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  • View in gallery

    (a) Topography and (b) land-use categories over the Osaka–Kyoto urban area. Also shown in (a) are the locations of the analyzed sites: Osaka (Os), Hirakata (Hi), Kyoto (Ky), and Shionomisaki (Sh).

  • View in gallery

    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.

  • View in gallery

    Average diurnal variation of wind, surface air temperature, and net heat input into the atmospheric column from 0500 to 1700 JST for the 10 HT events (1, 5–12, and 14 Aug 2007) in (a) Osaka and (b) southern Kyoto. The first line of vectors is the observed wind, and the second line is the simulated wind. The circles represent the observed temperature, and the black solid line is the simulated temperature. The green solid line is the net heat input into the atmospheric column from the morning QC. The blue solid line denotes the time-integrated upward sensible heat flux from the ground surface QH. The red solid line is the net heat input due to the heat flux convergence QCONV.

  • View in gallery

    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).

  • View in gallery

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

  • View in gallery

    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.

  • View in gallery

    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.

  • View in gallery

    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).

  • View in gallery

    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).

  • View in gallery

    Surface weather charts at 0900 JST on (a) 10 and (b) 12 Aug 2007.

  • View in gallery

    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.

  • View in gallery

    (a) Horizontal distribution of the wind (vectors) at 1500 JST and potential temperature difference (colors) between 1500 and 0500 JST 10 Aug 2007 at the 850-hPa pressure level. (b) Vertical cross section of the potential temperature difference between 1500 and 0500 JST 10 Aug 2007 along line A–B shown in Fig. 1a for the NO-SFCF case.

  • View in gallery

    (a) Horizontal distribution of the wind (vectors) and potential temperature (colors) at the 850-hPa pressure level at 1500 JST 12 Aug 2007 for the CTRL case. (b) Vertical cross section of vertical velocity (colors) and potential temperature (contours) at 1500 JST 12 Aug 2007 along line C–D shown in Fig. 1a for the CTRL case.

  • View in gallery

    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.

  • View in gallery

    Horizontal distribution of simulated trace concentration (m3 m−3) 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.

  • View in gallery

    Vertical cross section of the tracer concentration (colors) and potential temperature (contours) for the NO-SFCF case at (a) 0900, (b) 1200, and (c) 1500 JST 12 Aug 2007 along line C–D shown in Fig. 1a.

  • View in gallery

    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 five other events without temperature increase (5–7, 9, and 10 Aug 2007) from the CTRL case. The crosses are for all of the HT events during August from 1990 to 2011, except for the 10 HT events noted during August 2007.

  • View in gallery

    (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.

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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

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  • 1 Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Japan
  • | 2 Department of Biosphere–Geosphere Science, Okayama University of Science, Okayama, Japan
  • | 3 Center for Computational Sciences, University of Tsukuba, Ibaraki, Japan
  • | 4 Department of Environment System, Rissho University, Kumagaya, Japan
  • | 5 Graduate School of Science and Engineering, Meisei University, Hino, Japan
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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.

Current affiliation: Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan.

Corresponding author address: Yuya Takane, Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. E-mail: takane.yuya@aist.go.jp

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.

Current affiliation: Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan.

Corresponding author address: Yuya Takane, Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan. E-mail: takane.yuya@aist.go.jp

1. Introduction

Urban high-temperature (HT) events in the summer present a social problem in Japan, and many researchers have studied the actual conditions and formation mechanisms of these events (e.g., Sakurai et al. 2009; Takane and Kusaka 2011).

Some cities in the Osaka and Kyoto Prefectures (Fig. 1a) have had the highest August mean daily maximum temperature over the past 30 yr (1981–2010) in Japan, as computed by the Japan Meteorological Agency (2011). The August mean daily maximum temperatures in the cities of Osaka (34.68°N, 135.52°E) and Kyoto (35.02°N, 135.73°E) are 33.4° and 33.3°C, respectively. These values are higher than those of Otemachi in Tokyo (31.1°C) and Nagoya City (32.8°C).

Fig. 1.
Fig. 1.

(a) Topography and (b) land-use categories over the Osaka–Kyoto urban area. Also shown in (a) are the locations of the analyzed sites: Osaka (Os), Hirakata (Hi), Kyoto (Ky), and Shionomisaki (Sh).

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Osaka has the highest gross city product of Japanese cities and faces Osaka Bay (Fig. 1). Kyoto is located about 40 km northeast of Osaka City (Fig. 1). In this study, we refer to the area including these cities as the Osaka–Kyoto urban area.

The actual conditions associated with HT events in the Osaka–Kyoto urban area have been investigated using observational data based on the administrative district (e.g., Moriyama et al. 2002). These studies focused on the HT events at the local scale (10–30 km 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 induce 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 reconsider 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 through sensitivity experiments, backward trajectory analysis, and Euler forward tracer analysis. The results will not only help to elucidate the formation mechanism of the HT events but will also be applicable to HT events that may occur in other regions with complex terrain.

2. Observations

a. Observation locations and selection of HT events

The surface air temperatures at 61 stations in the Osaka–Kyoto urban area were measured to investigate the actual conditions during HT events. Observational sites were selected that were neither Automated Meteorological Data Acquisition System (AMeDAS) sites [AMeDAS is operated by the Japan Meteorological Agency (JMA)] nor the air pollution monitoring station operated by the Ministry of the Environment. For our measurements, a portable thermistor thermometer (Ondotori Jr. RTR-52; T&D Company, Ltd.) with a radiation shelter was used. The air temperatures over each site were measured every 30 s during the period of 1–14 August 2007.

During this period, we chose the following conditions to define an HT event: 1) daily maximum temperature over 33.4°C at Osaka or 33.3°C at Kyoto, which are the 30-yr climatological means of the August daily maximum temperatures; 2) sunshine duration in excess of 6 h at Osaka and Kyoto; 3) wind speeds below 15 m s−1 at 850 hPa above Wajima, Shionomisaki, and Yonago at 0900 and 2100 Japan standard time (JST); and 4) a typical summer pressure pattern. Using these criteria, 10 events (1, 5–12, and 14 August 2007) were selected.

Here, we describe the climatological relevance of the above 10 events. The scatter diagrams between the daily maximum surface air temperature in Kyoto and the 850-hPa temperature at 0900 JST at Shionomisaki station for HT events in August from 1990 to 2011 is shown in Fig. 2. The maximum surface air temperature and the temperature at the 850-hPa level averaged for the selected 10 events are 35.3° and 18.5°C, respectively, which are almost the same as the averages of all HT events during August from 1990 to 2011 (22 yr) (35.5° and 18.8°C, respectively). Moreover, the standard deviations of the maximum temperatures and the temperatures at the 850-hPa level of the selected 10 events are 1.1° and 1.6°C, respectively, which also compare well to those in all HT events during August 1990–2011 (1.4° and 1.4°C, respectively). Thus, the 10 events selected in the present study are considered to be reasonably representative of HT events in August over the 22 yr studied.

Fig. 2.
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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

b. Results

Figure 3a shows diurnal variations of surface air temperature and horizontal wind from 0500 to 1700 JST, which are averaged over the 10 HT events in Osaka. The minimum and maximum surface air temperatures are 26.2°C at 0500 JST and 33.0°C at 1200 JST, respectively. The diurnal temperature range corresponds to 6.8°C. A west-southwesterly wind (sea breeze) is observed from 1100 to 1700 JST in Osaka. In the southern area of Kyoto, the minimum surface air temperature is 25.4°C at 0500 JST, which is 0.8°C lower than that of Osaka (Fig. 3b). On the other hand, the maximum temperature is 34.4°C at 1500 JST, which is 1.4°C higher than that of Osaka. The diurnal temperature range corresponds to 9.0°C, which is 2.2°C higher. A southerly wind prevails from 1000 to 1700 JST in Kyoto.

Fig. 3.
Fig. 3.

Average diurnal variation of wind, surface air temperature, and net heat input into the atmospheric column from 0500 to 1700 JST for the 10 HT events (1, 5–12, and 14 Aug 2007) in (a) Osaka and (b) southern Kyoto. The first line of vectors is the observed wind, and the second line is the simulated wind. The circles represent the observed temperature, and the black solid line is the simulated temperature. The green solid line is the net heat input into the atmospheric column from the morning QC. The blue solid line denotes the time-integrated upward sensible heat flux from the ground surface QH. The red solid line is the net heat input due to the heat flux convergence QCONV.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Here, we describe a relation between horizontal distributions of surface air temperature and surface wind. At 1000 JST, temperatures over 31°C are observed around Osaka (Fig. 4a, black circle). Around this time, weak winds are dominant in the Osaka–Kyoto urban area (Fig. 4b). However, by 1500 JST, a southwesterly sea breeze develops and an HT area with temperatures exceeding 34°C forms in the inland area (extending from Hirakata and southern Kyoto; indicated by black circle in Fig. 4c). An area of approximately 30-km radius over southern Kyoto experiences HT events. This study is the first to observe these at the resolution level used herein.

Fig. 4.
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).

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

3. Numerical simulation

a. Description of numerical simulation

To quantitatively investigate the formation mechanism of HT, a series of numerical simulations was conducted by the Weather Research and Forecasting (WRF) model, version 3.0.1.1 (Skamarock et al. 2008). The WRF model is very versatile and has been applied to numerical studies of local wind and thermal environments in urban areas (e.g., Liu et al. 2006; Lo et al. 2007; Miao et al. 2009; Grossman-Clarke et al. 2010; Chen et al. 2011; Kusaka et al. 2012). The model domain shown in Fig. 1 covers western Japan, which includes the Osaka–Kyoto urban area that is the focus of our study. The domain consists of 150 grid points in the x and y directions. We set the horizontal grid spacing to 2 km and the model top to 50 hPa with 42 vertical sigma levels. Time integration was continuously conducted from 31 July to 15 August 2007. The initial and boundary conditions were provided from the JMA mesoscale analysis (MANAL) dataset with 10-km horizontal and 3-h temporal resolution (atmosphere), National Centers 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: the Dudhia simple shortwave radiation scheme (Dudhia 1989), the Rapid Radiative Transfer Model longwave radiation scheme (Mlawer et al. 1997), the WRF single-moment three-class (WSM3) cloud microphysics scheme (Hong et al. 2004; Dudhia 1989), the Noah land surface model (Chen and Dudhia 2001), the single-layer Urban Canopy Model (UCM; Kusaka et al. 2001; Kusaka and Kimura 2004a,b), and the Yonsei University (YSU) atmospheric boundary layer scheme (Hong et al. 2006). No cumulus parameterization was used. The UCM considers the urban geometry, green fraction, and anthropogenic heat emission with diurnal variation at the urban grid. For parameter values (green fraction, building height, building coverage ratio, sky-view factor, and daily mean anthropogenic heat emission), we used the average values for the center of the Osaka urban area.

b. Surface air temperature and wind reproduced by the WRF model

Figure 3a indicates that the WRF model reproduces the diurnal variations of surface wind and temperature from the observations at Osaka. However, the model underestimates the daily maximum temperature of 33.0°C by 0.4°C. In southern Kyoto, the simulated wind and temperature agree with the observations; however, the model underestimates the daily maximum temperature by 0.6°C.

Figure 5c shows horizontal distributions of simulated average surface air temperature at 1500 JST for the 10 HT events. The simulated HT region agrees well with the observed region (cf. Fig. 4c and Fig. 5c). The simulated surface wind also agrees with the observation (cf. Fig. 4d and Fig. 5d).

Fig. 5.
Fig. 5.

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

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

The horizontal distributions of mean bias and root-mean-square error (RMSE) averaged over 10 HT events using hourly data covering 0000–2300 JST are shown in Figs. 6a and 6b, respectively. The mean biases the HT region range from −0.5° to +0.5°C, and the RMSE are ~1°C. These results indicate that the WRF model reproduces the temperature in the HT region well. On the other hand, the WRF model overestimates the temperature in the southern area of Osaka (Fig. 6a). These area differences in mean bias and RMSE between the HT region and the southern area of Osaka mainly arise from the inability of the model to reproduce temperatures during the nighttime especially in the early morning hours: the WRF model overestimates temperature at night in south Osaka relative to the HT region (Fig. 7). A possible reason for the overestimation of temperature during the nighttime in south Osaka is the overestimation of urban parameters using the UCM described in section 3a. Low-rise buildings mainly cover the southern area of Osaka, whereas medium- and high-rise buildings mainly cover the Osaka–Kyoto urban area. The domain-averaged mean bias and RMSE of 61 stations are −0.1° and 1.4°C, respectively. Figure 6c shows the horizontal distribution of the correlation coefficient between the observed and simulated hourly air temperatures for the 10 HT events. The correlation coefficient for the HT region is over +0.94, which is larger than that in the coastal region around the Osaka–Kyoto urban area. The domain-averaged correlation coefficient of 61 stations is +0.86, with significance at the 95% level (Fig. 8a). The above results mean that the WRF model is applicable to our analyses.

Fig. 6.
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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Fig. 7.
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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Fig. 8.
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).

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

4. Heat budget analysis for an atmospheric column

To quantitatively evaluate the mechanism underlying the temperature change in the mixed layer, we conducted a heat budget analysis on an atmospheric column. The top of the atmospheric column is set to a height of approximately 1500 m based on the mixed layer height in the southern region of Kyoto (not shown). The net heat input into the atmospheric column from the morning QC, the time-integrated sensible heat flux from the ground surface QH, and the net heat input due to the heat flux convergence QCONV are defined in the following equations (e.g., Kusaka et al. 2000; Ohashi and Kida 2002):
e1
e2
e3

In these equations, θ0 and θ1 are the potential temperatures at 0500 JST and at the respective times (0600–1700 JST), and ZG and ZR are the ground surface and height of the atmospheric column. The quantity cp is the specific heat of the air (J kg−1 K−1), and ρ is the density of dry air (kg m−3). In this analysis, we assumed a Boussinesq approximation (density is constant with height) following previous studies [Kusaka et al. (2000); Ohashi and Kida (2002) on the relationship between the heat island and local winds under the summer clear-sky conditions]. Note that the difference between the QC calculated by the above simple heat budget analysis assuming a Boussinesq approximation and the QC determined by a precise analysis considering the density variation with height is only about 5% averaged for the HT events in southern Kyoto. Therefore, the above approximation is reasonable in this analysis. The QC results indicate the net heat input into the atmospheric column is from ZG to ZR. The H is the sensible heat flux from the ground surface, and QH indicates the time-integrated H from t0 to t1. The QCONV represents the advection and diffusion of heat from the sides and top of the column and diabatic heating by water vapor condensation and radiation. However, condensation does not occur in the present simulation, and the temperature change by radiation is small. Thus, QCONV can be assumed to be the net heat input due to the heat flux convergence by the advection and diffusion of heat in the present study. As shown in Fig. 3, QC, QH, and QCONV are calculated over all grids in the analysis domain for the 10 HT events. Here, we mainly discuss the results of Osaka and southern Kyoto. Note that although the results of Osaka and southern Kyoto are each based on only one model grid column, values of QC, QH, and QCONV averaged over several model grids around Osaka and southern Kyoto are approximately the same.

Figure 9 shows the horizontal distributions of QC, QH, and QCONV at 1500 JST averaged for the 10 HT events. In the inland area including southern Kyoto (indicated by the white circle in Fig. 9a), QC is over 10 MJ m−2, which is higher than for the coastal area including Osaka. On the other hand, QH over the inland area is smaller than that of the coastal area (Fig. 9b). These results indicate that the relatively high value of QC in the inland area (Fig. 9a) is due to the high value of QCONV in this area (Fig. 9c).

Fig. 9.
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).

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Here, we focus on the diurnal variation of the heat budget in Osaka and southern Kyoto. In Osaka, QC increases from 0500 to 1300 JST and remains constant after 1300 JST (Fig. 3a). By 1500 JST, QH reaches 7.2 MJ m−2, which is 0.4 MJ m−2 larger than QC, while QCONV is −0.4 MJ m−2 at the same time. A possible reason for the negative value of QCONV is the penetration of the cooler air mass associated with the sea breeze from Osaka Bay. In other words, the temperature increase (QC increase) in Osaka is mitigated by the penetration of the sea breeze. In southern Kyoto, the maximum QC is 11.2 MJ m−2 at 1500 JST (Fig. 3b). At the same time, QH and QCONV are 6.2 and 5.0 MJ m−2, respectively.

The value of QC at 1500 JST in southern Kyoto is 4.4 MJ m−2 higher than that in Osaka. Relative to the results from Osaka, the value of QH is 1.0 MJ m−2 smaller, and QCONV is 5.4 MJ m−2 larger. This relatively larger QCONV contributes to the difference in QC between the cities of southern Kyoto and Osaka. Since Osaka is near Osaka Bay, the temperature increase (QC increase) is mitigated by the penetration of the sea breeze, as described above. On the other hand, southern Kyoto is inland; therefore the temperature increase is not mitigated by the sea breeze, but rather is enhanced by factors presented in section 5.

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.

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.

Table 1.

Table 2.

As in Table 1, but for Osaka.

Table 2.

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).

Fig. 10.
Fig. 10.

Surface weather charts at 0900 JST on (a) 10 and (b) 12 Aug 2007.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Fig. 11.
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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

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 daytime on the same day was approximately 1500 m, which is higher than the aforementioned 700 m. Thus, it is believed that the above-mentioned synoptic- and/or mesoscale temperature increases contributed to the HT events by diffusion and advection within the mixed layer.

Fig. 12.
Fig. 12.

(a) Horizontal distribution of the wind (vectors) at 1500 JST and potential temperature difference (colors) between 1500 and 0500 JST 10 Aug 2007 at the 850-hPa pressure level. (b) Vertical cross section of the potential temperature difference between 1500 and 0500 JST 10 Aug 2007 along line A–B shown in Fig. 1a for the NO-SFCF case.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

This temperature increase can also be simulated in 6 of 10 HT events (Table 3). In these six events, south-southwesterly winds were dominant in the Osaka–Kyoto urban area. It is considered that the sensible heat is transported from the tropics by circulation along the edge of the Pacific high, as well as from tropical cyclones that were present in the vicinity of Japan (not shown). The south-southwesterly synoptic-scale wind is considered to be a contributory factor for the synoptic- and/or mesoscale temperature increase(s).

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 1°C 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.

Table 3.

b. Backward trajectory analysis and Euler forward trace analysis

The following additional analysis suggested that another mechanism of the temperature increase associated with synoptic-scale wind contributes to the HT events. Here, we introduce the results of 12 August 2007.

Figure 10b shows the surface weather chart at 0900 JST 12 August, when the North Pacific anticyclone 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., 34.4°, 34.6°, and 34.7°). 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. 13.
Fig. 13.

(a) Horizontal distribution of the wind (vectors) and potential temperature (colors) at the 850-hPa pressure level at 1500 JST 12 Aug 2007 for the CTRL case. (b) Vertical cross section of vertical velocity (colors) and potential temperature (contours) at 1500 JST 12 Aug 2007 along line C–D shown in Fig. 1a for the CTRL case.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

To investigate the contribution of down-mountain flow to the increases in the surface temperature, a backward trajectory analysis was employed. Fifty air parcels were released from the lowest of the model grids over a 100-km2 area around southern Kyoto every hour from 1000 to 1500 JST for the 10 HT events. Air parcels were tracked back every 1 h using the three wind components: u, υ, and w.

The trajectories of 50 air parcels released at 1000, 1100, 1200, 1300, and 1400 JST indicate that many air parcels are transported from the southeastern side of the Kii Peninsula and above a 900-m height into southern Kyoto (Figs. 14a–e). The trajectory does not 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 potential temperature above the southeastern side of the Kii Peninsula flow in around southern Kyoto near the ground.

Fig. 14.
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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

To clearly show the evidence for air parcels above the southeastern side of the Kii Peninsula inflow near the ground surface around southern Kyoto, we performed an Eulerian forward tracer analysis. In this analysis, the release point of the tracers is a square region along the southeastern side of the Kii Peninsula (Figs. 15a–c, red solid line) at elevations ranging from 900 to 1500 m. The concentration of the initial tracer is 1.0 (m3 m−3), and they are released at 0700 JST 12 August. We calculated their advection and diffusion for every time step in the simulation. The tracers move northward and descend along the northern slope of the Kii Mountains to finally arrive at the surface near southern Kyoto (the HT area) at 1500 JST (Figs. 15a–c). This result is consistent with the backward trajectory analysis shown in Fig. 14. Incidentally, the tracers do not reach Osaka because of the sea breeze that penetrates into this area at 1500 JST (Fig. 15c).

Fig. 15.
Fig. 15.

Horizontal distribution of simulated trace concentration (m3 m−3) 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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

In addition, we also conducted backward trajectory analysis and an Eulerian forward tracer analysis for the NO-SFCF case, as with the CTRL case. The trajectories of 50 air parcels released at 1000, 1100, 1200, 1300, and 1400 JST show that many air parcels transported to southern Kyoto originate from upper levels (altitudes around 1200 m) over the ocean located on the southeastern side of the Kii Peninsula (not shown). This trajectory is similar to that of the CTRL case (Fig. 14). A similar result is obtained by an Eulerian forward trace analysis for the NO-SFCF case (Fig. 16). These results suggest that air parcels originating from upper levels over the ocean located on the southeastern side of the Kii Peninsula arrive near the ground surface around southern Kyoto, even without surface heat flux from the ground surface and development of a mixed layer over land. The above results indicate that the trajectories shown in Fig. 14 reveal airflow over the mountain.

Fig. 16.
Fig. 16.

Vertical cross section of the tracer concentration (colors) and potential temperature (contours) for the NO-SFCF case at (a) 0900, (b) 1200, and (c) 1500 JST 12 Aug 2007 along line C–D shown in Fig. 1a.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

Airflow over the mountain can be confirmed in 5 of the 10 HT events (Table 3). Additional analysis indicates that synoptic-scale winds appearing in the above five events are south-southeasterly and blow over the Kii Mountain Range (Fig. 17). This suggests that south-southeasterly synoptic-scale wind is needed for the temperature increase associated with the airflow over the mountain.

Fig. 17.
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 five other events without temperature increase (5–7, 9, and 10 Aug 2007) from the CTRL case. The crosses are for all of the HT events during August from 1990 to 2011, except for the 10 HT events noted during August 2007.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

The comparison in Tables 1 and 3 shows that the contribution rates of QCONV on the above five HT events are from 41% to 63%, which is higher than the 25%–42% range reported for the other five HT events that did not confirm airflow over the mountain. These results suggest that airflow over the mountain led to an increase in the contribution rate of QCONV and played an important role in the five HT events.

Scatter diagrams between the U and V components of winds at the 850-hPa level at Shionomisaki station for HT events during August from 1990 to 2011 are shown in Fig. 17. There were 40 south-southeasterly synoptic-scale wind events, which corresponds to 14.9% of all events. This result suggests that the temperature increase associated with airflow over the mountain in the Osaka–Kyoto urban area appears not only for the five HT events but also for other previous events.

c. Days with neither synoptic- and/or mesoscale temperature increase nor airflow over the mountains

Table 3 shows that two events (6 and 9 August) had neither synoptic- and/or mesoscale temperature increases nor airflow over the mountains. On 6 August 2007, the synoptic-scale wind was weak (Fig. 18b) and sunshine duration was long (10.7 h). In general, the thermally driven local circulation develops under such weather conditions. Also, penetration of the sea breeze to southern Kyoto is prevented before 1400 JST. These weather conditions are similar to those found in Ohashi and Kida (2002), which has reported that valley wind circulation is an important factor in temperature increases in the interior of the Osaka–Kyoto urban area, as mentioned in section 1. Similar conditions were also observed for 9 August 2007. For this reason, it is concluded that a valley wind circulation pattern contributed to the temperature increases over the interior of the Osaka–Kyoto urban area during the above-discussed two HT events.

Fig. 18.
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.

Citation: Journal of Applied Meteorology and Climatology 52, 8; 10.1175/JAMC-D-12-0116.1

6. Summary

Previously unreported actual conditions and formation mechanisms involved with the mesoscale high-temperature events over the Osaka–Kyoto urban area were investigated by using our observation network and the WRF model. We have specifically considered previously unstudied effects of synoptic-scale wind on the HT events through sensitivity experiments, backward trajectory analysis, and an Eulerian forward tracer analysis. The results are summarized as follows:

  1. Analysis of daytime temperatures observed during the 10 HT events over the Osaka–Kyoto urban area indicated the highest temperatures were found over southern Kyoto (area with no AMeDAS observation sites).

  2. The heat budget analysis of the atmospheric column shows that the daytime temperature increase (QC) of 11.2 MJ m−2 over the average for the 10 HT events in southern Kyoto is due to the sensible-heat diffusion generated from the urban ground surface QH at the contribution rate of 54% and the sensible-heat advection and diffusion supplied from the sides and top of the column QCONV at a rate of 46% of all sensible heat supplied.

  3. The value of QC at 1500 JST in southern Kyoto is 4.4 MJ m−2 higher than that in Osaka. Relative to the results from Osaka, the value of QH is 1.0 MJ m−2 smaller and QCONV is 5.4 MJ m−2 larger. This relatively larger QCONV contributes to the difference in QC between the cities of southern Kyoto and Osaka. Osaka is near Osaka Bay; hence, the temperature increase (QC increase) is mitigated because of the penetration of the sea breeze. On the other hand, southern Kyoto is inland and therefore the temperature increase is not mitigated but rather is enhanced by factors iv and v below and/or a thermally driven local circulation.

  4. A sensitivity experiment with no surface heat fluxes shows that synoptic- and/or mesoscale temperature increases associated with south-southwesterly synoptic-scale wind arriving from the tropics, contributes to 6 of 10 HT events by diffusion and advection within the mixed layer of the Osaka–Kyoto urban area.

  5. Backward trajectory and Eulerian forward tracer analyses show that temperature increase with airflow over the mountain occurs in the Osaka–Kyoto urban area during 5 of the 10 HT events. Additional analysis suggests that a south-southeasterly synoptic-scale wind is needed for the airflow over the mountain. Moreover, the airflow over the mountain led to an increase in the contribution rate of QCONV and played an important role in the five HT events.

Factors iii and iv above mean that the synoptic-scale wind under the typical summer pressure pattern contributes to the HT events in the Osaka–Kyoto urban area. These factors are different from the thermally driven local circulation patterns suggested by previous studies. This result will not only help to elucidate the formation mechanism of HT events but will also be applicable to HT events that may occur in other regions with complex terrain.

Acknowledgments

We thank three anonymous reviewers for providing valuable comments that helped us to improve the manuscript. We thank Dr. Asuka Suzuki-Parker of the University of Tsukuba for her help in improving this manuscript. We also thank Dr. Wei Wang of the National Center for Atmospheric Research for her helpful advice on Euler forward tracer analysis. The present study was supported by the Research Program on Climate Change Adaptation (RECCA). This research was also partially supported by the Environment Research and Technology Development Fund (S-8) of the Ministry of the Environment, Japan. Numerical simulations for the present work have been carried out under the Interdisciplinary Computational Science Program at the Center for Computational Sciences, University of Tsukuba. The free software package Generic Mapping Tools (GMT) was used in drawing the figures.

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