only viable process for maintaining a low-level warm anomaly in clear

69 air. Descent is typically observed in the form of unsaturated downdrafts underneath the anvil canopy

70 of long-lived MCSs. Chen and Frank (1993) showed the presence of a wake low in a region of

71 low-tropospheric subsidence on the edge of a simulated MCS. In mature tropical cyclones, instead,

4 72 observed subsidence-induced warm anomalies are often attributed to vortex tilting (Halverson

73 et al. 2006; Heymsfield et al. 2006; Shelton and Molinari 2009). The relationship between wind

74 shear and asymmetric warm anomalies in TCs has been investigated by Tao and Zhang (2019),

75 which describe how the alignment of a mid-tropospheric and upper-tropospheric warm anomalies

76 is often seen as the storm approaches the onset of rapid intensification. A small number of studies

77 focuses on the role of subsidence during TC genesis and almost exclusively investigated the role of

78 subsidence within MCSs. Dolling and Barnes (2012b) and Dolling and Barnes (2012a) describe

79 how subsidence helped the formation of a lower-tropospheric warm core in TS Humberto (2001)

80 by inducing a hydrostaticManuscript pressure drop and by capping the boundary layer, allowing for a buildup

81 of high equivalent potential temperature air, which was later ingested in the nascent eyewall.

82 Stossmeister and Barnes (1992) document the formation of a second circulation center in TS Isabel

83 (1982) underneath a region of mesoscale subsidence. Finally, subsidence warming during TC

84 genesis was also captured by dropsondes released in typhoons Megi and Fanapi during the Impact

85 of Typhoons on Ocean in Pacific (ITOP) field campaign (Kerns and Chen 2015).

86 Is the presence of long-lived MCSs the only pathway to low-level subsidence warming during

87 TC genesis? In this study, we focus on the genesis of TS Cindy (2017) to describe how subsidence-

88 induced warm anomalies in the lower troposphere can support TC formation in a very different

89 dynamic and thermodynamic context. TS Cindy developed from a poorly-organized, broad cy-

90 clonic disturbance embedded in a high-shear, low mid-tropospheric relative humidity environment.

91 Airborne observations collected during the NASA Convective Processes Experiment (CPEX) field

92 campaign reveal a low-level warm anomaly formed within a shallow cyclonic circulation in a

93 cloud-free region, well removed from convective clusters and their associated anvils. Using a com-

94 bination of aircraft observations and a convection-resolving, fully-coupled atmosphere-wave-ocean

95 simulation, we address three main questions: a) what are the spatial and temporal characteristics

5 96 of the subsidence-induced temperature perturbation, b) does the subsidence-induced warming pro-

97 duce a significant pressure perturbation during TC genesis? and c) what mechanisms drive the

98 descent? The thermodynamic and kinematic properties of the disturbance are investigated using

99 dropsonde and airborne wind lidar retrievals, complemented by a high-resolution model simulation

100 to overcome their limited spatial and temporal sampling. The model simulation is then employed

101 to perform backward trajectory analysis and to diagnose the driving mechanisms of subsidence.

102 The paper is organized as follows: section 2 is an overview of the meteorological evolution of TS

103 Cindy, the data and methodology employed in the study are described in section 3. The results are

104 presented in section 4,Manuscript 5, 6, 7 and 8. A discussion of the results and their implications is included

105 in section 9.

106 2. Overview of Tropical Storm Cindy

107 The National Hurricane Center (NHC) describes Cindy as a large, sprawling TS that formed

108 on 20 June. Its genesis was preceded by the interaction of two consecutive tropical waves with a

109 Central-American Gyre (CAG). The first wave reached the Caribbean Sea on 15 June, while the

110 second wave moved into the basin on 18 June. The second wave featured deep convection on

111 its eastern flank (Fig.1a) along with low-level cyclonic vorticity. The interaction with the CAG

112 produced a cyclonic disturbance in the central Gulf of Mexico on 19 June 2017 characterized by

−1 113 an elongated circulation, with wind speed exceeding 17 m s (Berg 2018). At this stage, the

114 convective activity was primarily focused along the eastern flank of the disturbance (Fig.1b).

115 On 20 June, multiple low-level vorticity local maxima merged into a coherent center as the deep

116 convection attained a curved structure around it (Fig.1c), prompting the NHC to declare the genesis

117 of TS Cindy at 1800 UTC 20 June 2017. TS Cindy formed in an unfavorable environment: an

118 upper-level cut-off in the northwestern Gulf of Mexico advected mid-tropospheric drier air into

6 119 the Gulf and enhanced the vertical wind shear in the area prior to TC genesis. As a result, TS

120 Cindy was characterized by a broad surface wind field, with an exposed low-level circulation and

121 asymmetrically-distributed convection (Fig.1c).

122 On the next day, deep convection rotated towards the NW quadrant (Fig.1d) as TS Cindy

123 intensified to reach a peak intensity of 50 kt on 21 June 0000 UTC. TS Cindy made landfall at

124 0700 UTC on 22 June just west of Cameron, LA. While inland, TS Cindy weakened to TD status

125 and finally dissipated on 24 June 0600 UTC in the mid-Atlantic states. The impact of TS Cindy

126 was primarily due to excessive rainfall: widespread accumulations in the 7-10 in. range were

127 measured in south-easternManuscript Mississippi, southwestern Alabama and part of the Florida panhandle,

128 with a maximum of 18.69 in. at Ocean Springs, MS.

129 3. Data and methods

130 This study uses a combination of aircraft observations from the CPEX field campaign, satellite

131 observations, and a fully coupled atmosphere-wave-ocean high resolution numerical simulation.

132 For model comparison purposes, the track and intensity of TS Cindy are obtained by linearly inter-

133 polating the Best-Track dataset (Landsea and Franklin 2013) to hourly interval. High-frequency,

134 2-minute storm center fixes from the NHC are used for the CPEX observations analysis.

135 a. CPEX Field Campaign − Aircraft Observations

136 The CPEX field campaign took place in the North Atlantic-Gulf of Mexico-Caribbean Sea

137 region in May-June 2017. It was designed to study convective processes in the tropics using the

138 NASA DC-8 aircraft observations (Chen and Zipser 2018). Four research flights from 17-21 June

139 were conducted to capture the development of TS Cindy from its precursor tropical wave in the

140 western Caribbean Sea. On 17 June and 19 June, the flights targeted some of the convective

7 141 elements embedded in the precursor disturbance. On 20 June, the NASA DC-8 airplane sampled

142 the kinematic and thermodynamic structure of TS Cindy right as the disturbance was classified as

143 TS by the NHC (Fig.2a). Its mature phase was captured by the 21 June mission (Fig.2b).

144 The wind and thermodynamic profiles analyzed in this study were obtained from the Doppler

145 Aerosol WiNd lidar (DAWN) and by Yankee Environmental System (YES) dropsondes (Black et al.

146 2017). DAWN is a coherent-detection, wind-profiling lidar system that emits a pulsed signal with

147 a 2-micron wavelength (Kavaya et al. 2014). The instrument retrieves wind speed and direction at

148 a vertical resolution of approximately 65 m. YES dropsondes measure wind and thermodynamic

149 variables as they descendManuscript through the atmosphere at a 2 Hz frequency. YES dropsondes have

150 been previously employed during the Tropical Cyclone Intensity (TCI) field campaign (Doyle et al.

151 2017). In the 20 June CPEX mission, 254 DAWN wind profiles were obtained and 16 dropsondes

152 were launched (Fig.2a). 28 dropsondes and 528 DAWN wind profiles were recorded in the 21 June

153 CPEX mission (Fig.2b).

154 b. UWIN-CM simulation

155 The Unified Wave INterface-Coupled Model (UWIN-CM, Chen et al. 2013; Chen and Curcic

156 2016) is employed to perform a fully-coupled atmosphere-wave-ocean simulation of TS Cindy.

157 The UWIN-CM consists of the Weather Research and Forecasting model (WRF, Skamarock et al.

158 2008), the University of Miami Wave Model (UMWM, Donelan et al. 2012) and the Hybrid

159 Coordinate Ocean Model (HYCOM, Bleck 2002). The WRF model is configured with an outer

160 domain with two nested grids with horizontal grid spacings of 12, 4, and 1.3 km, respectively.

161 There are 44 vertical levels. The outer domain covers an area of 6,468 km (E-W) x 4,320 km (N-S).

162 The inner-most 1.3-km nest (523 km x 523 km) is storm-following (Fig.3). The Kain-Fritsch

163 cumulus scheme (Kain 2004) is used in 12-km outer domain, while in both the 4- and 1.3-km nests

8 164 convection is resolved explicitly. For all domains, the WRF single-moment, 6-class microphysics

165 scheme (WMS6, Hong and Lim 2006) and the YSU PBL scheme (Hong et al. 2006) are used.

166 The horizontal grid spacing of UMWM is 4 km and 36 frequency bins are used in the spectral

167 computations. HYCOM is run at a 1/25 °(approximately 4 km) horizontal grid spacing and 41

168 vertical levels. Initial and boundary conditions for WRF are provided by the ERA5 reanalysis

169 (Hersbach and Dee 2016), while the HYCOM model is initialized from HYCOM global analysis

170 fields. The simulation is initialized at 1200 UTC 19 June and terminates at 0000 UTC 23 June.

171 The 1.3-km moving nest is initialized 6 hours into the simulation at 1800 UTC 19 June. Analysis

172 nudging is applied everyManuscript 6 hours to the wind fields in the 12-km WRF domain to improve the

173 simulated track of TS Cindy.

174 For the analysis of the storm structure, the UWIN-CM WRF output is linearly interpolated onto

175 constant height levels with a vertical spacing of 50 m. A storm-tracking algorithm is used to

176 calculate the storm position and intensity using hourly model output. The algorithm locates the

177 850 hPa geopotential height minimum and calculates the storm intensity as the corresponding

178 minimum sea-level pressure and maximum wind speed. Following the NHC official report, the

179 time of TC genesis in the simulation is taken to be 1800 UTC 20 June 2017.

180 c. Hydrostatic Pressure Perturbation

181 The sea-level pressure perturbation (SLP’) due to changes in the virtual temperature (Tv) is

182 calculated using the hydrostatic equation along with the ideal gas law, in a manner consistent

0 183 with Stossmeister and Barnes (1992); Dolling and Barnes (2012a); Kerns and Chen (2015). SLP

184 at each model grid point can be estimated using equation 1, where the Top Of the Atmosphere

185 (TOA) is the upper limit of integration and the overbar denotes domain-averaged quantities. The

9 186 contribution from a specific atmospheric layer (e.g. 1-2 km) can be calculated by changing the

187 limits of integration in equation 1.

Z TOA 0 −g p(z) p(z) SLP = − dz (1) 0 Rd Tv (z) Tv (z) 

188 d. Trajectory Analysis

189 Backward trajectories are calculated by adapting a code developed for the Cloud

190 Model 1 (CM1, Bryan and Fritsch 2002) to work on WRF output. The origi-

191 nal code is availableManuscript at https://github.com/tomgowan/trajectories/blob/master/

192 trajectoriesCM12ndorder.ipynb. It is based on the work of Miltenberger et al. (2013)

193 and uses a second-order, semi-implicit discretization in space and time. The backward trajectory

194 calculations are performed on 10-minute model output from a twin numerical simulation of TS

195 Cindy, restarted at 0600 UTC 20 June 2017 from the parent experiment described in section 3b.

196 Three sets of parcel trajectories are initialized within the subsidence-induced warm anomaly at the

197 elevations of 2500, 2100 and 1700 m.

198 4. UWIN-CM Simulation of TS Cindy

199 The storm track and intensity TS Cindy in the UWIN-CM simulation are evaluated against the

200 Best-Track dataset (Landsea and Franklin 2013). The minimum SLP is used for the comparison as

201 it is a more reliable measure of the storm position and intensity than peak winds, in particular in

202 the early stages of development. As shown in Fig.3, the observed track is sufficiently well captured

203 by the model simulation up to the NHC genesis time (1800 UTC 20 June). Later on, the simulated

204 track oscillates around the observed one, showing an anticipated and accentuated recurving to

205 the west. The average track error is 104.7 km. TS Cindy is slower in the simulation than in the

10 206 observation: it makes landfall approximately 80 km east and 7 hours later than the observed storm

207 (Figs.3, 4a). The UWIN-CM simulated minimum SLP closely tracks the observation until about

208 0600 UTC 22 June when the observed Cindy made landfall, whereas the simulated storm remained

209 over the ocean because of its slower motion (Fig.4a). As a result, the simulated minimum SLP at

210 landfall is 6 hPa lower than observed. Overall, the root mean square error (RMSE) of the minimum

211 SLP from 1200 UTC 19 June to 1200 UTC 22 June is 2.9 hPa.

212 One of the most prominent large-scale environmental conditions during the development of TS

213 Cindy is strong wind shear. To assess the UWIN-CM simulation of the large-scale environment,

214 we compute the deep-layerManuscript (200-850 hPa) wind shear, averaged in a 200-800 km annulus around

215 the storm and compare it to the one calculated from the ERA5 reanalysis. Prior to TC genesis

216 (vertical dashed line), the UWIN-CM simulation correctly reproduces the observed high-wind

−1 217 shear environment, with values well above 20 m s (Fig.4b). After 00 UTC 21 June, the ERA5

−1 218 wind shear declines rather steadily, reaching a minimum of 11 m s at 12 UTC 22 June. The

219 UWIN-CM wind shear compares well with the ERA5 one, with an overall RMSE of just 1.3 m

−1 220 s , however there are two differences: the observed reduction in wind shear is anticipated by

221 approximately 6 hours and the wind shear is generally weaker after TC genesis between 0000 UTC

222 21 June and 0000 UTC 22 June.

223 The excessive shear reduction coincides with the storm recurving to the west and slowing down.

224 We speculate this could be associated with a more efficient rearrangement of the upper-level flow

225 in the simulation possibly due to diabatic effects. Given the similarity between the observed and

226 simulated track and intensity during the CPEX flights (Fig.3, Fig.4a) and the fact that most of the

227 analysis is performed in storm-relative coordinates, we do not expect these differences to influence

228 the results presented in this study.

11 229 5. Subsidence warming

230 Subsidence in an atmospheric layer is revealed by an enhanced dew point temperature depression

231 and increased static stability, often large enough to produce a temperature inversion. Its footprint

232 on the skew T-log(p) diagram is the so-called thermodynamic “onion profile” (Zipser 1977; Houze

233 1977). In this section we examine the evidence of organized subsidence during the genesis of TS

234 Cindy in the CPEX observations and UWIN-CM simulation.

235 a. Observations from the CPEX dropsondes

236 The 20 June CPEXManuscript mission reveals important thermodynamic features involved in the genesis

237 of TS Cindy. Three dropsondes were released shortly after the TS classification by the NHC at

238 1908, 1948 and 2029 UTC 20 June (square markers in Fig.5) in an area largely cloud-free, where

239 the lower-tropospheric temperature was higher than in the surrounding environment. All these

240 dropsondes are located within 100 km from the storm center (24, 69 and 54 km away respectively)

241 and indicate the presence of low-level subsidence warming within the developing disturbance. The

242 thermodynamic diagram in Fig.6a shows that the subsidence is maximized around the 800-825 hPa

243 level, where the dew point temperature depression approaches 10 °C and a cyclonic circulation

244 exists (wind barbs in Fig.5). Below that level, an approximately isothermal layer extends down

245 to 900 hPa. A second inversion is also observed at 600 hPa. In the mid-to-upper troposphere,

246 the dry-air intrusion associated with the trough is revealed by dew point temperature depressions

247 exceeding 20 °C. On 20 June, 13 dropsondes were released in the near and far environment, more

248 than 100 km away from the storm in the drier environment along the western flank of the storm or

249 in proximity of precipitating clouds in the eastern flank. Their average temperature and dew-point

250 temperature profiles (Fig.6b) are in stark contrast with those near the storm center: they do not

251 show signs of organized low-level subsidence such as deep layers of temperature inversion and

12 252 enhanced dew-point temperature depression. The temperature perturbation within the developing

253 TS Cindy is estimated by subtracting the mean temperature profile of the environment (Fig.6b)

254 from that of the inner disturbance ( Fig.6a). The result is shown in Fig.6c: a positive anomaly of

255 3.84 °C is collocated with the subsidence at 825 hPa, while a second anomaly (3.97 °C) is present

256 just below 600 hPa. The dropsondes data is scarce above 450 hPa but the temperature perturbation

257 profile suggests that the system does not have a well-defined warm core above 500 hPa. The

258 observed thermodynamic structure of TS Cindy indicates that near its genesis time (1900 UTC

259 20 June) a subsidence-induced positive temperature perturbation, maximized at 800-825 hPa, is

260 located within a developingManuscript cyclonic circulation.

261 The thermodynamic structure of TS Cindy changed remarkably after its genesis. The dropsondes

262 launched during the 21 June CPEX mission (Fig.7) reveal that, in its mature stage, TS Cindy is

263 characterized by a warm anomaly close to its center that extends from 890 hPa to the upper

264 troposphere, with a maximum of 2.4 °C located at approximately 650 hPa. Some shallow inversion

265 layers can be observed in the individual dropsondes, as it can be expected next to the convection

266 of a developing eyewall. The average thermodynamic profiles, however, do not exhibit a clear

267 subsidence signature, suggesting that the sinking motion lacks the strength and the organization

268 observed on 20 June.

269 b. UWIN-CM simulation - Temperature Perturbation

270 To assess the presence of low-level warming in the simulation of TS Cindy, we calculate the

271 temperature perturbation at each vertical level by subtracting the corresponding domain-average

272 temperature and search for its maximum value within 100 km from the storm center. The time-

273 height diagram in Fig.8a shows how the maximum temperature perturbation evolves from 0000

274 UTC 20 June to 0000 UTC to 22 June. Prior to 1200 UTC 20 June, the disturbance is characterized

13 275 by a moderately positive temperature perturbation of 2-4 °C between 2000-8000 m. These warm

276 features are generally short lived and lack a coherent vertical structure. Starting at 1200 UTC 20

277 June, the maximum temperature perturbation sharply increases in the 4000 - 4500 layer and extends

278 downward to approximately 2000 m. A second pulse of lower-tropospheric warming starts before

279 0000 UTC 21 June. These warm anomalies have common characteristics: they are maximized

280 just above 2000 m, have magnitudes exceeding 5.5 °C and originate above 4000 m. In the lower

281 troposphere (around 2000 m), the maximum temperature perturbation grows by more than 3.5 °C

282 in 12 hours. The model simulation thus indicates that during the genesis of TS Cindy subsidence

283 produced a coherent,Manuscript long-lasting warm anomaly within the developing disturbance. Later on 21

284 June, the model portraits a structure characterized by a more elevated temperature perturbation,

285 largely consistent with the dropsondes collected during the 21 June CPEX mission.

286 The importance of subsidence in the genesis of TS Cindy is further suggested by the time series

287 of minimum sea-level pressure (Fig.8b): the growth of the lower tropospheric warm anomaly

288 is accompanied and followed by a 7 hPa pressure fall from 1001 hPa to 994 hPa. Once the

289 warm anomaly dissipates, the minimum sea-level pressure stabilizes around 994 hPa and remains

290 stationary for several hours afterwards.

291 6. Kinematic structure

292 As discussed in section 5, both the observations and the model simulation indicate that a

293 subsidence-induced warm anomaly in the lower troposphere occurred prior and during the genesis

294 of TS Cindy between 1200 UTC 20 June and 1000 UTC 21 June. To understand the context in

295 which the subsidence occurs, we analyze the circulation associated with the system along three

296 vertical cross sections at different phases:

297 1) Pre-Subsidence (0800 UTC 20 June)

14 298 2) During Subsidence (1900 UTC 20 June)

299 3) Post-Subsidence (2000 UTC 21 June)

300 To do so, we complement the CPEX DAWN wind profiles and dropsondes with the UWIN-CM

301 simulation. CPEX observations are used to validate the simulated structure of TS Cindy. The

302 model simulation provides a complete picture where CPEX observations are scarce: above the

303 flight level, in between dropsondes and in regions of strong lidar attenuation. Due to the absence

304 of airborne measurements, only model fields are presented for phase 1. For phases 2 and 3, the

305 CPEX flights legs (1855Manuscript - 1922 UTC 20 June and 1950-2022 UTC 21 June) are reproduced in the

306 model output.

307 The cyclonic circulation associated with the precursor disturbance is clearly evident in the model

308 cross section (Fig.9a, b). In phase 1, the circulation was confined in the lowest 3 km of the

309 troposphere and featured a 120 km-wide area of low winds at its center. A southwesterly jet located

310 between 9 and 12 km approached the disturbance from the west, imposing a large wind shear

311 gradient over the disturbance (Fig.9a).

312 Phase 2 was characterized by persistent subsidence within the disturbance. The 20 June CPEX

313 flight leg intersected the storm center at 1900 UTC, shortly after the NHC classified the system

314 as TS Cindy. The model cross section (Fig.9c, d) and CPEX observations (Fig.10a, c) show a

315 similar slanted region of low winds at the center of the circulation along with a mid-tropospheric jet

316 located at 7 km. Compared to phase 1, the low-level cyclone has strengthened. Both observations

−1 317 and modeling suggest that the wind speed exceeds 25 m s on the western flank of the circulation

318 (Fig.10d). In this stage, the upper-level jet is directly above the disturbance (Fig.10c). A notable

319 difference can be seen approximately 7 km above the disturbance: while both CPEX observations

320 and model simulation indicate the presence of a mid-tropospheric jet, its position is not entirely

15 321 consistent. Airborne wind measurements show the maximum located directly above the western

322 flank of the circulation whereas in the UWIN-CM it is located above the center of the disturbance.

323 On 21 June the CPEX flight leg passed to the south of the storm center after the bulk of the

324 subsidence had occurred. Since the DAWN lidar retrievals suffered from severe attenuation in

325 the middle troposphere, the analysis of phase 3 relies more on the model simulation. Following

326 the period of sustained low-level subsidence, the model indicates that TS Cindy is now more

327 axisymmetric (Fig.9e) with a deeper cyclonic circulation whose radius of maximum wind has

328 reduced to approximately 60 km. The model cross section indicates that an eyewall-like wind

329 structure is also present.Manuscript There are however discrepancies between the observed and the model

330 storm structure, in particular the dropsondes indicate lower wind speeds between 4-6 km compared

331 to the model cross-section (Fig.10d).

332 7. Key Processese contributing to subsidence

333 To better understand the physical processes that contributed to subsidence warming in TS Cindy,

334 we analyze the spatial and temporal evolution of the air parcels that undergo substantial warming

335 during the genesis of TS Cindy. We do so by performing a backward trajectory analysis as discussed

336 in section 3d. Three sets of trajectories are initialized within the subsidence-induced warm anomaly

337 in the lower troposphere at 1800 UTC 20 June at the elevations of 2500, 2100 and 1700 m, from

338 parcels whose temperature exceeds 18, 20, 22 °C respectively (Fig.11). The three initial levels are

339 selected to represent the behavior of parcels near the top, the center and the bottom of the warm

340 anomaly.

341 Backward trajectories reveal that intense subsidence begins for all three sets of parcels between

342 1100 UTC and 1300 UTC 20 June at an elevation between 3-3.5 km. As shown in Fig.12, prior

343 to starting their descent, the parcels originated primarily from two distinct airstreams: one rising

16 344 from the lowest levels of the troposphere, and one emanating from the mid-levels. Once the parcels

345 reach the northwest quadrant of the storm, they begin subsiding towards the lower troposphere; in

346 doing so, they become increasingly warmer than their surrounding environment, reaching terminal

347 values above 6 °C (Fig.12b). The bulk of the subsidence is confined along the western flank of

348 the storm, during a 5-hour period between 1300 UTC and 1800 UTC. We can estimate some key

349 characteristics of the subsidence experienced by the air parcels. The median descent for parcels

350 initialized at 1700, 2100, and 2500 m is estimated to be 1090 m, 1130 m and 1380 m respectively,

351 with an accompanying temperature increase of 9.9 °C, 10.3 °C and 12 °C. The resulting lapse rates

−1 −1 352 along the trajectoriesManuscript vary between 8.7 °C km and 9.1 °C km , with sinking rates between -6 −1 −1 353 cm s and -7.6 cm s .

354 The known mechanism supporting subsidence within MCSs or squall lines relies on the evapo-

355 ration of hydrometeors in the stratiform precipitation region (e.g. Houze 1977; Zipser 1977; Chen

356 and Frank 1993). The trajectories show that the subsidence is focused along the western flank

357 of the circulation, downstream of the precipitating region (Figs.12, and 13). Parcels initiate their

358 descent underneath the anvil clouds associated with the convection on the northern flank of the

359 storm (Fig.12). In that region, the evaporation of hydrometeors can help initiate the subsidence.

360 Although this process may have contributed to the initial descending motion of the air parcels

361 in the stratiform region, the continued descending motion in the nearly convection-free region

362 may be forced by other processes. The prolonged descent of positively buoyant parcels suggests

363 the presence of a thermally indirect circulation. The presence of this ageostrophic circulation is

364 diagnosed via the Pettersen kinematic frontogenesis as defined in Bluestein (1993). As shown in

365 Fig.11a, a large temperature gradient is present in the lower troposphere in the northwest quad-

366 rant of the storm. Colder air wrapping around the developing cyclonic circulation gives rise to a

367 localized band of frontogenesis in the lower troposphere (Fig.14a). The position of this banded

17 368 region is consistent with the convection observed in the GOES infrared imagery just few hours

369 later (Fig.5). During subsidence, the parcels are located downstream of such band, in a region

370 characterized by weak frontolysis (Fig.14a). The cross section through the frontolytical region

371 shows that it is associated with an area of subsidence (i.e. negative vertical velocity) in which the

372 parcels are embedded (Fig.14b). Such a mechanism relies on the divergence and deformation of

373 the flow, acting on the existing horizontal temperature gradient, which force subsidence to restore

374 thermal wind balance. It is worth noting that a similar mechanism has been proposed to explain the

375 descent of mid-tropospheric air in the sting jets of deep, marine extra-tropical cyclones (Schultz

376 and Sienkiewicz 2013;Manuscript Martínez-Alvarado et al. 2014; Coronel et al. 2016).

377 8. Decreasing sea-level pressure from subsidence warming

378 Descent produces low level warming and drying, resulting in increased static stability and

379 enhanced dew point temperature depression. When the subsidence is sufficiently organized, a

380 surface meso-low can emerge (Zipser 1977; Chen and Frank 1993). As discussed in section 3c,

381 a change of virtual temperature in the atmospheric column will result in a pressure perturbation.

382 The presence of low-level cyclonic vorticity can determine whether the pressure perturbation is

383 retained or dissipated by gravity waves. In this section, we investigate the pressure perturbation

384 induced by the subsidence in the UWIN-CM simulation.

385 The lower tropospheric warming and drying within TS Cindy is shown in Fig.15. Early in the

386 genesis stage (0900 UTC), the disturbance does not display a well-organized warm anomaly. As

387 the subsidence develops within the disturbance, however, the temperature rapidly increases in the

388 inner vortex, reaching values above 18 °C at 1800 UTC 20 June. The warm anomaly is located

389 very close to the SLP minimum and is then advected along its southern and eastern flank over

390 time. This temperature perturbation is retained for several hours following TC genesis up to 0000

18 391 UTC 21 June. As the dropsondes in Fig.10a suggests, subsidence-induced warming also results

392 in a consistent drying of the air mass, with low relative humidity values and an enhanced dew

393 point temperature depression. The simulated temperature perturbation is indeed associated with

394 a significant dew point temperature depression (Fig.15, middle column). At 0900 UTC 20 June,

395 the dew point temperature depression is smaller than 2 °C over most of the domain and does not

396 display a coherent spatial organization. In response to the persistent subsidence, the dew point

397 temperature depression increases near the center of the disturbance and exceeds 8 °C at 1800 UTC.

398 A primary objective of this study is to quantify the direct contribution of the subsidence warming

399 to the TC genesis inManuscript Cindy. To estimate how much of the total pressure perturbation is accounted

400 for by the low-level warm anomaly, we compute the hydrostatic pressure perturbation by integrating

401 Equation 1 for two atmospheric layers: i) the SLP pressure perturbation is computed by integrating

402 Eq.1 from the surface to the top of the atmosphere, ii) the pressure perturbation due to the low-level

403 warming is estimated by integrating Eq.1 from 1500 m to 3500 m. As shown in Fig.15 (right

404 column), the genesis of TS Cindy is associated with a deepening of the SLP perturbation: as the

405 storm acquires a more axisymmetric look, the total SLP perturbation grows from -3 hPa at 0900

406 UTC 20 June to -6 hPa at 1800 UTC 20 June and -7 hPa at 0000 UTC 21 June. This 4 hPa drop

407 from is consistent with the deepening of the SLP minimum displayed in Fig.10b. The pressure

408 perturbation due to the subsidence-induced warm anomaly during TC genesis is shown by the black

409 contours in figure 15. At 0900 UTC 20 June, its contribution is very limited. As the warming

410 occurs, the pressure perturbation in the 1500 - 3500 layer grows up to 3.2 hPa. At 1800 UTC 20

411 June it accounts for more than 50 % of the total perturbation. As most of the warming occurs prior

412 to 1800 UTC 20 June, the contribution of the 1500-3500 m layer does not grow significantly further

413 and remains approximately 3 hPa. Throughout the genesis, the 1500-3500 m pressure perturbation

19 414 is closely colocated with the position of the SLP minimum and contributes significantly to the

415 deepening and axisymmetrization of the low-pressure system.

416 9. Summary and Conclusion

417 The presence of subsidence-induced low-level warm anomalies has often been linked to a weaken-

418 ing of tropical disturbances due to suppressed convection and mixing of lower equivalent potential

419 temperature air into the eyewall (Shelton and Molinari 2009). It is usually argued that subsidence

420 is due to the interaction between its circulation and the environmental flow (Heymsfield et al.

421 2006; Halverson et al.Manuscript 2006) and results from the differential advection of cyclonic vorticity by the

422 environmental flow. Conversely, this study builds on Stossmeister and Barnes (1992), Dolling and

423 Barnes (2012b), Dolling and Barnes (2012a) and Kerns and Chen (2015) to argue that subsidence-

424 induced low-level warming can be a key process supporting tropical cyclogenesis. In this study

425 we investigate the genesis of TS Cindy from a broad, cyclonic circulation that moved from the

426 Caribbean Sea into the southern Gulf of Mexico (Fig.1), in an environment characterized by high

427 vertical wind shear and low mid-tropospheric moisture.

428 Throughout the genesis process, the structure of the disturbance changed from an initially shallow,

429 broad, asymmetric vortex (Fig.16a) to a vertically aligned, axisymmetric cyclone with a tighter

430 eyewall-like feature around its warm core (Fig.16c). Both the CPEX observations and UWIN-CM

431 modeling indicate that the subsidence-induced low-level warming occurred and persisted within

432 the shallow cyclonic disturbance as it organized into a TS. The temperature perturbation estimated

433 to be between 3.8 °C and 6 °C respectively and maximized in the atmospheric layer between 1500-

434 3500 m. The vertical location of the subsidence-induced warm anomaly in TS Cindy is consistent

435 with what previous studies have found in other developing TCs (Halverson et al. 2006; Heymsfield

436 et al. 2006; Dolling and Barnes 2012a). Its magnitude is also comparable to that observed by

20 437 Stossmeister and Barnes (1992) in TS Isabel (1982), Heymsfield et al. (2006) in TS Chantal,

438 by Dolling and Barnes (2012a) in hurricane Humberto and by Shelton and Molinari (2009) in

439 hurricane Claudette.

440 Backward trajectories calculated from the UWIN-CM simulation indicate that subsidence is

441 focused in the western flank of the disturbance (Fig.16b). Air parcels located in the northern sector

442 of the storm at an elevation between 3-3.5 km start to descend as they exit a precipitating region and

−1 443 move into a drier environment. The subsidence rates are calculated to be between -6 cm s and

−1 444 -7.6 cm s . By integrating the hydrostatic equation we estimate that the lower tropospheric warm

445 anomaly accounts forManuscript approximately 3 hPa or 50% of the overall SLP depression when the storm

446 was classified as TS by the NHC. Such a pressure perturbation is also consistent with previous

447 studies. Stossmeister and Barnes (1992) estimated from TS Isabel that a 3-4 km deep layer, having

448 a temperature perturbation of 2 to 3 °C, would result in a pressure perturbation of 2 hPa. Similarly,

449 Dolling and Barnes (2012a) calculated that a 7 °C temperature perturbation would result in a 5 hPa

450 pressure perturbation.

451 We describe how subsidence can produce a significant pressure perturbation in a disturbance

452 vastly different from a typical long-lived MCS, one where a pre-existing, shallow and broad

453 cyclonic circulation is present but lacks the spatial organization required for TC classification. Due

454 to the shallow nature of the cyclonic disturbance during TC genesis, the deep-layer shear did not

455 produce an appreciable vortex tilt in TS Cindy, as confirmed by both the observed and modeled

456 storm structure, hence the subsidence cannot be interpreted as a response to vortex tilting. We

457 suggest instead that the subsidence results from two processes. The parcels initiate their descent

458 underneath the anvil clouds associated with the convection on the northern flank of the storm,

459 there the evaporation of hydrometeors can initiate the subsidence, as described by Houze (1977),

460 Zipser (1977) and Chen and Frank (1993). As the parcels move away from the precipitation, the

21 461 subsidence is supported by a thermally indirect circulation linked to an area of weak frontolysis

462 along the western flank of the storm. A similar process has been proposed as the driving mechanism

463 behind sting jets in deep, marine extratropical cyclones (Schultz and Sienkiewicz 2013; Martínez-

464 Alvarado et al. 2014; Coronel et al. 2016). We acknowledge that our analysis does not rule out

465 possible concurring processes that might drive the subsidence.

466 TC genesis events, especially in the early part of the hurricane season or in the subtropics often

467 feature environments characterized by large wind shear, dry mid-tropospheric air and horizontal

468 temperature gradients in the lower troposphere. It is therefore possible that tropical or subtropical

469 disturbances embeddedManuscript in these environments could undergo a similar physical process. This

470 study thus allows us to better understand the occurrence of subsidence during the genesis of TCs

471 in unfavorable environments by providing a comprehensive analysis of the subsidence-induced

472 temperature, humidity and pressure anomalies that could guide more targeted aircraft observations

473 during future events. Additional studies are needed to understand how frequently this process is

474 observed and its storm-to-storm variability.

475 Acknowledgments. We thank the CPEX science team for their support during the field campaign,

476 especially Dr. G. D. Emmitt and Mr. S. Greco for providing the DAWN wind data. The authors

477 are thankful to Dr. Ed Zipser and two anonymous reviewers’ whose constructive comments and

478 suggestions helped improve the manuscript. This research was supported by two NASA research

479 grants, CPEX (80NSSC18K0185) and CYGNSS (80NSSC18K0713).

480 Data availability statement. All the CPEX aircraft observations used in this study are publicly

481 available on the CPEX data repository (https://tcis.jpl.nasa.gov/data/cpex/). The

482 ERA-5 reanalysis data can be obtained from the CDS repository (DOI:10.24381/cds.bd0915c6).

22 483 Access to the UWIN-CM simulation output along with the analysis codes on the Prof. Chen’s

484 group server can be easily obtained by contacting the corresponding author ([email protected]).

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32 693 LIST OF FIGURES

694 Fig. 1. Evolution of TS Cindy as observed by the GOES-East IR temperature at a) 00 UTC 18 June, 695 b) 1430 UTC 19 June, c) 1800 UTC 20 June, and d) 1530 UTC 21 June 2017...... 35

696 Fig. 2. CPEX flight tracks (magenta line) and dropsondes location (black markers) for the CPEX 697 missions on 20 June (a), 21 June (b). GOES-East visible imagery at 1945 UTC 20 June (a) 698 and at 2000 UTC 21 June (b)...... 36

699 Fig. 3. UWIN-CM simulated SST (color, °C) at 1800 UTC on 20 June. The inner nested WRF grids 700 of 4-km (solid) and 1.3-km (dashed) are marked. Note that 1.3-km grid is storm-following 701 during the simulation. The simulated track of TS Cindy (solid blue line) is compared to the 702 Best-Track (solid black line), with dots marking every 12 hours. The dashed lines denote the 703 tracks prior to TS genesis...... 37

704 Fig. 4. a) TS Cindy intensity: UWIN-CM simulation (blue) and Best-Track (black). The dashed 705 lines denote the intensity of TS Cindy prior to genesis. The TC genesis time according 706 to the NHC is marked by the vertical black dashed line. The June 20th and 21st CPEX 707 missions are denoted by the grey shaded strips. The landfall times are marked by the vertical 708 Manuscript dashed-dotted lines. b) 200-850 hPa wind shear: UWIN-CMs simulation (blue) and ERA5 709 reanalysis (black)...... 38

710 Fig. 5. GOES-East brightness temperature at 1930 UTC and dropsondes locations during the CPEX 711 flight on 20 June.. White and magenta barbs denote the 980 hPa and 400 hPa wind respec- 712 tively. The markers are color-coded according to the 800 hPa temperature. Square markers 713 denote dropsondes characterized by subsidence. The white cross denotes the TS center from 714 the NHC at 2000 UTC 20 June...... 39

715 Fig. 6. Observed skew T-log(p) diagrams from the dropsondes launched during the CPEX mission 716 on 20 June 2017.: a) within 100 km from storm center. The mean profile is shown by the 717 thick lines. b) outside of 100 km from the storm center. c) Vertical profile of the temperature 718 perturbation within TS Cindy...... 40

719 Fig. 7. Fig, 7, Same as in Fig. 6, except for the 21 June 2017 CPEX mission...... 41

720 Fig. 8. UWIN-CM simulated a) time-height diagram of maximum temperature perturbation within 721 100 km from the center of TS Cindy. b) Time series of simulated minimum SLP. The time of 722 TS classification by the NHC is marked by the vertical dashed line. The gray shaded strips 723 denote the duration of the 20 June, 21 June CPEX missions...... 42

724 Fig. 9. UWIN-CM simulated kinematic structure of TS Cindy: pre-subsidence stage (08 UTC 20 725 June - a,b), subsidence stage (19 UTC 20 June - c, d), post-subsidence phase (19 UTC 21 726 June - e, f). Panels a, c, e show SLP contours (black), 200-850 hPa wind shear (color) and 727 the cross section paths. Panels b, d, f show the horizontal wind speed (shaded) and wind 728 vectors along the corresponding cross sections...... 43

729 Fig. 10. DAWN and dropsondes observed horizontal wind speed (colors) and direction (vectors) cross 730 sections: a, c) 1855 - 1922 UTC flight leg from the 20 June CPEX mission, b, d) 1950 - 2022 731 UTC flight leg from the 21 June CPEX mission. The location of the dropsondes is marked by 732 the triangle markers. The cross-sections are displayed from west to east. The corresponding 733 flight leg path and GOES-East IR brightness temperature are provided in the left column. 734 The NHC 2-minute center fix at the time of the flight leg is denoted by the white cross marker . 44

33 735 Fig. 11. UWIN-CM simulated SLP (contours) and temperature (colors) at 2500, 2100, 1700 m. The 736 white dots denote the initial location of the parcels at their respective vertical levels. All 737 backward trajectories are initialized at 1800 UTC 20 June...... 45

738 Fig. 12. Storm-relative parcel trajectories from 1000 to 1800 UTC on 20 June 2017 and cloud ice −1 739 mixing ratio 0.0001 gkg isosurface (grey shaded). The trajecotries are color-coded by 740 temperature in a) and temperature perturbation in b)...... 46

741 Fig. 13. UWIN-CM simulated cloud-top temperature (colors), SLP (contours) and parcels locations 742 at: a) 1200 UTC 20 June, b) 1800 UTC 20 June. The white dots with magenta edges denote 743 the parcel positions corresponding to the initial and end times of the subsidence shown in 744 Fig. 12...... 47

745 Fig. 14. UWIN-CM simulated a) 2-2.5 km average 2D kinematic frontogenesis (shaded) at 1500 UTC 746 20 June and SLP (black contours). b) 2D kinematic frontogenesis (colors, only negative 747 values are plotted) and vertical velocty (contours) along the XY cross section. The vertical 748 velocity and the frontogensis fields have been smoothed by 4 passes of a 9-point filter for 749 visual purposes.Manuscript The white dots with magenta edges denote the location of the parcels. . . . 48 750 Fig. 15. UWIN-CM simulated evolution of: left panels) SLP (contours) and 2500 m temperature 751 (colors); middle panels) SLP (contours) and 2500 m dew point temperature depression 752 (colors); right panels) hydrostatic pressure perturbation for the full atmosphere (colors) and 753 for the 1500-3500 m layer (black contours, only negative values are plotted)...... 49

754 Fig. 16. Schematic of the TC genesis of TS Cindy: a) pre-subsidence phase: interaction between the 755 cyclonic disturbance with an upper-level trough in strong SW shear; b) during genesis phase: 756 subsidence along the upshear flank of the storm produces a lower tropospheric temperature 757 anomaly (W); c) post-subsidence phase: TS Cindy has a tighter cyclonic circulation with a 758 deeper warm core (W) ...... 50

34 Manuscript

759 Fig. 1. Evolution of TS Cindy as observed by the GOES-East IR temperature at a) 00 UTC 18 June, b) 1430

760 UTC 19 June, c) 1800 UTC 20 June, and d) 1530 UTC 21 June 2017.

35 Manuscript

761 Fig. 2. CPEX flight tracks (magenta line) and dropsondes location (black markers) for the CPEX missions on

762 20 June (a), 21 June (b). GOES-East visible imagery at 1945 UTC 20 June (a) and at 2000 UTC 21 June (b).

36 Manuscript

763 Fig. 3. UWIN-CM simulated SST (color, °C) at 1800 UTC on 20 June. The inner nested WRF grids of 4-km

764 (solid) and 1.3-km (dashed) are marked. Note that 1.3-km grid is storm-following during the simulation. The

765 simulated track of TS Cindy (solid blue line) is compared to the Best-Track (solid black line), with dots marking

766 every 12 hours. The dashed lines denote the tracks prior to TS genesis.

37 Manuscript

767 Fig. 4. a) TS Cindy intensity: UWIN-CM simulation (blue) and Best-Track (black). The dashed lines denote

768 the intensity of TS Cindy prior to genesis. The TC genesis time according to the NHC is marked by the vertical

769 black dashed line. The June 20th and 21st CPEX missions are denoted by the grey shaded strips. The landfall

770 times are marked by the vertical dashed-dotted lines. b) 200-850 hPa wind shear: UWIN-CMs simulation (blue)

771 and ERA5 reanalysis (black).

38 Manuscript

772 Fig. 5. GOES-East brightness temperature at 1930 UTC and dropsondes locations during the CPEX flight on

773 20 June.. White and magenta barbs denote the 980 hPa and 400 hPa wind respectively. The markers are color-

774 coded according to the 800 hPa temperature. Square markers denote dropsondes characterized by subsidence.

775 The white cross denotes the TS center from the NHC at 2000 UTC 20 June.

39 Manuscript

776 Fig. 6. Observed skew T-log(p) diagrams from the dropsondes launched during the CPEX mission on 20 June

777 2017.: a) within 100 km from storm center. The mean profile is shown by the thick lines. b) outside of 100 km

778 from the storm center. c) Vertical profile of the temperature perturbation within TS Cindy.

40 Manuscript

Fig. 7. Fig, 7, Same as in Fig. 6, except for the 21 June 2017 CPEX mission.

41 Manuscript

779 Fig. 8. UWIN-CM simulated a) time-height diagram of maximum temperature perturbation within 100 km

780 from the center of TS Cindy. b) Time series of simulated minimum SLP. The time of TS classification by the

781 NHC is marked by the vertical dashed line. The gray shaded strips denote the duration of the 20 June, 21 June

782 CPEX missions.

42 Manuscript

783 Fig. 9. UWIN-CM simulated kinematic structure of TS Cindy: pre-subsidence stage (08 UTC 20 June - a,b),

784 subsidence stage (19 UTC 20 June - c, d), post-subsidence phase (19 UTC 21 June - e, f). Panels a, c, e show SLP

785 contours (black), 200-850 hPa wind shear (color) and the cross section paths. Panels b, d, f show the horizontal

786 wind speed (shaded) and wind vectors along the corresponding cross sections. 43 Manuscript

787 Fig. 10. DAWN and dropsondes observed horizontal wind speed (colors) and direction (vectors) cross sections:

788 a, c) 1855 - 1922 UTC flight leg from the 20 June CPEX mission, b, d) 1950 - 2022 UTC flight leg from the 21

789 June CPEX mission. The location of the dropsondes is marked by the triangle markers. The cross-sections are

790 displayed from west to east. The corresponding flight leg path and GOES-East IR brightness temperature are

791 provided in the left column. The NHC 2-minute center fix at the time of the flight leg is denoted by the white

792 cross marker

44 793 Fig. 11. UWIN-CM simulated SLP (contours) and temperature (colors) at 2500, 2100, 1700 m. The white

794 dots denote the initial location of the parcels at their respective vertical levels. All backward trajectories are

795 initialized at 1800 UTC 20 June. Manuscript

45 796 Fig. 12. Storm-relativeManuscript parcel trajectories from 1000 to 1800 UTC on 20 June 2017 and cloud ice mixing ratio −1 797 0.0001 gkg isosurface (grey shaded). The trajecotries are color-coded by temperature in a) and temperature

798 perturbation in b).

46 Manuscript

799 Fig. 13. UWIN-CM simulated cloud-top temperature (colors), SLP (contours) and parcels locations at: a)

800 1200 UTC 20 June, b) 1800 UTC 20 June. The white dots with magenta edges denote the parcel positions

801 corresponding to the initial and end times of the subsidence shown in Fig. 12.

47 802 Fig. 14. UWIN-CM simulated a) 2-2.5 km average 2D kinematic frontogenesis (shaded) at 1500 UTC 20 803 June and SLP (black contours).Manuscript b) 2D kinematic frontogenesis (colors, only negative values are plotted) and 804 vertical velocty (contours) along the XY cross section. The vertical velocity and the frontogensis fields have

805 been smoothed by 4 passes of a 9-point filter for visual purposes. The white dots with magenta edges denote the

806 location of the parcels.

48 Manuscript

807 Fig. 15. UWIN-CM simulated evolution of: left panels) SLP (contours) and 2500 m temperature (colors);

808 middle panels) SLP (contours) and 2500 m dew point temperature depression (colors); right panels) hydrostatic

809 pressure perturbation for the full atmosphere (colors) and for the 1500-3500 m layer (black contours, only negative

810 values are plotted).

49 Manuscript

811 Fig. 16. Schematic of the TC genesis of TS Cindy: a) pre-subsidence phase: interaction between the cyclonic

812 disturbance with an upper-level trough in strong SW shear; b) during genesis phase: subsidence along the upshear

813 flank of the storm produces a lower tropospheric temperature anomaly (W); c) post-subsidence phase: TS Cindy

814 has a tighter cyclonic circulation with a deeper warm core (W)

50