Impacts of frontal SST gradient on the formation of axially. asymmetric thermal structure of a tropical cyclone:

Transcription

1 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 4 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 Submitted to Monthly Weather Review in April 2015, as a potential contribution to the Special Collection Climate Implications of Frontal-Scale Air-Sea Interaction Revised in September, 2015 *Corresponding author address: Fukiko Takehi, Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1, Komaba, Meguro-ku, Tokyo, , Japan. takehi@atmos.rcast.u-tokyo.ac.jp Current affiliation: Office of Observation Systems Operation, Japan Meteorological Agency, Tokyo, Japan Additional affiliation: APL, JAMSTEC, Yokohama, Japan 1

2 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. 2

3 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 ). 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 3

4 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 4

5 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 ; 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 5

6 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 (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; 6

7 114 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 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. 7

8 132 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 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 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 ( Within the domain [ N, E], the data are available on a grid at each of the 16 pressure levels, 146 while precipitation and surface meteorological data are provided on a 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 8

9 150 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 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, E] and Minami-daito-jima [25.83 N, E] stations by JMA. The data were 163 obtained from the JMA website ( 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) 9

10 168 (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 [ N, 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 height levels, including nine levels below 1.5 km in altitude. The three lowest levels are at m, 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 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 10

11 186 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 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 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 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 11

12 204 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 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, E] on 19 May 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 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 12

13 222 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 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 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. 13

14 240 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 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 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 14

15 258 temperature (θθ) distribution at the level of 4,135 m (equivalent to the 600-hPa level) was nearly 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 θθ distribution at the level of 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 d) 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 15

16 276 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 θθ field thus 291 highlights the asymmetric structure in the θθ field itself (Fig. 8a). 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: 16 (1)

17 294 θθ diff = abs θθ 3.5 east θθ 3.5 west, (1) 295 where θθ 3.5 east(west) denotes an instantaneous value of θθ at 3.5 east (west) of the TC center. 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 θθ diff calculated for each level and time step is shown in the along-track section in Fig. 9a. It is 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) 17

18 denotes 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 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 (θθ ), which is defined as local deviations from the basic state 326 (θθ ), based on the following thermodynamic equation in height coordinates: θθ JJJJ = CC pp TT (VV θθ) ww, (2) 327 where CC pp R the specific heat of dry air at constant pressure, TT temperature, VV 328 horizontal wind velocity and ww vertical velocity. Otherwise the notation is standard. Diabatic 18

19 329 heating/cooling JJ has been estimated as the residual of (2), where the time tendency of θθ on 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, E] and the other to the east 335 [25-27 N, 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 θθ anomalies at the 290-m level for 1200 UTC on 28 May, when the 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 19

20 347 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 (VV θθ) = VV θθ VV θθ (VV θθ ). (3) 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 20

21 365 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 VV θθ was dominant in the anomalous advection at the 290-m level (Fig. 11a), while the 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 b). 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 21

22 383 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 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 22

23 401 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 , 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. 23

24 419 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 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)

25 437 b. Impact of the OFZ on the axially asymmetric thermal structure of the TC 438 Figure 13a shows horizontal distribution of θθ anomalies at the 160-m level and their 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 to 0000 UTC 01 June 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 25

26 455 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 VV θθ (Fig. 14a, d, 14g) and (VV θθ ) (Fig. 14b, 14e, 14f) contribute substantially to the anomalous cold 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 26

27 473 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 27

28 491 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 ), 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 θθ and anomalous 502 wind VV to VV θθ, we evaluate how much the domain-averaged value of VV θθ at the m level to the west of the TC center in the CTL experiment (Table 2b) could be altered if 504 either θθ or VV within the particular domain in the CTL experiment were replaced with the 505 corresponding field simulated in the SMTH experiment. Through this hypothetical evaluation, 506 the value of VV θθ in the CTL experiment would decrease by 0.10 K hour -1 (26 %) if θθ 507 were replaced, while the corresponding decrease would be only half in magnitude if VV were 508 replaced. The evaluation thus suggests certain importance of the OFZ for generating stronger 28

29 509 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 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 29

30 527 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 30

31 545 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 31

32 563 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 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 32

33 581 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 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 In the JMA-MSM analysis, a band 598 of positive temperature anomalies extended northeastward from the vicinity of the warm core of 33

34 599 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. 34

35 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 ( 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 and by Japanese Ministry of 631 Environment through Environment Research and Technology Development Funds 2A-1201 and References 635 Bond, N. A., M. F. Cronin, and M. Garvert, 2010: Atmospheric sensitivity to SST near the 35

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41 723 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 1 ) and (d) upward SHF (W m 2 ), and (e) equatorward gradients of seven-day running 730 mean SST and 290 m potential temperature (K (100 km) -1 ) 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 UTC on 28 May). 41

42 735 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 1 ) and (d) upward SHF (W m 2 ), and (e) equatorward gradients of seven-day running 741 mean SST and 290-m potential temperature (K (100 km) -1 ). western domain eastern domain (a) Heat budget [K (hour) 1 ] JJJJ CC pp TT (VV θθ) (wwwwww/ ) ( θθ )/ (b) Decomposition of anomalous horizontal advection [K (hour) 1 ] VV θθ VV θθ (VV θθ ) (c) 290-m anomalous southerly wind velocity [m s 1 ] (d) Anomalous upward SHF [W m 2 ] υυ (SHF) (e) Equatorward gradient of seven-day running mean [K (100km) 1 ] θθ (SST)

43 742 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 UTC on 28 May). CTL MSMTH SMTH (a) Heat budget [K (hour) 1 ] (JJJJ (CC pp TT)) (VV θθ) (wwwwww/ ) ( θθ )/ (b) Decomposition of anomalous horizontal advection [K (hour) 1 ] VV θθ VV θθ VV θθ (c) 160-m anomalous southerly wind velocity [m s 1 ] (d) Anomalous upward SHF [W m 2 ] υυ (SHF) (e) Equatorward gradient of the seven-day mean [K (100km) 1 ] θθ SST

44 750 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 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 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 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 1 ) based on the JMA-MSM analysis and SST (, colored) based on 766 MGDSST. Blue and red squares denote the positions of radiosonde stations at 767 Ishigaki-jima and Minami-daito-jima, respectively. 44

45 768 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) hpa thickness (a) at 1200 UTC on 28 May and (b) at 0000 UTC on 29 May in (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 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 hPa level at 1200 UTC on 28 May 2011 at (a) Ishigaki-jima [24.33 N, E] 783 (blue) and (b) Minami-daito-jima [25.83 N, E] (red). Isotherms (black; every ), 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 45

46 786 corresponding profiles based on the JMA-MSM analysis from the 1000-hPa level to the hPa level at (a) [24.3 N, E] (blue) and (b) [25.8 N, E] (red) are 788 superimposed. 789 Fig. 8. Longitudinal cross sections across the TC center (26.4 N, black dashed lines) at 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 θθ diff (colored) following the TC center from 0000 UTC on 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 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 1 ) and equatorward 799 gradient of potential temperature (colored), in addition to equatorward SST gradient (K 800 (100 km) 1 ) plotted below the section with red line. 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 46

47 804 and cooling tendencies (red and blue solid contours, respectively) are superimposed 805 (value: ± 0.08, 0.16, 0.24 K (hour) 1 ). The positions of the TC center at the reference 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 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 May 2011 (contoured every 1 K), superimposed on (a) anomalous wind (vector, m s 1 ) 815 and VV θθ (colored) at the same level, or (b) seven-day running mean SST (colored), 816 based on the JMA-MSM analysis. The cross signs and rectangles are the same as in Fig 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 hpa level) and SST both averaged from 0000 UTC 25 May to 0000 UTC 1 June 47

48 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 May, with the central pressure of (a) 955 hpa, (b) 967 hpa, (c) 971 hpa and (d) 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 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. 48

49 840 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 ) potential temperature (contoured for every 1 K) and the resultant anomalous 843 thermal advection VV θθ (colored as indicated at the bottom of the figure) simulated 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 (VV θθ ) (colored). (c) As in (a), but the 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 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 UTC, respectively, on the same day at [24.3ºN, ºE]. Isotherms, dry adiabats, 857 and moist pseudo-adiabats are drawn with thin lines for every 1, 3K, and 2K. 49

50 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

51 Fig. 2. JMA surface weather charts for (a) 1200 UTC on 28 May and (b) 0000 UTC on 29 May 884 in

52 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 X denotes the surface TC center at 898 each time

53 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 May in 2011, superimposed on seven-day averaged fields (0000 UTC on 25 May through UTC on 1 June) of 200-hPa wind (arrows; plotted only if the speed exceeds m s 1 ) based on the JMA-MSM analysis and SST (, colored) based on MGDSST. Blue 907 and red squares denote the positions of radiosonde stations at Ishigaki-jima and 908 Minami-daito-jima, respectively. 53

54 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. 54

55 Fig. 6. Horizontal thermal structure of Songda based on the JMA-MSM analysis. (a-b) hpa thickness (a) at 1200 UTC on 28 May and (b) at 0000 UTC on 29 May in (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

56 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 hPa level at 1200 UTC on 28 May 2011 at (a) Ishigaki-jima [24.33 N, E] 946 (blue) and (b) Minami-daito-jima [25.83 N, E] (red). Isotherms (black; every ), 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 hPa level at (a) [24.3 N, E] (blue) and (b) [25.8 N, E] (red) are 951 superimposed. 56

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

58 Fig. 9. (a) Along-track section of θθ diff (colored) following the TC center from 0000 UTC on 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 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 1 ) and equatorward 979 gradient of potential temperature (colored), in addition to equatorward SST gradient (K 980 (100 km) 1 ) plotted below the section with red line. 58

59 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 59

60 985 and cooling tendencies (red and blue solid contours, respectively) are superimposed 986 (value: ± 0.08, 0.16, 0.24 K (hour) 1 ). The positions of the TC center at the reference 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 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

61 Fig. 11. Seven-day running mean potential temperature at the 290-m level for 0900 UTC on May 2011 (contoured every 1 K), superimposed on (a) anomalous wind (vector, m s 1 ) 1010 and VV θθ (colored) at the same level, or (b) seven-day running mean SST (colored), 1011 based on the JMA-MSM analysis. The cross signs and rectangles are the same as in Fig

62 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 hpa level) and SST both averaged from 0000 UTC 25 May to 0000 UTC 1 June 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. 62

63 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 (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 63

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

65 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 ) potential temperature (contoured for every 1 K) and the resultant anomalous 1061 thermal advection VV θθ (colored as indicated at the bottom of the figure) simulated 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 (VV θθ ) (colored). (c) As in (a), but the 65

66 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

67 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 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 UTC, respectively, on the same day at [24.3ºN, ºE]. Isotherms, dry adiabats, 1100 and moist pseudo-adiabats are drawn with thin lines for every 1, 3K, and 2K. 67