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1764 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 52

Effects of Synoptic-Scale Wind under the Typical Summer Pressure Pattern on the Mesoscale High-Temperature Events in the Osaka and Kyoto Urban Areas by the WRF Model

YUYA TAKANE* Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Japan

YUKITAKA OHASHI Department of Biosphere–Geosphere Science, Okayama University of Science, Okayama, Japan

HIROYUKI KUSAKA Center for Computational Sciences, University of Tsukuba, Ibaraki, Japan

YOSHINORI SHIGETA Department of Environment System, Rissho University, Kumagaya, Japan

YUKIHIRO KIKEGAWA Graduate School of Science and Engineering, Meisei University, Hino, Japan

(Manuscript received 1 May 2012, in ﬁnal form 19 February 2013)

ABSTRACT

The actual conditions of mesoscale summer high temperatures (HTs) recorded in the Osaka–Kyoto urban area of Japan were investigated using an observation network. The daytime temperatures observed on 10 HT events in this area were the highest in the southern area of Kyoto [area with no Automated Meteorological Data Acquisition System (AMeDAS) observation sites]. To quantitatively evaluate the formation mechanisms of HT events, a heat budget analysis on an atmospheric column was conducted using the Weather Research and Forecasting (WRF) model. The results showed that over the HT area the daytime column temperature increased as a result of sensible-heat diffusion generated from the urban surface at the contribution rate of 54% and as a result of the sensible-heat advection and diffusion supplied from the sides and at the top of the column at the rate of 46% of all sensible heat supplied. To clarify previously unreported effects of synoptic-scale winds under typical summer pressure patterns on the HT events, a sensitivity experiment with no surface heat fluxes, backward trajectory analysis, and Euler forward tracer analysis was performed. These analyses yielded the following findings: 1) sensible heat at the synoptic scale and/or mesoscale was transported from the tropics by circulation patterns along the edge of the Pacific high as well as from tropical cyclones that were present in the vicinity of Japan and 2) airflow over the Kii Mountains also contributes to the HT events.

1. Introduction * Current afﬁliation: Research Institute for Environmental Man- agement Technology, National Institute of Advanced Industrial Urban high-temperature (HT) events in the summer Science and Technology, Ibaraki, Japan. present a social problem in Japan, and many researchers have studied the actual conditions and formation mech- Corresponding author address: Yuya Takane, Research Institute anisms of these events (e.g., Sakurai et al. 2009; Takane for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, and Kusaka 2011). Tsukuba, Ibaraki 305-8569, Japan. Some cities in the Osaka and Kyoto Prefectures (Fig. 1a) E-mail: [email protected] have had the highest August mean daily maximum

DOI: 10.1175/JAMC-D-12-0116.1

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in the horizontal), which is smaller than the mesoscale (about 100 km) that covers the Osaka–Kyoto urban area. Understanding the actual conditions of HT events in the Osaka–Kyoto urban area requires mesoscale analysis. There are few studies in which the formation mechanisms of HT events in the Osaka–Kyoto urban area have been investigated using numerical models (e.g., Ohashi and Kida 2002, 2004; Kitao et al. 2010). Ohashi and Kida (2002, 2004) found that HT events in the Osaka–Kyoto urban area were formed by a downward flow of thermally driven local circulations that developed in this area. However, their simulations did not include synoptic- scale wind effects. Synoptic-scale winds under the typical summer pressure pattern (described in section 4) occasionally in- duce airflow over Chubu Mountain, which contributes to HT events in the Tokyo metropolitan area in Japan (e.g., Takane and Kusaka 2011). This mechanism should be considered as a possible cause of the HT events in the Osaka–Kyoto urban area, which is surrounded by complex terrain. However, this mechanism has not been considered in previous studies. Hence, there are still many unknown aspects regarding the formation mechanism of HT events in the Osaka–Kyoto urban area. The purpose of this study is to assess previously unreported actual conditions and to quantitatively re- consider the formation mechanism of HT events in the Osaka–Kyoto urban area. We focus on the summer of 2007 in Japan. We will specifically consider previously unstudied effects of synoptic-scale wind on HT events

FIG. 1. (a) Topography and (b) land-use categories over the through sensitivity experiments, backward trajectory Osaka–Kyoto urban area. Also shown in (a) are the locations of analysis, and Euler forward tracer analysis. The results the analyzed sites: Osaka (Os), Hirakata (Hi), Kyoto (Ky), and will not only help to elucidate the formation mechanism Shionomisaki (Sh). of the HT events but will also be applicable to HT events that may occur in other regions with complex terrain. temperature over the past 30 yr (1981–2010) in Japan, as computed by the Japan Meteorological Agency (2011). 2. Observations The August mean daily maximum temperatures in the a. Observation locations and selection of HT events cities of Osaka (34.688N, 135.528E) and Kyoto (35.028N, 135.738E) are 33.48 and 33.38C, respectively. These values The surface air temperatures at 61 stations in the are higher than those of Otemachi in Tokyo (31.18C) and Osaka–Kyoto urban area were measured to investigate Nagoya City (32.88C). the actual conditions during HT events. Observational Osaka has the highest gross city product of Japanese sites were selected that were neither Automated Me- cities and faces Osaka Bay (Fig. 1). Kyoto is located teorological Data Acquisition System (AMeDAS) sites about 40 km northeast of Osaka City (Fig. 1). In this [AMeDAS is operated by the Japan Meteorological study, we refer to the area including these cities as the Agency (JMA)] nor the air pollution monitoring sta- Osaka–Kyoto urban area. tion operated by the Ministry of the Environment. For The actual conditions associated with HT events in our measurements, a portable thermistor thermome- the Osaka–Kyoto urban area have been investigated ter (Ondotori Jr. RTR-52; T&D Company, Ltd.) with using observational data based on the administrative a radiation shelter was used. The air temperatures over district (e.g., Moriyama et al. 2002). These studies fo- each site were measured every 30 s during the period of cused on the HT events at the local scale (10–30 km 1–14 August 2007.

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FIG. 2. Scatter diagrams of the daily maximum surface air temperature in Kyoto vs the 850-hPa temperature at Shionomisaki station at 0900 JST for the HT events during August from 1990 to 2011. The open squares indicate the 10 HT events selected in the present study (1, 5–12, and 14 Aug 2007). The times signs indicate all of the HT events during August from 1990 to 2011 except for the 10 HT events noted during August 2007.

During this period, we chose the following conditions to define an HT event: 1) daily maximum temperature over 33.48C at Osaka or 33.38C at Kyoto, which are the 30-yr climatological means of the August daily maximum temperatures; 2) sunshine duration in excess of 6 h 2 at Osaka and Kyoto; 3) wind speeds below 15 m s 1 at 850 hPa above Wajima, Shionomisaki, and Yonago at FIG. 3. Average diurnal variation of wind, surface air temperature, and net heat input into the atmospheric column from 0500 to 0900 and 2100 Japan standard time (JST); and 4) a typical 1700 JST for the 10 HT events (1, 5–12, and 14 Aug 2007) in (a) summer pressure pattern. Using these criteria, 10 events Osaka and (b) southern Kyoto. The first line of vectors is the ob- (1, 5–12, and 14 August 2007) were selected. served wind, and the second line is the simulated wind. The circles Here, we describe the climatological relevance of the represent the observed temperature, and the black solid line is the above 10 events. The scatter diagrams between the daily simulated temperature. The green solid line is the net heat input into the atmospheric column from the morning Q . The blue solid line maximum surface air temperature in Kyoto and the C denotes the time-integrated upward sensible heat flux from the 850-hPa temperature at 0900 JST at Shionomisaki station ground surface QH. The red solid line is the net heat input due to the for HT events in August from 1990 to 2011 is shown in heat flux convergence QCONV. Fig. 2. The maximum surface air temperature and the temperature at the 850-hPa level averaged for the se- b. Results lected 10 events are 35.38 and 18.58C, respectively, which are almost the same as the averages of all HT events Figure 3a shows diurnal variations of surface air during August from 1990 to 2011 (22 yr) (35.58 and 18.88C, temperature and horizontal wind from 0500 to 1700 JST, respectively). Moreover, the standard deviations of the which are averaged over the 10 HT events in Osaka. The maximum temperatures and the temperatures at the minimum and maximum surface air temperatures are 850-hPa level of the selected 10 events are 1.18 and 26.28C at 0500 JST and 33.08C at 1200 JST, respectively. 1.68C, respectively, which also compare well to those in The diurnal temperature range corresponds to 6.88C. A all HT events during August 1990–2011 (1.48 and 1.48C, west-southwesterly wind (sea breeze) is observed from respectively). Thus, the 10 events selected in the present 1100 to 1700 JST in Osaka. In the southern area of study are considered to be reasonably representative of Kyoto, the minimum surface air temperature is 25.48C HT events in August over the 22 yr studied. at 0500 JST, which is 0.88C lower than that of Osaka

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FIG. 4. Observed average (left) surface air temperature and (right) surface wind at (a),(b) 1000 and (c),(d) 1500 JST for the 10 HT events (1, 5–12, and 14 Aug 2007).

(Fig. 3b). On the other hand, the maximum temperature model, version 3.0.1.1 (Skamarock et al. 2008). The is 34.48C at 1500 JST, which is 1.48C higher than that of WRF model is very versatile and has been applied to Osaka. The diurnal temperature range corresponds to numerical studies of local wind and thermal environ- 9.08C, which is 2.28C higher. A southerly wind prevails ments in urban areas (e.g., Liu et al. 2006; Lo et al. 2007; from 1000 to 1700 JST in Kyoto. Miao et al. 2009; Grossman-Clarke et al. 2010; Chen Here, we describe a relation between horizontal dis- et al. 2011; Kusaka et al. 2012). The model domain tributions of surface air temperature and surface wind. shown in Fig. 1 covers western Japan, which includes the At 1000 JST, temperatures over 318C are observed around Osaka–Kyoto urban area that is the focus of our study. Osaka (Fig. 4a, black circle). Around this time, weak The domain consists of 150 grid points in the x and y winds are dominant in the Osaka–Kyoto urban area directions. We set the horizontal grid spacing to 2 km (Fig. 4b). However, by 1500 JST, a southwesterly sea and the model top to 50 hPa with 42 vertical sigma breeze develops and an HT area with temperatures ex- levels. Time integration was continuously conducted ceeding 348C forms in the inland area (extending from from 31 July to 15 August 2007. The initial and boundary Hirakata and southern Kyoto; indicated by black circle conditions were provided from the JMA mesoscale in Fig. 4c). An area of approximately 30-km radius over analysis (MANAL) dataset with 10-km horizontal and southern Kyoto experiences HT events. This study is the 3-h temporal resolution (atmosphere), National Centers ﬁrst to observe these at the resolution level used herein. for Environmental Prediction Final Analysis (FNL) data (land surface), and real-time global SST data (sea surface). The following schemes were used in the simulation: 3. Numerical simulation the Dudhia simple shortwave radiation scheme (Dudhia a. Description of numerical simulation 1989), the Rapid Radiative Transfer Model longwave radiation scheme (Mlawer et al. 1997), the WRF single- To quantitatively investigate the formation mecha- moment three-class (WSM3) cloud microphysics scheme nism of HT, a series of numerical simulations was con- (Hong et al. 2004; Dudhia 1989), the Noah land surface ducted by the Weather Research and Forecasting (WRF) model (Chen and Dudhia 2001), the single-layer Urban

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FIG. 5. As in Fig. 4, but for the simulated temperature and wind.

Canopy Model (UCM; Kusaka et al. 2001; Kusaka and surface wind also agrees with the observation (cf. Fig. 4d Kimura 2004a,b), and the Yonsei University (YSU) at- and Fig. 5d). mospheric boundary layer scheme (Hong et al. 2006). The horizontal distributions of mean bias and root- No cumulus parameterization was used. The UCM con- mean-square error (RMSE) averaged over 10 HT events siders the urban geometry, green fraction, and anthro- using hourly data covering 0000–2300 JST are shown in pogenic heat emission with diurnal variation at the Figs. 6a and 6b, respectively. The mean biases the HT urban grid. For parameter values (green fraction, build- region range from 20.58 to 10.58C, and the RMSE are ing height, building coverage ratio, sky-view factor, and ;18C. These results indicate that the WRF model re- daily mean anthropogenic heat emission), we used the produces the temperature in the HT region well. On the average values for the center of the Osaka urban area. other hand, the WRF model overestimates the temperature in the southern area of Osaka (Fig. 6a). These area b. Surface air temperature and wind reproduced by differences in mean bias and RMSE between the HT the WRF model region and the southern area of Osaka mainly arise Figure 3a indicates that the WRF model reproduces from the inability of the model to reproduce temper- the diurnal variations of surface wind and temperature atures during the nighttime especially in the early from the observations at Osaka. However, the model morning hours: the WRF model overestimates temper- underestimates the daily maximum temperature of ature at night in south Osaka relative to the HT region 33.08C by 0.48C. In southern Kyoto, the simulated wind (Fig. 7). A possible reason for the overestimation of and temperature agree with the observations; however, temperature during the nighttime in south Osaka is the the model underestimates the daily maximum temper- overestimation of urban parameters using the UCM ature by 0.68C. described in section 3a. Low-rise buildings mainly cover Figure 5c shows horizontal distributions of simulated the southern area of Osaka, whereas medium- and high- average surface air temperature at 1500 JST for the 10 HT rise buildings mainly cover the Osaka–Kyoto urban area. events. The simulated HT region agrees well with the The domain-averaged mean bias and RMSE of 61 sta- observed region (cf. Fig. 4c and Fig. 5c). The simulated tions are 20.18 and 1.48C, respectively. Figure 6c shows

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FIG. 6. Horizontal distributions of (a) mean bias, (b) RMSE, and (c) correlation coefficient between the observed and simulated hourly surface air temperatures averaged over the 10 HT events (1, 5–12, and 14 Aug 2007) using hourly data covering 0000–2300 JST. the horizontal distribution of the correlation coefficient 4. Heat budget analysis for an atmospheric column between the observed and simulated hourly air temperatures for the 10 HT events. The correlation coefficient To quantitatively evaluate the mechanism underlying for the HT region is over 10.94, which is larger than that the temperature change in the mixed layer, we con- in the coastal region around the Osaka–Kyoto urban ducted a heat budget analysis on an atmospheric col- area. The domain-averaged correlation coefficient of 61 umn. The top of the atmospheric column is set to a stations is 10.86, with significance at the 95% level (Fig. height of approximately 1500 m based on the mixed 8a). The above results mean that the WRF model is ap- layer height in the southern region of Kyoto (not shown). plicable to our analyses. The net heat input into the atmospheric column from

FIG. 7. Horizontal distributions of (a),(b) mean bias and (c),(d) RMSE averaged over the 10 HT events (August 1, 5–12, 14, 2007) using data for (a),(c) 0500 and (b),(d) 1500 JST data.

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FIG. 8. Scatter diagram of the observed air temperatures at 61 stations (x axis) vs the simulated air temperatures (y axis) at (a) 0000–2300, (b) 1000, and (c) 1500 JST for the 10 HT events (1, 5–12, and 14 Aug 2007).

21 21 the morning QC, the time-integrated sensible heat flux specific heat of the air (J kg K ), and r is the density of 23 from the ground surface QH, and the net heat input due dry air (kg m ). In this analysis, we assumed a Boussinesq to the heat flux convergence QCONV are defined in the approximation (density is constant with height) following following equations (e.g., Kusaka et al. 2000; Ohashi and previous studies [Kusaka et al. (2000); Ohashi and Kida Kida 2002): (2002) on the relationship between the heat island and ð local winds under the summer clear-sky conditions]. Note z R that the difference between the Q calculated by the Q 5 c r (u 2 u ) dz, (1) C C p 1 0 above simple heat budget analysis assuming a Boussi- zG ð nesq approximation and the QC determined by a precise t 1 analysis considering the density variation with height is Q 5 Hdt, and (2) H only about 5% averaged for the HT events in southern t0 Kyoto. Therefore, the above approximation is rea- 5 2 sonable in this analysis. The Q results indicate the net QCONV QC QH . (3) C heat input into the atmospheric column is from ZG to In these equations, u0 and u1 are the potential tem- ZR.TheH is the sensible heat flux from the ground peratures at 0500 JST and at the respective times (0600– surface, and QH indicates the time-integrated H from 1700 JST), and ZG and ZR are the ground surface and t0 to t1.TheQCONV represents the advection and dif- height of the atmospheric column. The quantity cp is the fusion of heat from the sides and top of the column

FIG. 9. Horizontal distributions of (a) the net heat input into the atmospheric column QC, (b) the time-integrated sensible heat ﬂux from the ground surface QH, and (c) the net heat input due to the heat ﬂux convergence QCONV, at 1500 JST averaged for the 10 HT events (1, 5–12, and 14 Aug 2007).

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TABLE 1. Values of QC, QH, and QCONV at 1500 JST for each of the HT events from the heat budget analysis of the atmospheric column over southern Kyoto. The values in parentheses represent the contributions of QH or QCONV to QC.

QCONV Surface air temperature at Temperature increase from 22 22 22 August day 2007 QC (MJ m ) QH (MJ m )(%) (MJ m ) (%) 1500 JST (8C) 0500 to 1500 JST (8C) 1 16.7 6.1 (37) 10.6 (63) 33.6 10.6 5 11.5 6.7 (58) 4.8 (42) 34.5 8.3 6 9.3 5.5 (59) 3.8 (41) 33.1 7.0 7 10.5 6.7 (64) 3.8 (36) 33.2 5.5 8 6.1 3.6 (59) 2.5 (41) 31.0 4.6 9 9.3 7.0 (75) 2.3 (25) 32.9 7.3 10 11.9 7.2 (61) 4.7 (39) 35.0 8.2 11 15.2 6.5 (43) 8.7 (57) 36.2 9.1 12 12.0 6.2 (52) 5.8 (48) 34.2 8.2 14 11.3 6.0 (53) 5.3 (47) 33.0 5.7 and diabatic heating by water vapor condensation and Here, we focus on the diurnal variation of the heat radiation. However, condensation does not occur in the budget in Osaka and southern Kyoto. In Osaka, QC present simulation, and the temperature change by ra- increases from 0500 to 1300 JST and remains constant diation is small. Thus, QCONV can be assumed to be the after 1300 JST (Fig. 3a). By 1500 JST, QH reaches 22 22 net heat input due to the heat ﬂux convergence by 7.2 MJ m , which is 0.4 MJ m larger than QC, while 22 the advection and diffusion of heat in the present study. QCONV is 20.4 MJ m at the same time. A possible As shown in Fig. 3, QC, QH, and QCONV are calculated reason for the negative value of QCONV is the pene- over all grids in the analysis domain for the 10 HT tration of the cooler air mass associated with the sea events. Here, we mainly discuss the results of Osaka and breeze from Osaka Bay. In other words, the tempera- southern Kyoto. Note that although the results of Osaka ture increase (QC increase) in Osaka is mitigated by the and southern Kyoto are each based on only one model penetration of the sea breeze. In southern Kyoto, the 22 grid column, values of QC, QH, and QCONV averaged maximum QC is 11.2 MJ m at 1500 JST (Fig. 3b). At 22 over several model grids around Osaka and southern the same time, QH and QCONV are 6.2 and 5.0 MJ m , Kyoto are approximately the same. respectively.

Figure 9 shows the horizontal distributions of QC, QH, The value of QC at 1500 JST in southern Kyoto is 22 and QCONV at 1500 JST averaged for the 10 HT events. 4.4 MJ m higher than that in Osaka. Relative to the re- 22 In the inland area including southern Kyoto (indicated sults from Osaka, the value of QH is 1.0 MJ m smaller, 22 22 by the white circle in Fig. 9a), QC is over 10 MJ m , and QCONV is 5.4 MJ m larger. This relatively larger which is higher than for the coastal area including Osaka. QCONV contributes to the difference in QC between the On the other hand, QH over the inland area is smaller cities of southern Kyoto and Osaka. Since Osaka is near than that of the coastal area (Fig. 9b). These results in- Osaka Bay, the temperature increase (QC increase) is dicate that the relatively high value of QC in the inland mitigated by the penetration of the sea breeze, as de- area (Fig. 9a) is due to the high value of QCONV in this scribed above. On the other hand, southern Kyoto is in- area (Fig. 9c). land; therefore the temperature increase is not mitigated

TABLE 2. As in Table 1, but for Osaka.

QCONV Surface air temperature at Temperature increase from 22 22 22 August day 2007 QC (MJ m ) QH (MJ m )(%) (MJ m ) (%) 1500 JST (8C) 0500 to 1500 JST (8C) 1 10.8 6.2 (57) 4.6 (43) 30.6 7.6 5 5.2 8.2 (158) 23.0 (—) 31.9 5.1 6 5.9 5.9 (100) 0.0 (0) 30.1 4.7 7 6.0 7.2 (120) 21.2 (—) 30.2 4.1 8 5.1 7.2 (141) 22.1 (—) 31.5 5.1 9 5.9 8.1 (137) 22.2 (—) 32.1 5.6 10 7.9 7.8 (99) 0.1 (1) 32.0 5.6 11 6.6 7.2 (109) 20.6 (—) 31.0 5.0 12 9.4 7.0 (74) 2.4 (26) 32.0 5.7 14 5.0 7.4 (148) 22.4 (—) 32.0 3.8

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The values of QC, QH, and QCONV at 1500 JST for each of the HT events in southern Kyoto and Osaka are summarized in Tables 1 and 2, respectively.

5. Effects of synoptic-scale wind under the typical summer pressure pattern As described in section 4, the temperature increase

(QC increase) in southern Kyoto was due to enhanced QCONV. Without synoptic-scale wind, the thermally driven local circulation is suggested as a primary source for

QCONV (Ohashi and Kida 2002). On the other hand, with synoptic-scale winds, it is not clear what contributes to QCONV. In this section, we investigate the effects of synoptic-scale winds under the typical summer pressure pattern of HT events. a. Experiment with no surface heat flux (case NO-SFCF) To investigate the effects of synoptic-scale wind on HT events, we conducted an experiment with no surface heat flux (case NO-SFCF). In the NO-SFCF case, surface-heat fluxes supplied from the ground surface at all grids are uniformly set to zero. In other words, QH is set to zero. Other model descriptions, such as the model physics and initial conditions, are unchanged. Here, we mainly discuss the results of 10 August 2007. Figure 10a shows the surface weather chart at 0900 JST 10 August 2007 where a North Pacific anticyclone covered Japan. The vertical profile of wind at the same time shows that a westerly wind appears below 1200-m height (Fig. 11a). Figure 12a shows the horizontal distribution of the wind at 1500 JST and the potential temperature difference between 1500 and 0500 JST 10 August 2007 at the 850-hPa level. Temperature increases appear not only above the Osaka–Kyoto urban area but also above the entire region. This temperature increase also appears above 700-m height (Fig. 12b). The CTRL case shows that the mixed layer height in southern Kyoto during the FIG. 10. Surface weather charts at 0900 JST on (a) 10 and daytime on the same day was approximately 1500 m, (b) 12 Aug 2007. which is higher than the aforementioned 700 m. Thus, it is believed that the above-mentioned synoptic- and/or cyclones that were present in the vicinity of Japan (not mesoscale temperature increases contributed to the shown). The south-southwesterly synoptic-scale wind HT events by diffusion and advection within the mixed is considered to be a contributory factor for the synoptic- layer. and/or mesoscale temperature increase(s). This temperature increase can also be simulated in 6 of 10 HT events (Table 3). In these six events, south- b. Backward trajectory analysis and Euler forward southwesterly winds were dominant in the Osaka– trace analysis Kyoto urban area. It is considered that the sensible heat is transported from the tropics by circulation along The following additional analysis suggested that another the edge of the Pacific high, as well as from tropical mechanism of the temperature increase associated with

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FIG. 11. Vertical proﬁles of potential temperature and wind at 0900 JST on (a) 10 and (b) 12 Aug 2007 at Shionomisaki station. The circles represent the observations, and the solid line is the simulated results. For each panel, the left vectors are the observed wind and the right vectors are the simulated wind.

synoptic-scale wind contributes to the HT events. Here, a 100-km2 area around southern Kyoto every hour we introduce the results of 12 August 2007. from 1000 to 1500 JST for the 10 HT events. Air par- Figure 10b shows the surface weather chart at 0900 cels were tracked back every 1 h using the three wind JST 12 August, when the North Pacific anticyclone components: u, y,andw. covered Japan. The vertical profile of wind at the same time shows that a southwesterly wind formed from near the surface of the ground to a height of 4000 m (Fig. 11b). Figure 13a shows the horizontal distribution of the horizontal wind and potential temperature at the 850-hPa level at 1500 JST 12 August 2007 for the CTRL case. Southeasterly synoptic-scale wind covers the entire region shown in Fig. 13a. The difference in the potential temperature between windward and leeward can be seen in the Kii Mountain Range. Figure 13b shows the vertical cross section of vertical velocity and potential temperature at 1500 JST on the same days along line C–D shown in Fig. 1a for the CTRL case. The downward flows are formed in the upper air over the area where the mountains are inclined leeward (e.g., 34.48,34.68, and 34.78). The potential temperature increases in that area. The similar distributions of downward flow and potential temperature can be seen from 0500 to 1500 JST. These results indicate that the temperature increase associated with the downward flow of airflow over the mountain occurs in the leeward area. FIG. 12. (a) Horizontal distribution of the wind (vectors) at 1500 To investigate the contribution of down-mountain flow JST and potential temperature difference (colors) between 1500 and 0500 JST 10 Aug 2007 at the 850-hPa pressure level. (b) Ver- to the increases in the surface temperature, a backward tical cross section of the potential temperature difference between trajectory analysis was employed. Fifty air parcels 1500 and 0500 JST 10 Aug 2007 along line A–B shown in Fig. 1a for were released from the lowest of the model grids over the NO-SFCF case.

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TABLE 3. The results of a sensitivity experiment with no surface heat ﬂuxes (NO-SFCF case) and backward trajectory analyses from the CTRL case for the 10 HT events (1, 5–12, and 14 Aug 2007). The open circles indicate the events with a synoptic- and/or mesoscale temperature increase of more than 18C above a height of 1500-m over the entire region shown in Fig. 8a from the NO-SFCF case. The dots indicate the events with airﬂow from over the mountain from the CTRL case.

August day Synoptic and/or mesoscale Airﬂow from during 2007 temperature increase over the mountain

1 s d 5 s 3 6 33 7 s 3 8 3 d 9 33 10 s 3 11 3 d 12 s d 14 s d

The trajectories of 50 air parcels released at 1000, FIG. 13. (a) Horizontal distribution of the wind (vectors) and potential temperature (colors) at the 850-hPa pressure level at 1500 1100, 1200, 1300, and 1400 JST indicate that many air JST 12 Aug 2007 for the CTRL case. (b) Vertical cross section of parcels are transported from the southeastern side vertical velocity (colors) and potential temperature (contours) at of the Kii Peninsula and above a 900-m height into 1500 JST 12 Aug 2007 along line C–D shown in Fig. 1a for the southern Kyoto (Figs. 14a–e). The trajectory does not CTRL case. go directly to the highest points of the Kii Mountains, but it does go over the mountain range’s northeastern side. These results mean that air parcels with high po- 1400 JST show that many air parcels transported to tential temperature above the southeastern side of the southern Kyoto originate from upper levels (altitudes Kii Peninsula flow in around southern Kyoto near the around 1200 m) over the ocean located on the south- ground. eastern side of the Kii Peninsula (not shown). This tra- To clearly show the evidence for air parcels above the jectory is similar to that of the CTRL case (Fig. 14). southeastern side of the Kii Peninsula inflow near the A similar result is obtained by an Eulerian forward ground surface around southern Kyoto, we performed an trace analysis for the NO-SFCF case (Fig. 16). These Eulerian forward tracer analysis. In this analysis, the re- results suggest that air parcels originating from upper lease point of the tracers is a square region along the levels over the ocean located on the southeastern side southeastern side of the Kii Peninsula (Figs. 15a–c, red of the Kii Peninsula arrive near the ground surface solid line) at elevations ranging from 900 to 1500 m. The around southern Kyoto, even without surface heat 2 concentration of the initial tracer is 1.0 (m3 m 3), and flux from the ground surface and development of a they are released at 0700 JST 12 August. We calcu- mixed layer over land. The above results indicate that lated their advection and diffusion for every time step the trajectories shown in Fig. 14 reveal airflow over the in the simulation. The tracers move northward and mountain. descend along the northern slope of the Kii Mountains Airflow over the mountain can be confirmed in 5 of to finally arrive at the surface near southern Kyoto the 10 HT events (Table 3). Additional analysis indicates (the HT area) at 1500 JST (Figs. 15a–c). This result is that synoptic-scale winds appearing in the above five consistent with the backward trajectory analysis shown events are south-southeasterly and blow over the Kii in Fig. 14. Incidentally, the tracers do not reach Osaka Mountain Range (Fig. 17). This suggests that south- because of the sea breeze that penetrates into this area southeasterly synoptic-scale wind is needed for the at 1500 JST (Fig. 15c). temperature increase associated with the airflow over In addition, we also conducted backward trajectory the mountain. analysis and an Eulerian forward tracer analysis for the The comparison in Tables 1 and 3 shows that the

NO-SFCF case, as with the CTRL case. The trajectories contribution rates of QCONV on the above ﬁve HT of 50 air parcels released at 1000, 1100, 1200, 1300, and events are from 41% to 63%, which is higher than

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FIG. 14. Backward trajectories of air parcels released from the lowest level on a model grid in southern Kyoto along with topography at (a) 1000, (b) 1100, (c) 1200, (d) 1300, (e) 1400, and (f) 1500 JST 12 Aug 2007 for the CTRL case. the 25%–42% range reported for the other five HT events c. Days with neither synoptic- and/or mesoscale that did not confirm airflow over the mountain. These temperature increase nor airflow over the results suggest that airflow over the mountain led to an mountains increase in the contribution rate of QCONV and played an important role in the five HT events. Table 3 shows that two events (6 and 9 August) had Scatter diagrams between the U and V components neither synoptic- and/or mesoscale temperature increases of winds at the 850-hPa level at Shionomisaki station nor airflow over the mountains. On 6 August 2007, the for HT events during August from 1990 to 2011 are synoptic-scale wind was weak (Fig. 18b) and sunshine shown in Fig. 17. There were 40 south-southeasterly duration was long (10.7 h). In general, the thermally synoptic-scale wind events, which corresponds to driven local circulation develops under such weather 14.9% of all events. This result suggests that the tem- conditions. Also, penetration of the sea breeze to southern perature increase associated with airflow over the Kyoto is prevented before 1400 JST. These weather mountain in the Osaka–Kyoto urban area appears not conditions are similar to those found in Ohashi and Kida only for the five HT events but also for other previous (2002), which has reported that valley wind circulation events. is an important factor in temperature increases in the

3 23 FIG. 15. Horizontal distribution of simulated trace concentration (m m ) along with topography for the CTRL case in the lowest level of the model grid at (a) 0900, (b) 1200, and (c) 1500 JST 12 Aug 2007. The red square indicates the area where the tracers were released.

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FIG. 17. Scatter diagrams of the U component (x axis) vs the V component (y axis) of the wind at the 850-hPa pressure level at Shionomisaki station at 0900 JST for the HT events during August from 1990 to 2011. The open squares indicate the five HT events with a temperature increase associated with airflow from over the mountain (1, 8, 11, 12, and 14 Aug 2007), and the gray circles show FIG. 16. Vertical cross section of the tracer concentration five other events without temperature increase (5–7, 9, and 10 Aug (colors) and potential temperature (contours) for the NO-SFCF 2007) from the CTRL case. The crosses are for all of the HT events case at (a) 0900, (b) 1200, and (c) 1500 JST 12 Aug 2007 along line during August from 1990 to 2011, except for the 10 HT events C–D shown in Fig. 1a. noted during August 2007.

22 interior of the Osaka–Kyoto urban area, as mentioned (QC) of 11.2 MJ m over the average for the 10 HT in section 1. Similar conditions were also observed for events in southern Kyoto is due to the sensible-heat 9 August 2007. For this reason, it is concluded that a diffusion generated from the urban ground surface valley wind circulation pattern contributed to the tem- QH at the contribution rate of 54% and the sensible- perature increases over the interior of the Osaka–Kyoto heat advection and diffusion supplied from the urban area during the above-discussed two HT events. sides and top of the column QCONV at a rate of 46% of all sensible heat supplied.

6. Summary (iii) The value of QC at 1500 JST in southern Kyoto 2 is 4.4 MJ m 2 higher than that in Osaka. Relative Previously unreported actual conditions and forma- to the results from Osaka, the value of QH is tion mechanisms involved with the mesoscale high- 2 2 1.0 MJ m 2 smaller and Q is 5.4 MJ m 2 larger. temperature events over the Osaka–Kyoto urban area CONV This relatively larger Q contributes to the dif- were investigated by using our observation network and CONV ference in Q between the cities of southern Kyoto the WRF model. We have speciﬁcally considered pre- C and Osaka. Osaka is near Osaka Bay; hence, the viously unstudied effects of synoptic-scale wind on the temperature increase (Q increase) is mitigated HT events through sensitivity experiments, backward tra- C because of the penetration of the sea breeze. On jectory analysis, and an Eulerian forward tracer analysis. the other hand, southern Kyoto is inland and there- The results are summarized as follows: fore the temperature increase is not mitigated but (i) Analysis of daytime temperatures observed during rather is enhanced by factors iv and v below and/or the 10 HT events over the Osaka–Kyoto urban area a thermally driven local circulation. indicated the highest temperatures were found over (iv) A sensitivity experiment with no surface heat ﬂuxes southern Kyoto (area with no AMeDAS observa- shows that synoptic- and/or mesoscale temperature tion sites). increases associated with south-southwesterly (ii) The heat budget analysis of the atmospheric col- synoptic-scale wind arriving from the tropics, con- umn shows that the daytime temperature increase tributes to 6 of 10 HT events by diffusion and

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FIG. 18. (a) Surface weather chart at 0900 JST 6 Aug 2007. (b) Vertical proﬁles of potential temperature and wind at 0900 JST 6 Aug 2007 at Shionomisaki station. The circles represent the observed result, and the solid line represents the simulated result. The left vectors represent the observed wind, and the right vectors show the simulated wind.

advection within the mixed layer of the Osaka– on Climate Change Adaptation (RECCA). This re- Kyoto urban area. search was also partially supported by the Environment (v) Backward trajectory and Eulerian forward tracer Research and Technology Development Fund (S-8) of analyses show that temperature increase with air- the Ministry of the Environment, Japan. Numerical sim- flow over the mountain occurs in the Osaka–Kyoto ulations for the present work have been carried out under urban area during 5 of the 10 HT events. Additional the Interdisciplinary Computational Science Program analysis suggests that a south-southeasterly synoptic- at the Center for Computational Sciences, University of scale wind is needed for the airflow over the moun- Tsukuba. The free software package Generic Mapping tain. Moreover, the airflow over the mountain led to Tools (GMT) was used in drawing the figures.

an increase in the contribution rate of QCONV and played an important role in the ﬁve HT events. REFERENCES

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