Impacts of Frontal SST Gradient on the Formation of Axially

1 Impacts of frontal SST gradient on the formation of axially

2 asymmetric thermal structure of a tropical cyclone:

3 A case study of a typhoon in the East China Sea

5 Fukiko Takehi, * Hisashi Nakamura, † and Takafumi Miyasaka

6 Research Center for Advanced Science and Technology, University of Tokyo, Tokyo, Japan

7 and

8 Mayumi K. Yoshioka

9 Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan

11 Submitted to Monthly Weather Review in April 2015, as a potential contribution to

12 the Special Collection “Climate Implications of Frontal-Scale Air-Sea Interaction”

13 Revised in September, 2015

14 15 16 17 *Corresponding author address: Fukiko Takehi, Research Center for Advanced Science and 18 Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo, 153-8904, Japan. 19 E-mail: [email protected] 20 Current affiliation: Office of Observation Systems Operation, Japan Meteorological Agency, 21 Tokyo, Japan 22 †Additional affiliation: APL, JAMSTEC, Yokohama, Japan 23

24 Abstract

25 A tropical cyclone (TC) is known to undergo substantial modifications in its thermal

26 structure as it approaches a deep baroclinic zone associated with a midlatitude westerly jet,

27 which is often collocated with an oceanic frontal zone (OFZ) with sharp meridional gradient in

28 sea-surface temperature (SST). This collocation often makes it difficult to isolate an influence, if

29 any, of the frontal SST gradient from that of a jet-associated free-tropospheric baroclinic zone

30 on the formation of axially asymmetric thermal structure of a TC in its extratropical transition.

31 The present study makes the first attempt to isolate the former influence by focusing on a

32 particular typhoon (Songda) that approached a prominent OFZ in the southern East China Sea

33 located far south of a midlatitude westerly jet. An investigation based on high-resolution

34 regional atmospheric analysis reveals that axially asymmetric thermal structure first emerged in

35 the planetary boundary layer as the typhoon approached the OFZ well before the corresponding

36 structure reached the mid-troposphere around the westerly jet. Thermodynamic analysis

37 indicates that a near-surface cool anomaly that constituted the axial asymmetry was generated to

38 the west of the typhoon center through cold advection largely by the strong northerlies across a

39 near-surface baroclinic zone along the frontal SST gradient. A set of experiments with a

40 cloud-resolving atmospheric model with different intensities of SST gradient confirms the

41 importance of the frontal SST gradient in enhancing the near-surface cold advection.

42 1. Introduction

43 It is well known that a tropical cyclone (TC) tends to transform itself into a

44 midlatitude weather system, as it approaches a midlatitude baroclinic zone associated with a

45 westerly jet. In this process called “extratropical transition (ET)” (Jones et al. 2003; Harr 2010;

46 Kitabatake 2012), a TC undergoes structural changes, including the formation of surface fronts

47 and associated heavy precipitation systems, in addition to expansion of the area of storm-force

48 winds and localized gusts (Kitabatake and Bessho 2008; Kitabatake 2012). In fact, some of the

49 decaying TCs evolve into rapidly developing extratropical cyclones (Jones et al. 2003; Harr

50 2010). In addition to its usefulness for disaster mitigation and prevention, understanding the ET

51 process itself is scientifically intriguing as a transformation process from a tropical weather

52 system into a midlatitude one.

53 Previous studies focused primarily on particular ET processes occurring around a deep

54 baroclinic zone associated with an upper-tropospheric westerly jet (Klein et al. 2000; Ritchie

55 and Elsberry 2001; Kitabatake et al. 2007; Kitabatake 2008). Through analysis of satellite

56 imageries and output data from the Navy Operational Global Atmospheric Prediction System,

57 Klein et al. (2000) proposed a conceptual model for ET interactions influenced by both a

58 near-surface baroclinic zone and a midlatitude westerly jet, which were then verified by Ritchie

59 and Elsberry (2001) through idealized experiments. Operationally, ET is defined rather

60 subjectively by using satellite imagery and other observations available (Kitabatake 2008). In

61 fact, Jones et al. (2003) pointed out that there is no universal definition of ET. Evans and Hart

62 (2003) nevertheless proposed an objective parameter for the ET onset that measures axial

63 asymmetries in thermal structure of a storm. This parameter was defined as the asymmetry of

64 thickness between the 900 and 600-hPa levels measured in the direction perpendicular to the

65 instantaneous storm motion, and the value of this parameter is supposed to increase during ET.

66 In the evaluation of this parameter, however, thermal asymmetry in the near-surface layer below

67 the 900-hPa level is not included, despite the formation of 925-hPa thermal asymmetry tends to

68 precede that at the 600-hPa level in the composite of 274 TCs observed over the western North

69 Pacific (WNP) by Kitabatake (2011).

70 It is also known that high sea-surface temperature (SST) over 26°C is necessary for the

71 generation and maintenance of TCs (e.g., Gray 1975; Emanuel 1986), and a role of SST on TCs

72 in the ET process has been examined. For example, a numerical experiment by Ritchie and

73 Elsberry (2001) with idealized SST distribution elucidates how the lowering of SST into the

74 midlatitudes affects a TC at the initiation of its ET. Specifically, reduction in heat and moisture

75 supply from the ocean leads to the weakening of deep convection within the inner core of a TC.

76 Its warm core thus weakened becomes tilted under the vertically sheared westerlies, enhancing

77 axial asymmetries in convective precipitation. On the basis of global reanalysis data, Kitabatake

78 (2011) found that ET of a TC over the WNP (i.e., typhoon) is often completed within a

79 midlatitude baroclinic zone characterized by strong westerly shear between the 925 and 200-hPa

80 levels, including a warm maritime region where SST exceeds 24°C.

81 In the midlatitude ocean there are regions referred to as “oceanic frontal zones

82 (OFZs)”, where warm and cool currents are confluent to enhance SST gradient locally. From a

83 potential-vorticity perspective (Hoskins et al. 1985), near-surface baroclinicity associated, for

84 example, with surface air temperature (SAT) gradient is essential for baroclinic development of

85 extratropical cyclones. Recent studies have revealed that a sharp decline in sensible heat supply

86 from the ocean across an OFZ efficiently restores SAT gradient against the relaxing effect by

87 extratropical cyclones to allow their recurrent development (Nakamura et al. 2004; Taguchi et al.

88 2009; Hotta and Nakamura 2011). For individual cyclones the sensible heat exchange with the

89 ocean acts as thermal damping, while moisture supply from the warm current is important for

90 their growth (Nakamura et al. 2004). Therefore, sharp SST gradient across an OFZ can

91 influence a TC during its ET and its redevelopment as an extratropical cyclone. In fact, it has

92 been pointed out that frontal SST gradient is one of the environmental factors that can affect

93 structural transformation of a TC during its ET (Fig. 11 in Jones et al. 2003). Through numerical

94 experiments with different SST conditions around the Kuroshio Extension (KE) east of Japan,

95 Wada et al. (2013) suggested the influence of an OFZ that modulates the thermal structure of

96 Typhoon “Choi-wan” in the course of its ET. They found the influence of the OFZ within the

97 planetary boundary layer (PBL) outside of the inner-core region of the TC, in addition to

98 possible influence of a nearby stationary rain front. Still, specific impacts of frontal SST

99 gradients have not been fully clarified, because most of the previous studies investigated the ET

100 process under a baroclinic zone associated with a westerly jet.

101 This study attempts to identify the impacts of a frontal SST gradient through the

102 investigation of a particular TC that approached a well-defined OFZ, located far south of a

103 westerly jet. There is a tendency for an eddy-driven westerly jet to be collocated with a

104 midlatitude OFZ (Nakamura et al. 2004), which makes it difficult to distinguish the role of a

105 near-surface baroclinicity associated with an OFZ in the ET process from that of

106 free-tropospheric baroclinicity associated with the westerlies. In the KE region, for example, the

107 collocation of the westerlies with a prominent OFZ frequently occurs except in August (Sampe

108 and Xie 2010). Thus it is not often that a typhoon encounters frontal SST gradient associated

109 with the KE prior to its interaction with a westerly jet. In fact, when the typhoon Choiwan

110 reached the KE region on September 20, 2009, 200-hPa westerly wind speed exceeded 25 m s–1

111 (not shown). Although the August situation appears to be suited for our purpose, high-resolution

112 data necessary for capturing the structure of TCs away from landmasses are severely limited in

113 the KE region. A high-resolution data set provided by the Japan Meteorological Agency (JMA;

114 see section 2 for details) is not available east of 150°E. To avoid any serious influence from the

115 main island of Japan, we can select only those typhoons whose centers moved around 145°E for

116 our analysis. Unfortunately, no such typhoon was observed in August during the period since

117 2007 in which the particular data set is available. Furthermore, no operational radiosonde

118 observations are carried out in the KE.

119 To circumvent these difficulties, the present study focuses on Typhoon “Songda”,

120 which developed in early summer of 2011 and then approached a well-defined OFZ in the

121 southern East China Sea (ECS), which is located far south of the westerly jet. This situation

122 provided a unique opportunity for us to extract direct influence of the OFZ on the typhoon

123 before affected by the westerlies and associated deep baroclinic zone. Utilizing a dataset of

124 regional meso-scale objective analysis available only for the vicinity of Japan, we reveal how

125 important the OFZ was in the formation of zonally asymmetric thermal structure in the

126 near-surface layer of the TC at the initial stage of its ET. Then, the particular importance of the

127 OFZ is assessed through a set of sensitivity experiments with a cloud-resolving regional

128 atmospheric model with different SST distributions prescribed as the model lower-boundary

129 condition. It should be stressed that the purpose of the present study is to clarify the near-surface

130 structural changes of the particular TC occurring under the direct influence of frontal SST

131 gradient as an initiation of its ET but not to examine the entire course of the ET.

132 The rest of this paper is organized as follows. Section 2 describes the meso-scale

133 analysis and our numerical experiments. An overview of Songda is provided in section 3.

134 Section 4 describes time evolution of the asymmetric thermal structure of Songda, whose

135 formation is diagnosed thermodynamically in section 5. Results of our numerical experiments

136 are shown in section 6, before further discussions and a summary of the present study are given

137 in section 7.

138

139 2. Data and Model Experiments

140 a. Data

141 In the present study, three-hourly analysis and hourly precipitation forecast by the

142 JMA Meso-Scale Model (JMA-MSM; JMA 2007) are utilized as reference data. The data were

143 obtained from the Research Institute for Sustainable Humanosphere (RISH) of Kyoto University

144 (http://database.rish.kyoto-u.ac.jp/arch/glob-atmos/). Within the domain [22.4 ° -47.6 ° N,

145 120°-150°E], the data are available on a 0.125° ×0.1° grid at each of the 16 pressure levels,

146 while precipitation and surface meteorological data are provided on a 0.0625° ×0.05° grid.

147 Among the 16 pressure levels, six levels are located below the 850-hPa level. In order to avoid

148 using any data extrapolated onto pressure levels below the surface, especially in the vicinity of a

149 TC center, we interpolated the JMA-MSM data from the pressure levels onto 19 height levels

150 with the aid of the surface data. Among the 19 levels, nine levels are below the 850-hPa level

151 and the 10 other levels roughly correspond to the 10 pressure levels of the JMA-MSM analysis

152 between the 800-hPa and 100-hPa levels. The data thus interpolated is convenient for

153 comparison with output data from our numerical experiments available on height coordinates.

154 Merged satellite and in-situ data Global Daily Sea Surface Temperature (MGDSST;

155 JMA 2007), used for the lower-boundary condition for JMA-MSM, is also utilized for our

156 analysis. The SST data originally on a 0.25° ×0.25° grid have been interpolated on the model

157 grid of JMA-MSM. Turbulent sensible heat flux from the sea surface was estimated locally, by

158 applying the COARE 3.0 algorithm (Fairall et al. 2003) to the MGDSST data and surface

159 meteorological valuables based on the JMA-MSM analysis.

160 To verify that the JMA-MSM analysis reproduces actual thermal structure of typhoon

161 Songda, we analyze operational radiosonde soundings conducted at Ishigaki-jima [24.33°N,

162 124.17°E] and Minami-daito-jima [25.83°N, 131.23°E] stations by JMA. The data were

163 obtained from the JMA website (http://www.data.jma.go.jp/obd/stats/etrn/upper/index.php).

164

165 b. Model experiments

166 To assess the impacts of strong SST gradient on structural changes of Songda, a cloud

167 resolving regional atmospheric model called Cloud Resolving Storm Simulator (CReSS)

168 (Tsuboki and Sakakibara 2002, 2007) is utilized for our numerical experiments. In this

169 non-hydrostatic model developed at the Hydrospheric Atmospheric Research Center of Nagoya

170 University, the basic equations with various physical parameterization schemes are solved in the

171 terrain-following coordinates. In the present study, the model domain is set for [11.02°-41.7°N,

172 116.02°-146.7°E] (Fig. 1). This setting of the model domain does not seem to severely affect our

173 analysis that focuses on the structure of the storm approaching the oceanic frontal zone in the

174 southern East China Sea. The horizontal resolution is 0.04° in both latitude and longitude, with

175 64 height levels, including nine levels below 1.5 km in altitude. The three lowest levels are at

176 50m, 160m, and 290m in altitude. The interval between two adjacent vertical levels is

177 interpolated with a cubic function. The rigid boundary is assumed for the uppermost model level

178 that is set at above 20 km.

179 Convection is represented explicitly with no cumulus parameterization, and a bulk

180 cold-rain microphysical scheme is employed. Subgrid-scale turbulence is parameterized with a

181 1.5-order closure scheme for turbulent kinetic energy (TKE), which is similar to the

182 Mellor-Yamada level-2 scheme (Mellor and Yamada 1974). Surface momentum and

183 sensible/latent heat fluxes are estimated with bulk formulae, based on 10-m wind speed, and

184 potential temperature and mixing ratio of water vapor at the 10-m level and the surface. Ground

185 surface temperature is calculated through bulk formulae (Louis et al. 1981) and a

186 one-dimensional thermal diffusion equation, while SST is prescribed as the boundary condition

187 as described below. Since CReSS is supposed to be integrated only for a few days, radiation

188 processes are included only for the heat balance at the surface.

189 The initial and lateral boundary conditions for our CReSS experiments were taken

190 from the JMA Global Spectral Model analysis. The six-hourly data on a 0.5° ×0.5° grid were

191 available at the RISH website. No bogus vortex was adopted in the initial condition for our

192 CReSS simulations. The integration period was from 0000 UTC on 24 May to 0000 UTC on 01

193 June 2011, and the hourly output was analyzed. The integration period longer than a week is

194 necessary to simulate the background state where the persistent impact of the OFZ is included.

195 It takes a couple of days a TC to spin up in the CReSS model.

196 A control experiment (CTL) with CReSS was conducted with high-resolution SST at

197 0000 UTC 24 May 2011 prescribed as the lower boundary condition (Fig. 1). The SST data with

198 resolution of 1/12° in both latitude and longitude was based on the JCOPE2 (Japan Coastal

199 Ocean Prediction Experiment 2) reanalysis (Miyazawa et al. 2009), into which both satellite and

200 in-situ measurements had been assimilated. In addition, two sensitivity experiments were

201 conducted with artificially modified SST fields. One of them is referred to as

202 “meridionally-smoothed experiment” (or MSMTH), which was carried out with a particular

203 SST distribution that had been obtained by applying meridional running mean with 10° width

204 to the distribution used for the CTL experiment (Taguchi et al. 2009). The other is referred to as

205 “smoothed experiment” (or SMTH), which was conducted with another SST distribution that

206 had been obtained by further applying zonal averaging to the smoothed distribution used for the

207 MSMTH experiment.

208

209 3. Overview of typhoon Songda (TY-1102)

210 According to the best track data of JMA, the disturbance that developed into Songda

211 (TY-1102) was first identified as a tropical depression at [8.3°N, 141.6°E] on 19 May 2011. In

212 moving northwestward, it attained its maximum intensity at 0600 UTC on 26 May with its

213 central pressure as low as 920 hPa and sustained wind of 105 kt. This peak intensity was

214 retained until 0000 UTC on 27 May. After reaching to the east of Taiwan, Songda moved

215 northeastward in increasing its transfer speed and losing its intensity (Fig. 2a). Reaching the

216 vicinity of the Baiu/Meiyu front (Fig. 2b), Songda completed its ET off the Shikoku Island of

217 Japan at 0600 UTC on 29 May.

218 The completion of ET is evident in an infrared (IR) cloud image for 0000 UTC on 29

219 May (Fig. 3b). It clearly indicates that convective activity was almost diminished around the

220 cyclone center, while cloud bands corresponding to warm and cold fronts (Fig. 2b) were more

221 evident. At 1200 UTC on 28 May, when Songda was moving over the southern ECS, cloudiness

222 in its outer region is less enhanced in the western and southern sectors than in the eastern and

223 northern sectors (Fig. 3a), as a characteristic of the initial stage of ET (Klein et al. 2000).

224 Figure 4 shows the track of Songda from 0000 UTC on 28 May, when it entered the

225 domain of the JMA-MSM analysis, to 0600 UTC on 29 May, superimposed on the MGDSST

226 and 200-hPa wind both averaged over the seven-day period from 0000 UTC on 25 May to 0000

227 UTC on 01 June. At each time step, the cyclone center is defined as a particular JMA-MSM grid

228 point at which the JMA-MSM surface pressure minimizes. Figure 4 indicates that the track

229 based on our definition well corresponds to the best track by JMA as shown in Fig. 1. Figure 4

230 also indicates that Songda moved just to the southeast of an OFZ around 27°N, where

231 meridional SST gradient is particularly strong. The shallow ECS over a continental shelf is

232 cooled strongly by the winter monsoon, generating sharp SST gradient with the warm Kuroshio

233 (Xie et al. 2002). Though warming rapidly into midsummer, SST over the ECS in late May is

234 still substantially lower than that along the Kuroshio (Fig. 4). Songda was closest to the OFZ

235 between 0900 and 1500 UTC on May 28. At that time, Songda was located away from

236 landmasses and far south of the free-tropospheric baroclinic zone below the upper-tropospheric

237 subtropical westerly jet (Fig. 4) associated with the surface Baiu/Meiyu Front (Fig. 2a).

238 As shown in Fig. 5a, the central pressure of the typhoon at 0900 UTC on 28 May

239 based on the JMA-MSM analysis is 10 hPa higher than that based on the JMA’s best track data.

240 Nevertheless, 925-hPa wind speeds based on the JMA-MSM analysis are 17.8 and 25.6 m s-1 at

241 the grid points closest to the Ishigaki-jima and Minami-daito-jima stations, respectively. These

242 values are reasonable if compared with the corresponding observations of 17 and 25 m s-1 by

243 radiosonde soundings at the respective stations at 1200 UTC on the same day. This suggests that

244 the JMA-MSM model analysis can reproduce the typhoon reasonably well around 28 May and

245 it is useful for our analysis shown in the following sections.

246

247 4. Time evolution of thermal structure of Songda

248 In this section, we describe the time evolution of thermal structure of Songda utilizing

249 the JMA-MSM analysis. Following Hart (2003), we first examine the lower portion of the free

250 troposphere, focusing on 900-600 hPa thickness (Fig. 6). At 1200 UTC on 28 May (Fig. 6a),

251 when Songda approached the OFZ, the thickness field was nearly axisymmetric, although it was

252 slightly cooler in the northwestern outer region of the TC and its warm-core was slightly

253 displaced southward from its surface pressure center. By 0000 UTC on 29 May (Fig. 6b), when

254 the TC approached the westerly jet, the thermal asymmetry became obvious with cool air

255 intruding into the western side of the TC center. These results confirm the importance of the

256 free-tropospheric baroclinicity associated with the westerly jet for the formation of axially

257 asymmetric thermal structure of a TC, in agreement with the previous studies. In fact, potential

258 temperature ( ) distribution at the level of 4,135 m (equivalent to the 600-hPa level) was nearly

𝜃𝜃 259 axisymmetric at 1200 UTC on 28 May (Fig. 6c), when the TC was south of the westerly jet.

260 At the same time (Fig. 6d), however, the corresponding distribution at the level of

𝜃𝜃 261 981 m (equivalent to the 900-hPa level) exhibited distinct axial asymmetry with cooler air

262 prevailing on the western side of the TC center. The JMA-MSM analysis thus indicates that the

263 thermal asymmetry emerged first in the PBL (Fig. 6d) prior to its emergence in the free

264 troposphere (Fig. 6b), as confirmed by radiosonde soundings. As evident in Fig. 7a, temperature

265 below the 870-hPa level (equivalent to the 1200-m level) at 1200 UTC on 28 May based on

266 radiosonde soundings at Ishigaki-jima (on the western side of the TC center; blue square in Fig.

267 6d) was cooler by 2-4°C than at Minami-daito-jima (on the eastern side of the TC center; red

268 square in Fig. 6d), and this feature is well represented in the JMA-MSM analysis (Fig. 7b).

269 The skew-T type profiles in Fig. 7 based on both radiosonde soundings and

270 JMA-MSM analysis indicate that the thermal asymmetry at 1200 UTC on 28 May was evident

271 only below the 820-hPa level (nearly corresponding to the 1800-m level), although very dry air

272 was observed in the mid-troposphere between the 550-hPa and 300-hPa levels only on the

273 western side of the TC. The low-level thermal asymmetry was characterized by both a strong

274 inversion layer with subsidence between the 870-hPa and 820-hPa levels only on the western

275 side of the TC center and a greater lapse rate (i.e., weaker stratification) near the surface below

276 the 950-hPa level (equivalent to the 500-m level) on the western side than on the eastern side.

277 This near-surface thermal asymmetry of Songda is well depicted in a longitudinal section of

𝜃𝜃 278 across the TC center at 1200 UTC on May 28 (Fig. 8a). On the western side of the center,

𝜃𝜃 279 below 1 km was nearly uniform in the vertical, indicative of the well-developed mixed layer. On

280 its eastern side, by contrast, stratification was substantially stronger even in the near-surface

281 layer.

282 To investigate the relationship between the time evolution of the asymmetric thermal

283 structure of the TC and the basic state of atmospheric circulation and the SST distribution in

284 which the TC was embedded, we regard seven-day running-mean fields as the basic state of the

285 TC and instantaneous deviations from the mean as fluctuations or “anomalies” associated with

286 the TC. Hereafter we denote the seven-day running-mean fields with overbars ( ) and the � 287 anomalies with primes ( ′ ). A longitudinal cross section of potential temperature anomalies ′

𝜃𝜃 288 across the TC center well depicts the zonal asymmetry in the thermal structure of PBL (Fig. 8b),

289 characterized by cool anomalies on the western side of the TC center and warm anomalies on

290 the eastern side, in addition to a warm-core structure near the TC center. The ′ field thus

𝜃𝜃 291 highlights the asymmetric structure in the field itself (Fig. 8a).

𝜃𝜃 292 To further quantify the zonal asymmetry in the thermal structure of Songda, we

293 evaluated the following quantity in the anomaly field based on the JMA-MSM analysis:

(1) 16

′ ′ ′ 294 = abs . ° . ° , (1)

𝜃𝜃diff �𝜃𝜃3 5 east − 𝜃𝜃3 5 west� ′ ′ 295 where . ° ( ) denotes an instantaneous value of at 3.5° east (west) of the TC center. 𝜃𝜃3 5 east west 𝜃𝜃 296 The particular zonal distance was determined in recognition of the fact that the zonal

297 asymmetric thermal structure was significant in the outer region of TC (Fig. 8b). The quantity

298 ′ calculated for each level and time step is shown in the along-track section in Fig. 9a. It is

𝜃𝜃diff 299 evident that the thermal asymmetry first emerged in the PBL below the 1.5-km level before the

300 development of deeper asymmetric structure in the free-tropospheric thermal field.

301 The aforementioned changes in thermal structure of Songda occurred as it traveled

302 northeastward in the meridionally varying basic state. The meridional structure of the basic state

303 to the 3.5° west of the moving TC center is depicted in the along-track section in Fig. 9b.

304 Comparison between Figs. 9a and 9b reveals that the asymmetric thermal structure that first

305 emerged in the PBL within the shallow near-surface baroclinic zone anchored near the OFZ. As

306 indicated in Fig. 4, the OFZ forms along the northern flank of the Kuroshio, extending

307 northeastward from just north of Taiwan. The sharp cross-frontal SST gradient could therefore

308 influence the TC as early as it traveled east of Taiwan. The comparison also reveals that the

309 deep asymmetric thermal structure of the TC into the free troposphere developed in the deep

310 baroclinic zone below the subtropical jet core north of 28°N. Essentially the same results as in

311 Fig. 9 can be obtained in the ERA-Interim global atmospheric reanalysis (Dee et al. 2011)

312 produced by the European Centre for Medium-Range Weather Forecasts (not shown).

313 It may be noteworthy to describe the asymmetric wind structure in the PBL briefly.

314 The near-surface inflow layer, one of the fundamental structural characteristics of a mature TC,

315 was evident in the wind profile observed at Minami-daito-jima on the eastern side of the TC, but

316 less so at Ishigaki-jima on the western side of the TC (not shown). This zonal asymmetry in the

317 near-surface inflow structure might arise from the frontal SST gradient to the northwest of the

318 TC center. The cool PBL on the western side, for example, acted to suppress the development of

319 deep cumulus convection, resulting in the weakening of the inflow, although the detailed

320 analysis is beyond the scope of the present study.

321

322 5. Mechanisms for the thermal asymmetry formation

323 To understand the mechanisms for the formation of the axially asymmetric thermal

324 structure in the PBL of Songda in the vicinity of the OFZ, we diagnose the time tendency of

325 potential temperature anomalies ( ′), which is defined as local deviations from the basic state

𝜃𝜃 326 ( ), based on the following thermodynamic equation in height coordinates:

𝜃𝜃̅ ′ ′ ′ (2) = ( )′ , 𝜕𝜕𝜃𝜃 𝐽𝐽𝐽𝐽 𝜕𝜕𝜕𝜕 � 𝑝𝑝 � − 𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃 − �𝑤𝑤 � 327 where R denotes the𝜕𝜕𝜕𝜕 specific𝐶𝐶 𝑇𝑇heat of dry air at 𝜕𝜕𝜕𝜕constant pressure, temperature,

𝐶𝐶𝑝𝑝 𝑇𝑇 𝐕𝐕 328 horizontal wind velocity and vertical velocity. Otherwise the notation is standard. Diabatic

𝑤𝑤 18

329 heating/cooling has been estimated as the residual of (2), where the time tendency of on

𝐽𝐽 𝜃𝜃′ 330 the LHS was evaluated locally from its values at three hours earlier and later. Derivations

331 appeared in horizontal and vertical advection term has been evaluated by the central difference

332 method. Furthermore, the individual terms in (2) were evaluated in Table 1a from the

333 JMA-MSM data at the 290-m level (corresponding to the 975-hPa level) within the two specific

334 domains, one to the west of the TC center [26°-28°N, 121.25°-123°E] and the other to the east

335 [25°-27°N, 128.25°-130°E], separately. These domains referred to as the “western” and “eastern”

336 domains and encircled with red and blue lines in Figs. 10 and 11, respectively. If the evaluation

337 is performed in the maritime PBL, the adiabatic contribution of the anomalous vertical motion

338 represented in the last term on the RHS of (2) is negligible (Table 1a) except in the vicinities of

339 the surface Baiu/Meiyu front and terrain (not shown).

340 Figure 10a shows anomalies at the 290-m level for 1200 UTC on 28 May, when the

𝜃𝜃 341 zonal asymmetry in the thermal structure of the TC emerged only in the PBL (Figs. 6-9). As

342 shown in Fig. 10a, near-surface warm anomalies were observed over an extensive area of the

343 eastern portion of the TC, while cool anomalies were evident on the western side of the TC

344 center but limited to an “outer domain” more than 400km away from the center. These warm

345 and cool anomalies were consistent with the corresponding warming and cooling tendencies,

346 respectively, observed three hours earlier (0900 UTC on 28 May; contoured in Fig. 10b). These

347 tendencies that could contribute to the formation of the near-surface thermal asymmetry were

348 largely accounted for by anomalous horizontal temperature advection (Fig. 10c), which was

349 counteracted by anomalous diabatic heating/cooling (Fig. 10d). As shown in Table 1a,

350 anomalous diabatic heating and cooling offset as much as 80% and 70% of the anomalous

351 thermal advection within the western and eastern domains, respectively. The spatial distribution

352 of the anomalous diabatic heating/cooling was overall similar to that of anomalous sensible heat

353 flux (SHF) (Fig. 10e) rather than rainfall anomaly distribution (Fig. 10f), suggesting the primary

354 importance of the former. A notable exception is found to the north of the TC center, where the

355 anomalous warming tendency was yielded largely by anomalous diabatic heating (Fig. 10d),

356 which was partially offset by anomalous cool horizontal advection (Fig. 10c). In addition to the

357 enhanced SHF from the ocean (Fig. 10e), anomalous latent heat release associated with a large

358 amount of rainfall may also contribute positively to the anomalous heating (Fig. 10f).

359 The importance of the persistent near-surface baroclinicity associated with the oceanic

360 front in the anomalous thermal advection is then assessed by decomposing it into several

361 contributions as follows:

362 ( ) = ( ) . (3) ′ ′ ′ ′ ′ ′ − 𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃 −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ − 𝐕𝐕� ∙ 𝛁𝛁𝜃𝜃 − 𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃 363 The first term on the RHS represents an instantaneous contribution from wind anomalies acting

364 on the basic-state thermal gradient (or baroclinicity), the second term the corresponding

365 contribution from the basic-state wind acting on anomalous temperature gradient, and the third

366 term a nonlinear advection with wind and temperature anomalies. The individual terms in (3)

367 evaluated separately for the western and eastern domains are listed in Table 1b.

368 Our assessment based on the decomposition (3) reveals that the first linear term

369 was dominant in the anomalous advection at the 290-m level (Fig. 11a), while the ′ −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 370 nonlinear term was also important for the anomalous cold advection to the south of the TC

371 center (not shown). In the western domain, as much as 80% of the anomalous cool advection

372 was generated by the anomalous northerlies acting on the sharp basic-state temperature gradient

373 (Table 1b). Likewise, the anomalous southerlies acting on the basic-state temperature gradient

374 were dominant in generating the anomalous warm advection within the eastern domain. Since

375 the wind anomalies were comparable in magnitude between the two domains (Table 1c), the

376 stronger thermal advection in the western domain was attributable primarily to the stronger

377 gradient in the basic-state air temperature gradient associated with the OFZ (Table 1e and Fig.

378 11b).

379 Furthermore, the prominent cool advection rendered the PBL statically unstable,

380 leading to the enhancement of turbulent mixing and thereby the development the mixed layer

381 (Fig. 8a), in addition to the enhancement of SHF from the ocean (Fig. 10e and Table 1d). As

382 evident in Fig. 10e, just to the west of the TC center, the particularly strong northerlies induced

383 the pronounced enhancement of both the upward SHF over the warm Kuroshio water and cool

384 advection, which were mutually counteracting. To the east of the TC center, in contrast, the

385 anomalous southerlies enhanced warm advection, acting to stabilize the stratification within the

386 PBL, thereby induce anomalous downward SHF (Fig. 10e and Table 1d) and retard the

387 development of mixed layer (Fig. 8a). The suppressed heat exchange resulted in relatively weak

388 counteracting effect of warm advection anomaly and warming tendency in the nearer area to the

389 TC center. As indicated in Fig. 10e, the anomalous upward SHF to the west of the TC center

390 tended to be enhanced over the warm Kuroshio water south of the baroclinic zone. This

391 meridional SHF gradient acted to maintain the basic-state near-surface baroclinicity.

392

393 6. Numerical experiments

394 a. SST prescribed for the model experiments and simulated TC track

395 In this section, impacts of SST gradient across the OFZ in the southern ECS that could be

396 exerted on the zonal asymmetry in thermal structure of the TC Songda are assessed, through

397 numerical experiments with the cloud-resolving model CReSS. As mentioned in section 2b, the

398 experiments were conducted by prescribing three types of SST distributions as the model

399 lower-boundary condition. The SST field prescribed for the CTL experiment (Fig. 12b) is based

400 the high-resolution JCOPE2 analysis, which well represents the prominent OFZ along the

401 Kuroshio in the southern ECS. Compared to the MGDSST used for the JMA-MSM analysis, the

402 cross-frontal SST gradient in the JCOPE2 analysis is even stronger owing to its higher spatial

403 resolution and the enhanced penetration of the Kuroshio as far north as 30°N just southwest of

404 the Kyushu Island. The SST gradient represented in the JCOPE2 analysis has been artificially

405 smoothed only meridionally for the MSMTH experiment (Fig. 12c) and smoothed further

406 longitudinally for the SMTH experiment (Fig. 12d).

407 The TC track simulated in each of the experiments was determined by tracking the

408 surface pressure minimum, in the same manner as for the JMA-MSM analysis. As shown in Fig.

409 12, the three experiments overall reproduce the TC track based on the JMA’s best track

410 reasonably well, although the TC center at a given instance tends to be displaced slightly to the

411 south of its counterpart in the JMA’s best track, or the JMA-MSM analysis when Songda

412 approached the OFZ in the southern ECS (Fig. 5). If compared at the time when the TC

413 approaches the OFZ in the CTL experiment, the central pressure of the TC simulated in the CTL

414 experiment is higher by 12 (22) hPa that in the JMA-MSM analysis (that based on the JMA’s

415 best track data) (Fig. 5a). Nevertheless, no notable difference is found in the wind pattern

416 associated with the TC from one experiment to another (Figs. 12b-d). Likewise, any of the

417 experiments reproduces the upper-tropospheric westerly jet, well north of the OFZ over the

418 western and central portions of the ECS.

419 It should be stressed that the purpose of the experiments is not to reproduce the observed

420 intensity and wind speed of Songda. Rather, a comparison between these experiments is thus

421 meaningful just for qualitatively confirming the impact of frontal SST gradient on asymmetric

422 thermal structure, as indicated by the JMA-MSM analysis. However, the most notable

423 difference between the CReSS experiments and JMA-MSM analysis is found in the mixed layer

424 depth, which is severely underestimated in the CReSS model. As discussed later in more detail

425 (see Fig. 15), the mixed layer to the west of the TC center is as deep as 1km in the JMA-MSM

426 analysis, while it is 200 m or less in the CReSS experiments.

427 As shown in Fig. 5a, the central pressure of the TC in the CTL experiment is lower about

428 10 hPa if compared that in the MSMTH and SMTH experiments, in its mature phase (i.e., when

429 the TC is located around 20°N). This leads to the difference of the lower central pressure of the

430 TC in the CTL experiment approaching the OFZ in the southern ECS. It is speculated that

431 higher SST south of 20°N in the CTL experiment might contribute to the more developed TC in

432 that experiment (Fig. 5b). In contrast, there is no indication that the enhanced asymmetry in the

433 CTL experiment affect the TC intensity after ET, as it decayed rapidly in approaching another

434 OFZ in the northern ECS around 28°N-29°N (Fig. 12b). Furthermore, the impacts of SST

435 smoothing on the transfer speed of the TC appear to be rather weak (Fig. 5).

436

437 b. Impact of the OFZ on the axially asymmetric thermal structure of the TC

438 Figure 13a shows horizontal distribution of anomalies at the 160-m level and their

𝜃𝜃 439 time tendency simulated in the CTL experiment for 1700 UTC on 28 May, when the TC

440 simulated in each of the experiments is in the vicinity of the OFZ. The particular level is chosen,

441 in recognition of the substantial underestimation of the mixed layer depth in the CReSS

442 experiments. It is the lowest level at which the vertical advection term can be evaluated. The

443 anomalies are defined as deviations from the seven-day average from 0000 UTC 25 May 2011

444 to 0000 UTC 01 June 2011. The potential temperature tendency ( / ) was evaluated locally

𝜕𝜕𝜕𝜕′ 𝜕𝜕𝜕𝜕 445 from its values at three hours earlier and later, in the same manner as for the JMA-MSM

446 analysis. As in the JMA-MSM analysis (Fig. 10b), the CTL experiment (Fig. 13a) simulates

447 strong anomalous cooling tendency off Taiwan to the west of the TC center, although the

448 cooling occurs closer to the TC center than in the JMA-MSM analysis. The cooling tendency is

449 also simulated to the south of the TC center as in the analysis. This tendency is strongest in the

450 CTL experiment, which is consistent with the advection of the strongest cold anomaly simulated

451 to the west of the TC center by cyclonic winds.

452 To identify factors contributing to the simulated cooling tendency, we compare

453 area-mean values for the individual terms in (2) among the three experiments (Table 2a). The

454 area for the averaging was a rectangular domain [(± 1° in latitude from the TC center), (2°-3°

455 west of the TC center)], where the strongest cooling tendency is simulated in the CTL

456 experiment. The horizontal distributions of anomalies in horizontal advection, diabatic

457 heating/cooling and SHF are shown in Figs. 13b-d, respectively. As consistent with the

458 JMA-MSM analysis, the anomalous cooing tendency in the CTL experiment to the west of the

459 TC is basically determined as the residual of the dominating anomalous cold advection and the

460 offsetting diabatic heating anomaly. The latter is contributed to by the enhanced SHF from the

461 ocean under the anomalous northerlies (Table 2c-d and Fig. 13d). Compared to the CTL

462 experiment, both the anomalous cold advection and SHF are weaker in the MSMTH and SMTH

463 experiments (Table 2c-d and Figs. 13f, 13h, 13j and 13l), in each of which the SST gradient is

464 artificially smoothed.

465 The anomalous cold advection simulated is decomposed into the three components

466 expressed in (3). Results shown in Fig. 14 and Table 2b indicate that both (Fig. 14a, ′ −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 467 14d, 14g) and ( ) (Fig. 14b, 14e, 14f) contribute substantially to the anomalous cold ′ ′ ′ − 𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃 468 advection to the west of the TC center in the CReSS simulations. This result is rather

469 inconsistent with the result from the JMA-MSM analysis (Table 1b), where the anomalous

470 advection by the anomalous northerlies acting on the sharp basic-state temperature gradient is

471 dominant. This inconsistency seems to result from the differences in the PBL structure.

472 Figure 15 compares the vertical structure of the atmospheric boundary layer around

473 Ishigaki-jima among the observed sounding, JMA-MSM analysis and CReSS CTL experiment.

474 It should be noted that the positions of the sounding relative to the TC center are not exactly the

475 same among the observation, JMA-MSM analysis and CReSS experiment. It should also be

476 noted that no radiosonde measurement is available between the surface and 950-hPa level,

477 which can potentially lead to underestimation of temperature lapse rate in the near-surface layer.

478 Keeping these aspects in mind, one can recognize that the observed temperature profile is

479 reproduced reasonably well in the JMA-MSM analysis, including the mixed layer depth. This

480 would attribute to the strong (weak) vertical mixing in the JMA-MSM (the CReSS model).

481 Specifically, a subsidence inversion layer observed between the 875-hPa and 820-hPa levels

482 above the mixed layer is analyzed in JMA-MSM between the 850-hPa and 800-hPa levels. In

483 the CReSS simulation, by contrast, the corresponding layer is not obvious and just hinted as a

484 layer of reduced lapse late between the 930-hPa and 830-hPa levels. These differences found in

485 the temperature profiles suggest that vertical turbulent mixing is unrealistically weak and thus

486 the mixed layer is unrealistically shallow in the CReSS model.

487 As suggested by Fig.15, the mixed layer depth in the CReSS simulations seems much

488 less than 160 m. The heat budget analysis shown in Table 2 was thus likely performed above the

489 mixed layer top. If this is the case, thermal damping effect of anomalous SHF from the

490 underlying ocean cannot reach the altitude for our analysis. In fact, the time-mean meridional

491 temperature gradient in the CTL experiment is stronger only slightly than in the other two

492 experiments, despite the meridional gradient of underlying SST is nearly twice as strong (Table

493 2e and Fig. 14). Even in the CTL experiment, large temperature fluctuations are simulated even

494 just 160 m above the sea surface. At the particular time for our evaluation, longitudinal gradient

495 of temperature is enhanced temporarily between the TC warm core and a cold anomaly in the

496 outer region of the TC. Acting on this gradient, anomalous westerlies as an inflow toward the

497 TC center lead to instantaneous enhancement of the anomalous cold advection (Table 2b; Fig.

498 14), which may in turn contribute to the further enhancement of longitudinal temperature

499 gradient. It is noteworthy that a large contribution from this nonlinear thermal advection is

500 observed in the JMA-MSM analysis only above the mixed layer (not shown).

501 To assess the relative importance between the contributions from and anomalous

𝛁𝛁𝜃𝜃̅ 502 wind to , we evaluate how much the domain-averaged value of at the ′ ′ ′ 𝐕𝐕 −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 503 160-m level to the west of the TC center in the CTL experiment (Table 2b) could be altered if

504 either or within the particular domain in the CTL experiment were replaced with the ′ 𝜃𝜃̅ 𝐕𝐕 505 corresponding field simulated in the SMTH experiment. Through this hypothetical evaluation,

506 the value of in the CTL experiment would decrease by 0.10 K hour-1 (26 %) if ′ −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 𝜃𝜃̅ 507 were replaced, while the corresponding decrease would be only half in magnitude if were ′ 𝐕𝐕 508 replaced. The evaluation thus suggests certain importance of the OFZ for generating stronger

509 cold advection to the west of the TC in the CTL experiment. Above the mixed layer it also

510 suggests that the anomalous zonal advection associated with thus-generated cold anomalies

511 further enhances the cold advection.

512 It should be stressed again that the purpose of our CReSS experiments is not to reproduce

513 the observed intensity and wind speed of Songda. Rather, the purpose is to examine the

514 sensitivity of the influence of the background SST gradient on the thermal structure in the

515 near-surface layer of the TC.

516

517 7. Summary and discussion

518 The present study highlights the importance of sharp SST gradient across an OFZ that

519 acts to anchor a near-surface baroclinic zone, whose importance has been overlooked in most of

520 the previous studies on TCs and their ET process. Specifically, the present study has revealed

521 how the sharp SST gradient across the OFZ in the southern ECS in early summer yielded zonal

522 asymmetries in the thermal structure of typhoon Songda, which are confined mainly into the

523 PBL. The particular OFZ forms between the warm Kuroshio and the water of the shallow ECS

524 that still remains cool in early summer after strongly cooled off in winter by the monsoonal

525 winds. Since the particular OFZ is located well south of the deep baroclinic zone below the

526 westerly jet core, the particular impact of the OFZ was isolated in the JMA-MSM analysis as a

527 distinct initial signature of the ET of Songda before an axially asymmetric thermal structure

528 reached up into the mid-troposphere in the vicinity of the jet. The shallow thermal asymmetry in

529 the PBL bears certain similarities to the composited thermal structure of 274 TCs observed over

530 the WNP one day before the ET completion (Kitabatake 2011). Nevertheless, the shallow

531 thermal asymmetry may not be identified with a particular criterion for an ET onset defined by

532 Evans and Hart (2003), which largely measure thermal asymmetry in the free troposphere.

533 Previous studies, including Klein et al. (2000) and Ritchie and Elsberry (2001), have

534 shown that cooler (warmer) air on the western (eastern) side of a TC, as axially asymmetric

535 thermal structure characteristic of a TC in ET, is induced by cold (warm) advection with TC

536 circulation. The importance of this advective process acting on a near-surface baroclinic zone

537 associated with the particular OFZ has been verified in the present study in the formation of the

538 zonally asymmetric thermal structure in the PBL of Songda. As shown in a numerical

539 experiment by Ritchie and Elsberry (2001), enhanced SHF from the warm ocean into the cool

540 northerlies to the west of the TC center acts to offset the contribution from the anomalous cold

541 advection.

542 Our numerical experiments with a cloud-resolving model, CReSS, suggest, though in a

543 qualitative manner, a certain contribution of the frontal SST gradient to the generation of

544 near-surface cold anomalies on the western flank of the TC Songda. Although the intensity of

545 the TC in approaching the OFZ is substantially underestimated and the track of the TC center

546 differs slightly from the observation, the degree of zonal asymmetry in thermal structure of the

547 TC within the PBL is found sensitive to the strength of the SST gradient prescribed at the model

548 boundary. In the experiment with realistic frontal SST gradient, enhanced near-surface air

549 temperature gradient yields stronger cold advection and thereby stronger cold anomalies to the

550 west of the TC center than in the experiments with relaxed SST gradient.

551 Compared to the JMA-MSM analysis, however, the mixed layer depth is severely

552 underestimated in CReSS. Our analysis of the model simulation therefore corresponds to the

553 situation above the mixed layer, where nonlinear effect of large temperature fluctuations in

554 combination with strong inflow toward the TC center contributes substantially to the anomalous

555 cold advection. This process is indeed found important above the deeper mixed layer in the

556 JMA-MSM analysis, while it is less important within the mixed layer in the JMA-MSM, where

557 anomalous cold advection that yielded cold anomalies is generated primarily through the

558 TC-associated northerlies acting on the near-surface baroclinic zone anchored by the OFZ.

559 Unfortunately, the underestimation of the intensity of the TC and the mixed layer depth in

560 CReSS, latter of which arises probably from the suppressed turbulent mixing in the PBL,

561 prevents us from a quantitative evaluation of the impacts of the frontal SST gradient on the

562 asymmetric thermal structure in the PBL. The suppressed turbulent mixing in CReSS, if

563 compared to JMA-MSM, seems attributable to different subgrid-scale turbulence

564 parameterization schemes used in these models. The scheme adopted in CReSS is similar to the

565 Mellor-Yamada level-2 scheme, whereas JMA-MSM uses the improved Mellor-Yamada

566 (Mellor-Yamada-Nakanishi-Niino) Level 3 scheme (Hara 2007) with improved representation

567 of vertical mixing length (Nakanishi and Niino 2009).

568 To elucidate the importance of persistent near-surface baroclinicity anchored by the

569 OFZ for the formation of the near-surface thermal asymmetry of Songda, we regard seven-day

570 running mean fields as the basic state in which wind and thermal anomalies associated with the

571 TC are embedded. The choice of the averaging period is rather subjective, however. We have

572 nevertheless confirmed that no obvious differences emerge in anomaly fields of sea level

573 pressure, near-surface temperature and winds if the averaging period is varied from seven to 13

574 days (not shown). In other words, the signature of the near-surface baroclinic zone is robust as

575 anchored by the OFZ. Caution must therefore be exercised in discussing the asymmetric thermal

576 structure of a given TC if embedded in a strong baroclinic zone in its background state.

577 It should be pointed out that impacts of an OFZ on the thermal structure of a TC are

578 unlikely to be limited to the particular aspect highlighted in the present study. As conjectured

579 from numerical experiments by Wada et al. (2013), differential heat supply from the ocean

580 across an OFZ can act to anchor a near-surface baroclinic zone associated with a nearby

581 stationary atmospheric front. This effect has been shown to be important for the formation of a

582 midlatitude storm-track (Nakamura et al. 2004; Taguchi et al. 2009; Hotta and Nakamura 2011).

583 Some recent TC studies have explored possible influence of strong SST gradient on

584 re-intensification of post-ET storms. Through composites for 34 TCs in the North Atlantic, Hart

585 et al. (2006) showed that those post-ET storms that undergo stronger re-intensification tend to

586 move over regions with stronger SST gradient in the course of their ET than those that undergo

587 weaker re-intensification. Bond et al. (2010) conducted regional model experiments for

588 Typhoon Tokage, which moved across the OFZ along the KE in October 2004. By imposing a

589 warm or cool SST anomaly over a broad domain around the KE, they found significant

590 influence of SST gradient on the path of the TC and eventually its intensity and structure after

591 its ET. The present study has evaluated potential impacts of an OFZ on changes in thermal

592 structure in the PBL of a TC. However, possible influence of an OFZ on re-intensification of the

593 TC after its ET, which requires examination of influences of a free-tropospheric baroclinic zone

594 below a westerly jet, cannot be discussed in the present study. It remains as an important topic

595 for future study.

596 As shown in Fig. 2a, the stationary Baiu/Meiyu front was extending along the

597 Kuroshio southwest of the Kyushu Island on 28 May 2011. In the JMA-MSM analysis, a band

598 of positive temperature anomalies extended northeastward from the vicinity of the warm core of

599 Songda along the Kuroshio (Fig. 10a), which can be regarded as another aspect of zonally

600 asymmetric thermal structure of the TC. This band of warm air formed along the Baiu/Meiyu

601 front, along which a narrow band of enhanced diabatic heating also formed. In fact, Miyama et

602 al. (2012) found the organization of a convective rainband in early summer along the warm

603 Kuroshio in the ECS within a warm moist airflow towards the Baiu/Meiyu front to the north.

604 Many studies have pointed out the importance of moisture supply from the ocean in

605 intensification of a TC through convective precipitation, especially in the mature phase of a TC

606 (Emanuel 1986) and for TCs in their ET process (Thorncroft and Jones 2000). Moisture supply

607 from the ocean was greater on the western side of Songda, which may have led to locally

608 enhanced precipitation (not shown). Further investigation, including an evaluation of moisture

609 transportation and budget, is necessary for deeper and more comprehensive understanding of the

610 impacts of OFZs on TCs in their ET, especially on their inner-core structure.

611 The present study has focused on the PBL response of a particular TC to the frontal

612 SST gradient associated with an OFZ. Nevertheless, the upper ocean structure could also be

613 affected by such processes as mixing of cooler subsurface water entrained with the warmer

614 ocean mixed layer (Shay 2010). Investigation with an air-sea coupled modeling system would

615 be required to fully understand impacts of an OFZ on the formation of the asymmetric structure

616 of a storm undergoing ET.

617

618 Acknowledgments. The authors thank the editor and three anonymous reviewers for their sound

619 criticism and constructive comments on the earlier version of this paper. The authors also thank

620 Drs. K. Tsuboki and A. Sakakibara for allowing us to use CReSS and Drs. H. Niino, T. Iwasaki,

621 N. Kitabatake, A. Wada, M. Mori, A. Kuwano-Yoshida and K. Nishii for their variable

622 comments and suggestions. The MGDSST and JMA-MSM data were obtained

623 through ”Meteorological Research Consortium”, a framework for research cooperation between

624 JMA and Meteorological Society of Japan. We used Earth Simulator in support of Japan Agency

625 for Marine-Earth Science and Technology (JAMSTEC). The surface weather charts were

626 provided in the courtesy of JMA, and the IR cloud images by Multi-functional Transport

627 Satellite (MTSAT) -2 provided by JMA were available at the Kochi University

628 (http://weather.is.kochi-u.ac.jp/). This study is supported in part by Japanese Ministry of

629 Education, Culture, Sports and Science and Technology (MEXT) through Grants-in-Aid for

630 Scientific Research in Innovative Areas 2205 and 25287120 and by Japanese Ministry of

631 Environment through Environment Research and Technology Development Funds 2A-1201 and

632 2-1503.

633

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708 atmospheric model simulations for the 2003/4 cold season. J. Climate, 22, 6536-6560.

709 Thorncroft, C., and S. C. Jones, 2000: The extratropical transitions of hurricanes Felix and Iris

710 in 1995. Mon. Wea. Rev., 128, 947-972.

711 Tsuboki, K., and A. Sakakibara, 2002: Large-scale parallel computing of cloud resolving storm

712 simulator. High Performance Computing, H. P. Zima et al., Ed., Springer, 243-259.

713 ––––, and ––––, 2007: Numerical prediction of high-impact weather systems. The textbook for

714 Seventeenth IHP training course in 2007, HyARC, Nagoya University, Japan and

715 UNESCO, 273 pp.

716 Wada, A., N. Usui, and M. Kunii, 2013: Interactions between Typhoon Choi-wan (2009) and the

717 Kuroshio Extension system. Advances in Meteorology, 2013, 859810,

718 doi:10.1155/2013/859810.

719 Xie, S.-P., J. Hafner, Y. Tanimoto, W. T. Liu, H. Tokinaga, and H. Xu, 2002: Bathymetric effect

720 on the winter sea surface temperature and climate of the Yellow and East China Seas.

721 Geophys. Res. Lett., 29, 2228, doi:10.1029/2002GL015884.

722

723 LIST OF TABLES

724 Table 1. Evaluations of individual terms involved in the heat budget and related anomalous

725 thermal advection at the 290-m level, 0900 UTC on 28 May 2011, as area averaging over

726 the “western” and “eastern” domains relative to the TC center (marked with red and blue

727 lines, respectively, in Figs. 8 and 9). (a) Heat budget (K (hour)-1), (b) decomposition of

728 anomalous horizontal advection (K (hour)-1), anomalies in (c) 290 m southerly wind velocity

729 (m s ) and (d) upward SHF (W m ), and (e) equatorward gradients of seven-day running −1 −2

730 mean SST and 290 m potential temperature (K (100 km)-1).

731

732 Table 2. As in Table 1, but for the 160-m level in the three experiments with CReSS, for the

733 CTL (at 1700 UTC on 28 May), the MSMTH (at 1200 UTC on 28 May), and the SMTH (at

734 1700 UTC on 28 May).

735 Table 1. Evaluations of individual terms involved in the heat budget and related anomalous

736 thermal advection at the 290-m level, 0900 UTC on 28 May 2011, as area averaging over

737 the “western” and “eastern” domains relative to the TC center (marked with red and blue

738 lines, respectively, in Figs. 8 and 9). (a) Heat budget (K (hour)-1), (b) decomposition of

739 anomalous horizontal advection (K (hour)-1), anomalies in (c) 290-m southerly wind velocity

740 (m s ) and (d) upward SHF (W m ), and (e) equatorward gradients of seven-day running −1 −2

741 mean SST and 290-m potential temperature (K (100 km)-1).

western eastern

domain domain (a) Heat budget [K (hour) ] −1 0.54 -0.22 ′ �𝐽𝐽(𝐽𝐽⁄𝐶𝐶𝑝𝑝𝑇𝑇�) -0.66 0.30 ′ −( 𝐕𝐕 ∙ 𝛁𝛁/𝜃𝜃 ) -0.01 -0.00 ′ −(𝑤𝑤𝑤𝑤𝑤𝑤)/𝜕𝜕𝜕𝜕 -0.13 0.08 ′ (b) Decomposition of anomalous𝜕𝜕𝜃𝜃 𝜕𝜕𝜕𝜕horizontal advection [K (hour) ] −1 -0.55 0.38 ′ −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 0.02 -0.02 ′ −( 𝐕𝐕� ∙ 𝛁𝛁𝜃𝜃 ) -0.12 -0.06 ′ ′ ′ (c) 290-m anomalous southerly− 𝐕𝐕 ∙ 𝛁𝛁wind𝜃𝜃 velocity [m s ] −1 -16.5 17.1 ′ (d) Anomalous upward SHF [𝜐𝜐W m ] −2 (SHF) 42.7 -20.4 ′ (e) Equatorward gradient of seven-day running mean [K (100km) ] −1 0.92 0.61

−(𝜕𝜕SST𝜃𝜃̅⁄)𝜕𝜕𝜕𝜕 1.23 0.67 − 𝜕𝜕 ⁄𝜕𝜕𝜕𝜕 42

742 Table 2. As in Table 1, but for the 160-m level in the three experiments with CReSS, for the

743 CTL (at 1700 UTC on 28 May), the MSMTH (at 1200 UTC on 28 May), and the SMTH (at

744 1700 UTC on 28 May).

CTL MSMTH SMTH (a) Heat budget [K (hour) ] −1 ( ( ))′ 0.44 0.21 0.14 ′ 𝐽𝐽𝐽𝐽(⁄ 𝐶𝐶𝑝𝑝𝑇𝑇) -0.92 -0.43 -0.35 −( 𝐕𝐕 ∙ 𝛁𝛁/𝜃𝜃 ) 0.02 -0.01 -0.05 ′ −(𝑤𝑤𝑤𝑤𝑤𝑤)/𝜕𝜕𝜕𝜕 -0.46 -0.23 -0.25 ′ (b) Decomposition of anom𝜕𝜕𝜃𝜃alous𝜕𝜕𝜕𝜕 horizontal advection [K (hour) ] −1 ′ -0.38 -0.32 -0.24 ′ −𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 0.23 0.16 0.11 ′ −𝐕𝐕�′ ∙ 𝛁𝛁𝜃𝜃′ -0.77 -0.28 -0.22 (c) 160-m anomalous south−�erly𝐕𝐕 ∙ wind𝛁𝛁𝜃𝜃 � velocity [m s ] −1 ′ -10.3 -11.5 -8.9

(d) Anomalous upward SHF 𝜐𝜐[W m ] −2 (SHF)′ 30.9 27.8 9.3

(e) Equatorward gradient of the seven-day mean [K (100km) ] −1 0.85 0.79 0.62

− 𝜕𝜕SST𝜃𝜃̅⁄𝜕𝜕𝜕𝜕 1.23 0.77 0.65 − 𝜕𝜕��⁄𝜕𝜕𝜕𝜕 745

746

747

748

749

750 LIST OF FIGURES

751 Fig. 1. SST distribution ( , blue contours) prescribed as the lower-boundary condition for the

℃ 752 CTL experiment with CReSS within the domain for the CTL and other experiments.

753 The best track of Songda by JMA from 0000 UTC on 24 May to 1200 UTC on 30 May

754 (every 6h, open circles) and the domain shown in Figs. 4 and 12 (red contours) are

755 superimposed.

756 Fig. 2. JMA surface weather charts for (a) 1200 UTC on 28 May and (b) 0000 UTC on 29 May

757 in 2011.

758 Fig. 3. IR cloud images by Multi-functional Transport Satellite (MTSAT) -2 for (a) 1200 UTC

759 on 28 May and (b) 0000 UTC on 29 May in 2011. “X” denotes the surface TC center at

760 each time.

761 Fig. 4. The track of Songda based on the three-hourly JMA-MSM analysis (black line) and the

762 best track data by JMA (open circles) from 0000 UTC on 28 May to 0600 UTC on 29

763 May in 2011, superimposed on seven-day averaged fields (0000 UTC on 25 May

764 through 0000 UTC on 1 June) of 200-hPa wind (arrows; plotted only if the speed

765 exceeds 25 m s ) based on the JMA-MSM analysis and SST ( , colored) based on −1 ℃ 766 MGDSST. Blue and red squares denote the positions of radiosonde stations at

767 Ishigaki-jima and Minami-daito-jima, respectively.

768 Fig. 5. (a) Time series of the central surface pressure of Songda in the JMA-MSM analysis

769 (purple), CTL (red), MSMTH (green), and the SMTH (blue) experiments with CReSS,

770 along with the best track data by JMA (black), from 0000 UTC on 24 May to 1200 UTC

771 on 30 May. The abscissa indicates the latitude of the storm center. Closed dots indicate

772 the latitude and the central pressure at 0900 UTC on 28 May. (b) As in (a), but for the

773 SST averaged over the area 3° ×3° around the TC center.

774 Fig. 6. Horizontal thermal structure of Songda based on the JMA-MSM analysis. (a-b) 600-900

775 hPa thickness (a) at 1200 UTC on 28 May and (b) at 0000 UTC on 29 May in 2011.

776 (c-d) As in (a), but for potential temperature at the levels of (c) 4,135 m (nearly

777 equivalent to the 600-hPa level) and (d) 981 m (nearly equivalent to the 900-hPa level).

778 Black circle in each panel indicates the distance of 400 km from the TC center (cross).

779 Blue and red squares in (d) are the same as in Fig. 4.

780 Fig. 7. (a) Skew-T type plot of Potential temperature (thick solid) and dew-point temperature

781 (thick dashed) profiles based on radiosonde observations from the surface to the

782 100-hPa level at 1200 UTC on 28 May 2011 at (a) Ishigaki-jima [24.33°N, 124.17°E]

783 (blue) and (b) Minami-daito-jima [25.83°N, 131.23°E] (red). Isotherms (black; every

784 2 ), dry adiabats (green; every 5K), moist pseudo-adiabats (thin dashed blue; every

℃ 785 5K), and mixing ratio lines (dashed purple) are superimposed. (b) As in (a), but for the

786 corresponding profiles based on the JMA-MSM analysis from the 1000-hPa level to the

787 100-hPa level at (a) [24.3°N, 124.125°E] (blue) and (b) [25.8°N, 131.125°E] (red) are

788 superimposed.

789 Fig. 8. Longitudinal cross sections across the TC center (26.4°N, black dashed lines) at 1200

790 UTC on 28 May 2011 based on the JMA-MSM data. (a) Potential temperature and (b)

791 its anomalies from the seven-day running mean. Blue solid lines in (b) denote the

792 longitudes of 3.5° east and west from the TC center.

793 Fig. 9. (a) Along-track section of ′ (colored) following the TC center from 0000 UTC on

𝜃𝜃diff 794 28 May to 0600 UTC on 29 May in 2011, based on the JMA-MSM analysis. The

795 abscissa indicates the latitude of the TC center. Dashed lines signify the 600 and

796 900-hPa levels. Black shading denotes the topography of Taiwan and that of the Kyushu

797 and Shikoku Islands of Japan. (b) As in (a), but for the basic states at 3.5° west of the

798 TC center of westerly wind velocity (contoured for 10, 15, 20 m s ) and equatorward −1

799 gradient of potential temperature (colored), in addition to equatorward SST gradient (K

800 (100 km) ) plotted below the section with red line. −1

801 Fig. 10. (a-b) Potential temperature anomalies at 290 m (colored) from the seven-day running

802 mean for (a) 1200 UTC and (b) 0900 UTC on 28 May 2011, based on the JMA-MSM

803 analysis. In (a), the circle indicates 400 km distance from the TC center. In (b), warming

804 and cooling tendencies (red and blue solid contours, respectively) are superimposed

805 (value: ± 0.08, 0.16, 0.24 K (hour) ). The positions of the TC center at the reference −1

806 time and three hours earlier and later are marked with crosses. Red and blue rectangles

807 define the domains for area averaging. Anomalies are defied as local departures from

808 seven-day running mean for 1200/0900 UTC on 28 May. (c-e) As in (b), but for the (c)

809 horizontal advection, (d) diabatic heating, and (e) upward SHF, all of which can

810 contribute to the tendency in (b). In (e), the seven-day running mean temperature at the

811 290-m level (contoured every 1 K) is superimposed. (f) As in (a), but for hourly

812 precipitation anomalies from 0900 UTC to 1000 UTC on 28 May.

813 Fig. 11. Seven-day running mean potential temperature at the 290-m level for 0900 UTC on 28

814 May 2011 (contoured every 1 K), superimposed on (a) anomalous wind (vector, m s ) −1

815 and ′ (colored) at the same level, or (b) seven-day running mean SST (colored),

−𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 816 based on the JMA-MSM analysis. The cross signs and rectangles are the same as in Fig.

817 10.

818 Fig. 12. TC track (bold black line) based on the (a) JMA-MSM analysis, (b) CTL, (c) MSMTH,

819 and (d) the SMTH experiments with CReSS, along with the JMA best track (open

820 circles), superimposed on the wind at the 12,439 m-level (arrows; corresponding to the

821 200 hPa level) and SST both averaged from 0000 UTC 25 May to 0000 UTC 1 June

822 2011. The SST plotted is based on (a) MGDSST (b-d) JCOPE-2, prescribed as the lower

823 boundary condition for the (a) JMA-MSM analysis and (b-d) the CReSS model,

824 respectively. The corresponding surface pressure pattern is also superimposed with

825 black contours for (a) 0900 UTC, (b) 1700 UTC, (c) 1200 UTC and (d) 1700 UTC on

826 28 May, with the central pressure of (a) 955 hPa, (b) 967 hPa, (c) 971 hPa and (d) 975

827 hPa.

828 Fig. 13. (a) Potential temperature anomaly at the 160-m level at 1700 UTC on 28 May, as

829 deviations from the seven-day mean from 0000 UTC on 25 May to 0000 UTC on 01

830 June 2011 (colored), superimposed on anomalous warming/cooling tendency (red solid

831 and blue dashed contours, respectively, for ± 0.2, 0.4, 0.6 K hour-1) in the CTL

832 experiment. (b-d) As in (a), but for anomalies at 1700 UTC on 28 May (colored) in (b)

833 horizontal advection (K hour–1), (c) diabatic heating/cooling rate (K hour–1) and (d)

834 upward SHF (W m–2), as local deviations from their seven-day means. (e-h) As in (a-d),

835 respectively, but for the MSMTH experiment at 1200 UTC on 28 May. (i-l) As in (a-b),

836 respectively, but for the SMTH experiment at 1700 UTC on 28 May. In each panel, the

837 TC center at the reference time is indicated with a cross, and the corresponding

838 positions simulated 3-hour earlier and later are with smaller crosses. The red rectangle

839 in each panel indicates the domain for averaging.

840 Fig. 14. As in Fig. 13, but for (a) anomalous winds (vector, m s-1) at the 160-m level

841 superimposed on seven-day mean (from 0000 UTC on 25 May to 0000 UTC on 01 June

842 2011) potential temperature (contoured for every 1 K) and the resultant anomalous

843 thermal advection ′ (colored as indicated at the bottom of the figure) simulated

−𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 844 in the CTL experiment. (b) As in (a), but for anomalous winds superimposed on

845 potential temperature at 1700 UTC on 28 May (contoured for every 1 K) and the

846 resultant anomalous thermal advection – ( ′ ′)′ (colored). (c) As in (a), but the

𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃 847 seven-day mean potential temperature at the 160-m level, superimposed on prescribed

848 SST distribution (colored). The rectangles with red and blue lines denote the same

849 domains as indicated in the Fig. 11. (d-f) As in (a-c), respectively, but for the MSMTH

850 experiment at 1200 UTC on 28 May. (g-i) As in (a-b), respectively, but for the SMTH

851 experiment at 1700 UTC on 28 May.

852 Fig. 15. As in Fig. 7, but for the profiles of temperature (solid) and dew-point temperature

853 (dashed) below the 700-hPa level based on radiosonde observation at Ishigaki-jima for

854 1200 UTC on 28 May 2011 (thick black) and the corresponding profiles based on the

855 JMA-MSM analysis (thick red) and the CTL experiment (thick blue) for 1200 UTC and

856 2000 UTC, respectively, on the same day at [24.3ºN, 124.18ºE]. Isotherms, dry adiabats,

857 and moist pseudo-adiabats are drawn with thin lines for every 1 , 3K, and 2K.

℃ 49

858

859

860

861

862

863

864

865

866

867

868

869

870 Fig. 1. SST distribution ( , blue contours) prescribed as the lower-boundary condition for the

℃ 871 CTL experiment with CReSS within the domain for the CTL and other experiments. The

872 best track of Songda by JMA from 0000 UTC on 24 May to 1200 UTC on 30 May (every

873 6h, open circles) and the domain shown in Figs. 4 and 12 (red contours) are

874 superimposed.

875

876

877

878

879

880

881

882

883 Fig. 2. JMA surface weather charts for (a) 1200 UTC on 28 May and (b) 0000 UTC on 29 May

884 in 2011.

885

886

887

888

889

890

891

892

893

894

895

896 Fig. 3. IR cloud images by Multi-functional Transport Satellite (MTSAT) -2 for (a) 1200 UTC

897 on 28 May and (b) 0000 UTC on 29 May in 2011. “X” denotes the surface TC center at

898 each time.

899

900

901

902 Fig. 4. The track of Songda based on the three-hourly JMA-MSM analysis (black line) and the

903 best track data by JMA (open circles) from 0000 UTC on 28 May to 0600 UTC on 29

904 May in 2011, superimposed on seven-day averaged fields (0000 UTC on 25 May through

905 0000 UTC on 1 June) of 200-hPa wind (arrows; plotted only if the speed exceeds 25

906 m s ) based on the JMA-MSM analysis and SST ( , colored) based on MGDSST. Blue −1 ℃ 907 and red squares denote the positions of radiosonde stations at Ishigaki-jima and

908 Minami-daito-jima, respectively.

909

910

911

912

913

914

915

916

917

918

919

920

921 Fig. 5 (a) Time series of the central surface pressure of Songda in the JMA-MSM analysis

922 (purple), CTL (red), MSMTH (green), and the SMTH (blue) experiments with CReSS,

923 along with the best track data by JMA (black), from 0000 UTC on 24 May to 1200 UTC

924 on 30 May. The abscissa indicates the latitude of the storm center. Closed dots indicate

925 the latitude and the central pressure at 0900 UTC on 28 May. (b) As in (a), but for the

926 SST averaged over the area 3° ×3° around the TC center.

927

928 Fig. 6. Horizontal thermal structure of Songda based on the JMA-MSM analysis. (a-b) 600-900

929 hPa thickness (a) at 1200 UTC on 28 May and (b) at 0000 UTC on 29 May in 2011.

930 (c-d) As in (a), but for potential temperature at the levels of (c) 4,135 m (nearly

931 equivalent to the 600-hPa level) and (d) 981 m (nearly equivalent to the 900-hPa level).

932 Black circle in each panel indicates the distance of 400 km from the TC center (cross).

933 Blue and red squares in (d) are the same as in Fig. 4.

934

935

936

937

938

939

940

941

942

943 Fig. 7. (a) Skew-T type plot of Potential temperature (thick solid) and dew-point temperature

944 (thick dashed) profiles based on radiosonde observations from the surface to the

945 100-hPa level at 1200 UTC on 28 May 2011 at (a) Ishigaki-jima [24.33°N, 124.17°E]

946 (blue) and (b) Minami-daito-jima [25.83°N, 131.23°E] (red). Isotherms (black; every

947 2 ), dry adiabats (green; every 5K), moist pseudo-adiabats (thin dashed blue; every 5K),

℃ 948 and mixing ratio lines (dashed purple) are superimposed. (b) As in (a), but for the

949 corresponding profiles based on the JMA-MSM analysis from the 1000-hPa level to the

950 100-hPa level at (a) [24.3°N, 124.125°E] (blue) and (b) [25.8°N, 131.125°E] (red) are

951 superimposed.

952

953

954

955

956

957 Fig. 8. Longitudinal cross sections across the TC center (26.4°N, black dashed lines) at 1200

958 UTC on 28 May 2011 based on the JMA-MSM data. (a) Potential temperature and (b)

959 its anomalies from the seven-day running mean. Blue solid lines in (b) denote the

960 longitudes of 3.5° east and west from the TC center.

961

962

963

964

965

966

967

968

969

970

971

972

973 Fig. 9. (a) Along-track section of (colored) following the TC center from 0000 UTC on ′ 𝜃𝜃diff 974 28 May to 0600 UTC on 29 May in 2011, based on the JMA-MSM analysis. The

975 abscissa indicates the latitude of the TC center. Dashed lines signify the 600 and

976 900-hPa levels. Black shading denotes the topography of Taiwan and that of the Kyushu

977 and Shikoku Islands of Japan. (b) As in (a), but for the basic states at 3.5° west of the

978 TC center of westerly wind velocity (contoured for 10, 15, 20 m s ) and equatorward −1

979 gradient of potential temperature (colored), in addition to equatorward SST gradient (K

980 (100 km) ) plotted below the section with red line. −1

981

982 Fig. 10. (a-b) Potential temperature anomalies at 290 m (colored) from the seven-day running

983 mean for (a) 1200 UTC and (b) 0900 UTC on 28 May 2011, based on the JMA-MSM

984 analysis. In (a), the circle indicates 400 km distance from the TC center. In (b), warming

985 and cooling tendencies (red and blue solid contours, respectively) are superimposed

986 (value: ± 0.08, 0.16, 0.24 K (hour) ). The positions of the TC center at the reference −1

987 time and three hours earlier and later are marked with crosses. Red and blue rectangles

988 define the domains for area averaging. Anomalies are defied as local departures from

989 seven-day running mean for 1200/0900 UTC on 28 May. (c-e) As in (b), but for the (c)

990 horizontal advection, (d) diabatic heating, and (e) upward SHF, all of which can

991 contribute to the tendency in (b). In (e), the seven-day running mean temperature at the

992 290-m level (contoured every 1 K) is superimposed. (f) As in (a), but for hourly

993 precipitation anomalies from 0900 UTC to 1000 UTC on 28 May.

994

995

996

997

998

999

1000

1001

1002

1003

1004

1005

1006

1007

1008 Fig. 11. Seven-day running mean potential temperature at the 290-m level for 0900 UTC on 28

1009 May 2011 (contoured every 1 K), superimposed on (a) anomalous wind (vector, m s ) −1

1010 and ′ (colored) at the same level, or (b) seven-day running mean SST (colored),

−𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 1011 based on the JMA-MSM analysis. The cross signs and rectangles are the same as in Fig.

1012 10.

1013

1014

1015

1016

1017

1018

1019

1020

1021

1022

1023

1024

1025 Fig. 12. TC track (bold black line) based on the (a) JMA-MSM analysis, (b) CTL, (c) MSMTH,

1026 and (d) the SMTH experiments with CReSS, along with the JMA best track (open

1027 circles), superimposed on the wind at the 12,439 m-level (arrows; corresponding to the

1028 200 hPa level) and SST both averaged from 0000 UTC 25 May to 0000 UTC 1 June

1029 2011. The SST plotted is based on (a) MGDSST (b-d) JCOPE-2, prescribed as the lower

1030 boundary condition for the (a) JMA-MSM analysis and (b-d) the CReSS model,

1031 respectively. The corresponding surface pressure pattern is also superimposed with black

1032 contours for (a) 0900 UTC, (b) 1700 UTC, (c) 1200 UTC and (d) 1700 UTC on 28 May,

1033 with the central pressure of (a) 955 hPa, (b) 967 hPa, (c) 971 hPa and (d) 975 hPa.

1034

1035 Fig. 13. (a) Potential temperature anomaly at the 160-m level at 1700 UTC on 28 May, as

1036 deviations from the seven-day mean from 0000 UTC on 25 May to 0000 UTC on 01 June 2011

1037 (colored), superimposed on anomalous warming/cooling tendency (red solid and blue dashed

1038 contours, respectively, for ± 0.2, 0.4, 0.6 K hour-1) in the CTL experiment. (b-d) As in (a), but

1039 for anomalies at 1700 UTC on 28 May (colored) in (b) horizontal advection (K hour–1), (c)

1040 diabatic heating/cooling rate (K hour–1) and (d) upward SHF (W m–2), as local deviations from

1041 their seven-day means. (e-h) As in (a-d), respectively, but for the MSMTH experiment at 1200

1042 UTC on 28 May. (i-l) As in (a-b), respectively, but for the SMTH experiment at 1700 UTC on

1043 28 May. In each panel, the TC center at the reference time is indicated with a cross, and the

1044 corresponding positions simulated 3-hour earlier and later are with smaller crosses. The red

1045 rectangle in each panel indicates the domain for averaging.

1046

1047

1048

1049

1050

1051

1052

1053

1054

1055

1056

1057

1058 Fig. 14. As in Fig. 13, but for (a) anomalous winds (vector, m s-1) at the 160-m level

1059 superimposed on seven-day mean (from 0000 UTC on 25 May to 0000 UTC on 01 June

1060 2011) potential temperature (contoured for every 1 K) and the resultant anomalous

1061 thermal advection ′ (colored as indicated at the bottom of the figure) simulated

−𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃̅ 1062 in the CTL experiment. (b) As in (a), but for anomalous winds superimposed on

1063 potential temperature at 1700 UTC on 28 May (contoured for every 1 K) and the

1064 resultant anomalous thermal advection – ( ′ ′)′ (colored). (c) As in (a), but the

𝐕𝐕 ∙ 𝛁𝛁𝜃𝜃 65

1065 seven-day mean potential temperature at the 160-m level, superimposed on prescribed

1066 SST distribution (colored). The rectangles with red and blue lines denote the same

1067 domains as indicated in the Fig. 11. (d-f) As in (a-c), respectively, but for the MSMTH

1068 experiment at 1200 UTC on 28 May. (g-i) As in (a-b), respectively, but for the SMTH

1069 experiment at 1700 UTC on 28 May.

1070

1071

1072

1073

1074

1075

1076

1077

1078

1079

1080

1081

1082

1083

1084

1085

1086

1087

1088

1089

1090

1091

1092

1093

1094

1095 Fig. 15. As in Fig. 7, but for the profiles of temperature (solid) and dew-point temperature

1096 (dashed) below the 700-hPa level based on radiosonde observation at Ishigaki-jima for

1097 1200 UTC on 28 May 2011 (thick black) and the corresponding profiles based on the

1098 JMA-MSM analysis (thick red) and the CTL experiment (thick blue) for 1200 UTC and

1099 2000 UTC, respectively, on the same day at [24.3ºN, 124.18ºE]. Isotherms, dry adiabats,

1100 and moist pseudo-adiabats are drawn with thin lines for every 1 , 3K, and 2K.

℃ 67