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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 mecha- nisms 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 contri- bution 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 Ó 2013 American Meteorological Society Unauthenticated | Downloaded 10/06/21 11:15 PM UTC AUGUST 2013 T A K A N E E T A L . 1765 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 mecha- nisms 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 ther- mally 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. Unauthenticated | Downloaded 10/06/21 11:15 PM UTC 1766 JOURNAL OF APPLIED METEOROLOGY AND CLIMATOLOGY VOLUME 52 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.

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