Formation Mechanisms of the Extreme High Surface Air Temperature of 40.98C Observed in the Tokyo Metropolitan Area: Considerations of Dynamic Foehn and Foehnlike Wind

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

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

(Manuscript received 21 December 2010, in ﬁnal form 8 April 2011)

ABSTRACT

A record-breaking high surface air temperature in Japan of 40.98C was observed on 16 August 2007 in Kumagaya, located 60 km northwest of central Tokyo. In this study, the formation mechanisms of this extreme high temperature event are statistically and numerically investigated using observational data and the Weather Research and Forecasting (WRF) model. The extreme event is caused by a combination of two particular factors: 1) Persistent sunshine and a lack of precipitation during the seven consecutive days preceding 16 August 2007 were seen in Kumagaya. This was the 12th-longest stretch of clear-sky days in July and August from 1998 up to 2008. Persistent clear-sky days allow the ground surface to dry out, which produces an increase in sensible heat flux from the ground surface. This contributes to the extreme event, and its mechanism is qualitatively supported by the results of sensitivity experiments of soil moisture on surface air temperature. 2) A foehnlike wind appears in the numerical simulation, which is caused by diabatic heating with subgrid-scale turbulent diffusion and sensible heat flux from the ground surface when this airflow passes in the mixed layer over the Chubu Mountains and the inland of the Tokyo metropolitan area. Backward trajectory analysis and Lagrangian energy budget analysis show that the foehnlike wind plays a more important role in the extreme event than the adiabatic dynamic foehn pointed out by previous studies.

1. Introduction A 74-year-old record was broken on 16 August 2007, when a surface air temperature of 40.98C was observed The northern part of the Tokyo metropolitan area in at 1442 Japan standard time (JST) in Kumagaya, located Japan is famous for the frequent occurrence of extreme 60 km northwest of central Tokyo, which is surrounded high surface air temperatures of over 388C, causing an by the Chubu Mountains and the Pacific Ocean (Fig. 1). increase in heat strokes there (Fujibe 1998, 2004). In Several previous studies have tried to elucidate the this study, we define the events with a daily maximum formation mechanisms of the 16 August 2007 event. surface air temperature of over 388C as the extreme Sakurai et al. (2009) reported diurnal variations and hor- high temperature events, after the definition by Fujibe izontal distributions of surface air temperature and wind (1998). In the present region, the frequency of extreme field on 16 August using the data from surface-observation high temperature events and the daily maximum tem- and wind profiler networks operated by the Japan perature has been increasing over several decades (Fujibe Meteorological Agency (JMA). Shinohara et al. (2009) 1995, 1998, 2004). These increases are often discussed in conducted a numerical simulation and forward trajec- relation to global warming and urban heat island. tory analysis with no-diffusion-process using the JMA nonhydrostatic model. Sakurai et al. (2009) and Shinohara et al. (2009) attributed the extreme high temperature event to the following factors: 1) adiabatic compression Corresponding author address: Yuya Takane, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 due to a large-scale descending current under the anti- Tennoudai, Tsukuba, Ibaraki 305-8572, Japan. cyclone, 2) high surface air temperature in the early E-mail: [email protected] morning on 16 August, 3) no penetration of sea breeze

DOI: 10.1175/JAMC-D-10-05032.1

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2000). Type 2 is a dynamic (dry) foehn wind caused by adiabatic heating (von Ficker 1910; Scorer and Klieforth 1959; Arakawa 1969; Brinkmann 1973, 1974; Ikawa and Nagasawa 1989; Seibert 1990). Finally, type 3 is a phenomenon caused by airflow over a mountain and diabatic heating from the ground surface, which in turn has been heated by solar radiation (Kondo 1983; Kondo and Kuwagata 1984). In this study, we describe the type 3 as foehnlike wind. Surface air temperature in the coastal area of Toyama Bay often reaches high figures, associated with airflow over the Chubu Mountains when cyclones pass over the Sea of Japan. Ishizaki and Takayabu (2009) reported that the above types 1, 2, and 3 caused some of these high temperature events. Consequently, all the three types should be considered as the possible causes in the extreme high temperature event on 16 August 2007, over the Kanto Plain, located on the south side of the Chubu Mountains. The purpose of this study is to clarify quantitatively the formation mechanisms of the extreme high temperature event on 16 August 2007, specifically considering the contributions of the foehn and the foehnlike wind. These contributions will be quantitatively analyzed using backward trajectory analysis, Lagrangian energy budget analysis, Euler forward tracer analysis, and a sensitivity experiment of soil moisture on surface air temperature. The results will not only help to elucidate the formation mechanism of the extreme high surface air temperature events but will also be applicable to high temperature events that may occur in the regions with complex terrain.

FIG. 1. (a) Map of Japan, (b) topography, and (c) land-use categories in central Japan and site locations of the analyzed point: 2. Observed features of the extreme high surface Takada (Ta), Maebashi (Ma), Kumagaya (Ku), Saitama (Sa), Tokyo air temperature event on 16 August 2007 (To), and Tsukuba (Ts). The actual conditions of the extreme high temperature from Tokyo Bay, and 4) a dynamic foehn with north- event are at first investigated using the following obser- westerly airflow from the Chubu Mountains. They insisted vational data: weather charts, rawinsonde data, wind that the dynamic foehn was the most important factor in profiler network and data acquisition system (WINDAS), causing the extreme high temperature event. Watarai and data from the Automated Meteorological Data Ac- et al. (2009a,b) also agreed with their idea that the dy- quisition System (AMeDAS) network operated by the namic foehn is the most important factor from their JMA. simulation results. However, these conclusions are based Figure 3 shows the surface weather chart at 0900 JST on just a hypothesis and do not show clear and quantitative 16 August 2007, showing a North Pacific anticyclone evidence of the role of the dynamic foehn. Hence, there covered central Japan. Figure 4 shows the observed sur- are still many unknown aspects regarding the formation face air temperature and wind fields at 1440 JST on the mechanisms of the extreme high temperature event. same day. High surface air temperature regions of over Takahashi (1996) summarized three types of high 388C extend in and around Kumagaya and Saitama cities, temperature phenomena associated with airflow over which are north of the convergence zone between north- a mountain (Fig. 2). Type 1 is a thermodynamic (wet) northwesterly winds and southerly sea breezes. foehn wind caused by diabatic heating by water vapor Figure 5a shows the daily precipitation from 1 to condensation (Hann 1866, 1867; Barry 1992; Whiteman 16 August 2007 in Kumagaya. From 1 to 16 August,

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FIG. 2. Schematic representation of typical types of high temperature phenomena associated with airflow over a mountain [this figure is a modification of Takahashi (1996)].

precipitation occurred only on 5 August. Sunshine dura- Figure 8 shows the diurnal variation of surface air tion of more than 8 h was observed from 9 to 16 August temperature observed at Kumagaya on 16 August. The (Fig. 5b). On 16 August, the sunshine duration was 10.8 h. daily minimum and the daily maximum temperatures The daily maximum surface air temperature progressively were observed at 0519 JST (28.88C) and at 1442 JST increased from 9 to 11 August (Fig. 5c). The temperature (40.98C), respectively. decreased on 12 August, but increased again after that. Figure 6 shows the vertical profiles of potential temperature on 0900 JST from 13 to 16 August 2007, in 3. Numerical simulation Tsukuba, located 70 km east of Kumagaya. The poten- a. Description of numerical simulation tial temperature between 400- and 1500-m level gradu- ally increased by the day—particularly seen from 14 to The numerical model used in this study is the Advanced 15 August. On 15 August, an isentropic layer was seen Research Weather Research and Forecasting (WRF) around 1000 m. Above 2200 m, potential temperature model, version 3.0.1.1 (Skamarock et al. 2008). The WRF decreased from 14 to 16 August. The opposite variation model is very versatile and has been applied to numerical occurred in the layer between 400 and 1500 m. studies of local wind and thermal environments in urban Figure 7a shows the time–height cross section of wind areas (e.g., Kusaka and Hayami 2006; Liu et al. 2006; Lo during 0500–1900 JST 16 August 2007 at Kumagaya et al. 2007; Tsunematsu et al. 2009a,b; Chen et al. 2010; observed by the WINDAS. A north-northwesterly wind Grossman-Clarke et al. 2010; Kusaka et al. 2010). The blew from 2000 to 4000 m during this period. On the specifications of the WRF model numerical experiments other hand, northwest-westerly or weak winds were seen used in this study are summarized in Table 1. Table 2 near the surface, from 0500 JST to around 1330 JST. shows the land-use categories from the Geospatial In- After that, the wind direction changed from west-northwest formation Authority of Japan (GSI) used in this study. to north-northwest, and the north-northwesterly wind The physics of the WRF model used in the present nu- flow continued until 1900 JST. Similar diurnal variations merical experiment are shown in Table 3. Horizontal grid were observed at the surface (16.8-m height) at Kumagaya spacing is set at 2 km in the inner region. The model top is as well (Fig. 7b). The surface wind speed increased from set to be 50 hPa, and 42 vertical sigma levels are used. 0500 JST (sunrise) to 1500 JST and the daily maximum Time integration was continuously conducted from 1 to wind speed appeared at 1500 JST. 16 August 2007. The first day was considered as the

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FIG. 3. Surface weather chart at 0900 JST on 16 Aug 2007. model spinup. The initial and boundary conditions were created from the JMA–Mesoscale Objective Analysis Dataset (MANAL) data (atmosphere), National Cen- ters for Environmental Prediction–Final (NCEP-FNL) data (land surface), and the NCEP–Real-Time Global Sea Surface Temperature Analysis (RTG-SST) data (sea surface). The land-use and terrain-height dataset created by the GSI was used in the present study. Note that the numerical simulations in this study do not consider the difference of urban parameters per grid point. Thus, the simulations cannot sufﬁciently represent the area around observation site on the microscale.

b. Results of the numerical simulation FIG. 4. (a) Observed surface air temperature and (b) wind fields at 1440 JST 16 Aug 2007. Figures 7b and 8 indicate that the WRF model reproduces diurnal variations of observed surface wind and air temperature on 16 August in Kumagaya. The simulated For the wind speed, the model reproduces the obser- wind direction and speed are in good agreement with vations well, except at around 2000 and 3300 m. The those observed. Diurnal variation of surface air temper- simulated potential temperature is roughly in agreement ature is also reproduced well. However, the model un- with the observed temperature, although the model derestimates the daily maximum temperature of 40.98C overestimates the observations by about 1 K above by about 28C; the simulated daily maximum temperature 1200 m and 2 K below 500 m. is about 398C. Figure 10 shows the horizontal distributions of simu- Figure 9 shows the performance of the WRF model in lated surface air temperature and wind field at 1440 JST. reproducing vertical profiles of wind and potential The high surface air temperature region of over 388Cis temperature in Tsukuba at 0900 JST 16 August 2007. extended in and around Kumagaya and Saitama, and the The simulated wind direction above 1500 m is in good north-northwesterly wind covers the northern part of the agreement with observed direction, although the model Tokyo metropolitan area. The high surface air temperature failed to reproduce observations between 500 and 1500 m. region is in close agreement with the north-northwesterly

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FIG. 6. Vertical proﬁles of potential temperature at 0900 JST from 13 to 16 Aug 2007 in Tsukuba. The broken lines with dots represent 13 Aug, the dotted line with open squares is 14 Aug, the solid line shows 15 Aug, and the solid line with solid circles depicts 16 Aug.

model. Diurnal variation of the wind above 1200 m with a northwesterly wind component is roughly in agreement with the observations. Within the layer of 500– 1000 m, the west wind changes to a northwesterly wind with time. Wind speed increases with time. The diurnal variation of the wind in this layer is roughly in agreement with the observed variation, except for the simulated westerly wind in the early morning. The simulated potential temperature increases with time below 1700-m height associated with the northwest- westerly wind. An increase in the potential temperature is also seen from 1030 to 1230 JST below about 1000 m, and from 1330 JST to around 1500 JST at the surface. The increase in potential temperature results in the mixed layer developing to about 1400 m at around 1500 JST. Similar tendencies in the vertical proﬁle of potential temperature are simulated in Saitama (ﬁgure omitted). c. Heat budget analysis in the column atmosphere

FIG. 5. Time changes in (a) daily precipitation, (b) sunshine To quantitatively evaluate the mechanism underlying duration, and (c) daily maximum surface air temperature from 1 to the temperature change in the mixed layer, we conducted 16 Aug 2007 in Kumagaya. a heat budget analysis on the column atmosphere. The top of the column atmosphere is set to about 1400-m wind region. These results are similar to the observed height based on the results of Fig. 11. The cumulative results (Fig. 10 versus Fig. 4). heat in the column atmosphere (QC), the time-integrated Figure 11 shows a time–height cross section of hori- sensible heat flux from the ground surface (QH), and the zontal wind, the vertical component, and the potential cumulative heat flux convergence (QCONV) are defined temperature on 16 August in Kumagaya from the WRF as shown in the following equations:

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TABLE 1. Speciﬁcations of the WRF model used in the present numerical experiment. Here, NOAA is the National Oceanic and Atmospheric Administration.

Region 1 Domain Fig. 1b,c Horizontal resolution (km) 2.0 Grid number (x, y, z) 200, 200, 42 Integral time (h) 384 (0900 JST 1–17 Aug 2007) Initial and boundary conditions (atmosphere) JMA-MANAL (land) NCEP-FNL (sea) NOAA/NCEP RTG-SST Land use and terrain height GSI digital national land information

column atmosphere 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 lateral and FIG. 7. (a) Time–height cross section of horizontal wind (vectors) top of the column atmosphere and diabatic heating and vertical component (shading) observed by WINDAS. (b) Di- by water vapor condensation and radiation. However, urnal variation of surface wind direction and wind speed from 0500 to 1900 JST 16 Aug 2007 in Kumagaya. The circles represent the condensation does not occur in the present simulation, AMeDAS observed result and the solid line represents the simu- and the temperature change by radiation is small. Thus, lated result. QCONV can be assumed to be the cumulative heat ﬂux convergence in the present study. ð Figure 12 shows the results of the analysis: Q is zR CONV QC 5 cpr (u1 2 u0) dz, (1) larger than QH from 0500 to 1400 JST; QCONV contributes z G at least 50% of the increase in QC until 1500 JST. The ð results indicate that the temperature increase in the t1 QH 5 Hdt, and (2) mixed layer is caused not only by QH but also by t 0 QCONV, where QCONV is seen as a very important factor of the temperature increase. The present result is dif- Q 5 Q 2 Q . (3) CONV C H ferent from that of Kusaka et al. (2000), who conducted

Here, u0 and u1 are the potential temperatures at t0 (0500 JST) and at the respective times, and ZG and ZR TABLE 2. Land-use categories from GSI used in the present are the ground surface and height at the top of the column numerical experiment and U.S. Geological Survey (USGS). atmosphere, respectively. The quantity cp is the speciﬁc heat of air (J kg21 K21), and r is the density of dry air GSI used in the 23 present simulation USGS (kg m ). The QC indicates the cumulative heat in the Urban Urban and built-up land Paddy Irrigated cropland and pasture Grass Dryland cropland and pasture Cropland–grassland mosaic Cropland–woodland mosaic Grassland Shrubland Savanna Forest Deciduous broadleaf forest Deciduous needleleaf forest Evergreen broadleaf forest Evergreen needleleaf forest FIG. 8. Diurnal variation of surface air temperature from 0000 Mixed forest to 1900 JST 16 Aug 2007 in Kumagaya. The circles represent Water Water bodies the observed result and the solid line represents the simulated Herbaceous wetland result.

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TABLE 3. Physics of the WRF model used in the present numerical experiment.

Physics Reference Microphysics WRF single-moment 3-class (WSM3) Hong et al. (2004); Dudhia (1989) Radiation (longwave) Rapid Radiative Transfer Model (RRTM) Mlawer et al. (1997) Radiation (shortwave) Dudhia Dudhia (1989) Boundary layer Mellor–Yamada–Janjic (MYJ) Mellor and Yamada (1982); Janjic (2002) Surface layer MYJ (based on similarity theory; Monin Janjic (1996; 2002) and Obukhow 1954) Land surface (no urban area) Noah land surface model (LSM) Chen and Dudhia (2001) Land surface (urban area) Single-layer urban canopy model (UCM) Kusaka et al. (2001); Kusaka and Kimura (2004a,b)

a similar analysis in a flat area in the vicinity of Saitama, height during the daytime. Based on these data, sensible and obtained a negative QCONV value. In contrast, the heat advection with the northwest-westerly wind below present results are similar to those of Ohashi and Kida 2000 m is significant in increasing QCONV. (2002), who conducted a similar analysis in the Kyoto basin in western Japan. These results suggest that there is a relationship between QCONV and mountainous areas. 4. Considerations of dynamic foehn Sensible heat transfer from the top of the present column and foehnlike wind atmosphere by turbulent diffusion is small (figures omitted). a. Backward trajectory analysis and Lagrangian Heat transfer from the top of the column atmosphere by energy budget analysis advection is consistently seen and should be a factor to consider (Fig. 11). As described above, the northwest- A backward trajectory analysis is employed to quan- westerly wind blows consistently from the surface to 2000-m titatively investigate the contributions of the foehn and

FIG. 9. Vertical proﬁles of (a) wind direction, wind speed, and (b) potential temperature at 0900 JST 16 Aug 2007 in Tsukuba. The circles represent the observed result and the solid line represents the simulated result.

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FIG. 11. Time–height cross section of horizontal wind (vectors), vertical component (shading), and potential temperature (solid lines) on 16 Aug 2007 in Kumagaya from the WRF model.

high lines. The second main course consists of about 57%ofallthetrajectoriesandshowstheseairparcels are transported from Toyama Bay to the vicinity of Kumagaya (Fig. 13c). We deﬁned these 57 trajectories as course low lines. Lagrangian energy budget analysis is performed by calculating dry static energy (s;Jkg21) to determine which kind of winds blow along each course:

s 5 gZ 1 cpT, (4)

where gZ is the geopotential energy and cpT is the sensible heat energy. Generally, diabatic heating is produced by the following factors: water vapor condensation, radiation, subgrid-scale turbulent diffusion, and sensible heat flux from the ground surface. In the present analysis, water vapor condensation does not occur along either course. Thus, condensation does not contribute to the change in dry static energy s. Direct heating by radiation is small. FIG. 10. (a) Surface air temperature and (b) wind fields at 1440 JST Therefore, in the present analysis diabatic heating is 16 Aug 2007 from the WRF model. thought to be caused by subgrid-scale turbulent diffusion and sensible heat flux from the ground surface. Figure 14a shows the Lagrangian mean energy budget foehnlike wind associated with the northwest-westerly along the trajectories for course high lines. Geopotential wind on the extreme high temperature event. The tra- energy gZ shows that air parcels constantly pass through jectories of 100 air parcels were released from the lowest at about 2300-m height in the free atmosphere from 0500 level of the model grid in a square area of 400 km2 to 1110 JST. The parcels then eventually descend from the around Kumagaya at 1500 JST 16 August 2007. Air free atmosphere to the mixed layer. The decrease in gZ parcels on that day are tracked back every 10 min from causes cpT to increase. During this time, air parcels ap- 1500 to 0500 JST using wind components of u, y, and w. proach near the ground surface. Dry static energy s is al- Figure 13a shows the trajectories of 100 air parcels. most constant from 0500 to 1500 JST. However, it slightly The results indicate that the air parcels take mainly two decreases from 1110 to 1230 JST and increases from 1230 courses. The first main course consists of about 30% of to 1340 JST. This increase could be supported by subgrid- all the trajectories and shows these air parcels are trans- scale turbulent diffusion and sensible heat flux from the ported from the Sea of Japan to the vicinity of Kumagaya ground surface (Fig. 14b). From these findings, the airflow (Fig. 13b). Here, we define these 30 trajectories as course along course high lines is under an adiabatic process and

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FIG. 12. Time series of cumulative heat in Kumagaya. The solid line with solid circles represents the cumulative heat in column atmosphere QC. The dotted line denotes the time-integrated sensible heat flux from ground surface QH. The solid line with open circles represents the cumulative heat flux convergence QCONV. corresponds to a dynamic foehn wind. It should be noted that the mean temperature in the air parcels at 1500 JST is about 36.18C (Table 4), which is lower than the simulated surface air temperature at 1500 JST in Kumagaya. Figure 15a shows the Lagrangian mean energy budget along the trajectories for course low lines. Geopotential energy gZ shows that parcels pass through at the 900– 1000-m height in the free atmosphere from 0500 to 0830 JST. After that, the air parcels penetrate from the free atmosphere into the mixed layer. At around 1100 JST, the parcels arrive near the mountain surface and eventually reach the vicinity of Kumagaya, with descending. The decrease in gZ causes cpT to increase. Dry static energy s is not constant from 0500 to 1500 JST. It is almost constant before 1000 JST and then begins to increase. As a result, dry static energy s increases to about 15000 J kg21,which indicates that diabatic heating contributes to the increase in s. From these findings, we can conclude that the north- northwesterly airflow on 16 August corresponds to a combination of types 2 and 3 (hereinafter called type 4) shown in Fig. 16. It should be noted that the air parcels mean temperature at 1500 JST is about 38.18C(Table4). This temperature is higher than these of course high lines and close to the simulated surface air temperature in Kumagaya. b. Euler forward tracer analysis and measurement of the sensitivity of surface air temperature FIG. 13. Backward trajectories of parcels released from the lowest level in a model grid around Kumagaya. (a) All trajectories to soil moisture and (b) a compartment of course high lines and (c) course low lines (see text for explanation of terms used). The trajectories are Since the backward trajectory analysis does not cal- occupied by course high lines (about 30%), course low lines culate subgrid-scale turbulence diffusion every time step (57%), and other courses (17%). Black represents 0500–0700, in the simulation, Euler forward tracer concentration blue is 0700–0900, green is 0900–1100, red is 1100–1300, and pink analysis is performed to investigate the contribution is 1300–1500 JST. of subgrid-scale turbulence diffusion for the course low lines. The release point of the tracers is a square area of

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21 FIG. 14. (a) The mean energy variation (J kg ). The y axis in- FIG. 15. As in Fig. 14, but for course low lines shown in Fig. 13c. dicates energy (J kg21), and the secondary y axis indicates the terrain height and mixed-layer height (zi) along the trajectories for course high lines shown in Fig. 13b. The solid lines indicate gZ, turbulent diffusion and sensible heat flux from the moun- dotted lines show c T, the white circles show s, and broken lines p tainsurfaceontheheatingofairflowalongthecourse with dots show zi (all lines are based on anomalies at 1500 JST). Shades represent terrain height along the trajectories. (b) The low lines. The initial time of the sensitively experi- time-integrated sensible heat flux from the ground surface H ments; case SMOIS_DRY and case SMOIS_WET are (MJ m22) along the trajectories. 0900 JST 15 August. The soil moisture content at the initial time of the cases SMOIS_DRY and SMOIS_WET are defined as the following equations: 3600 km2 around Toyama Bay from 1 to 8 of vertical model grid number (roughly equal below 1200-m height), which corresponds to the starting point of airflow along QD 5Q 2 0:15 and (5) course low lines (Fig. 17a). The tracers are released at 0500 JST 16 August. The advection and diffusion of tracers QW 5Q10:15. (6) are calculated every time step in the simulation. Figure 17a shows the horizontal distribution of the Here, QD and QW are soil moisture content at the initial 3 23 simulated tracer concentration (m m ) in the lowest time for cases SMOIS_DRY and SMOIS_WET, re- level of the model grid at 1500 JST 16 August. The tracers spectively. The Q is soil moisture content at 0900 JST cover not only the Chubu Mountains but also cover the 15 August for the control simulation (case CTRL). inland part of the Tokyo metropolitan area, including Note that the above modification is applied to the all Kumagaya. Figure 17b shows the vertical cross section of grid points higher than 200 m above sea level, which the tracers. The tracers reach 2600 m in height in some places. High-concentration areas of above 0.4 are seen only near the ground surface. Second, the sensitivity of surface air temperature at Kumagaya to soil moisture in Chubu Mountain area is numerically examined to verify the effects of subgrid-scale

TABLE 4. The mean energy values (s, gZ, and cpT), the time- integrated sensible heat ﬂux from the ground surface H, and the mean temperature of the air parcels at 1500 JST along the trajectories for lines; course high and course low.

s gZ cpT H Temperature (J kg21) (J kg21) (J kg21) (MJ m22) (8C) High lines 310 455 20 310 435 2.5 36.1 FIG. 16. Schematic representation of type-4 foehnlike wind shown Low lines 312 452 20 312 432 3.4 38.1 in the present study.

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FIG. 17. (a) Horizontal distribution of simulated tracer concentration (m3 m23) in the lowest level of the model grid at 1500 JST 16 Aug 2007. The red dashed line indicates the released area of the tracers. (b) Vertical cross section of the trace concentration at 1500 JST 16 Aug along line A–B shown in Fig. 1b. roughly corresponds to the Chubu mountainous area FIG. 18. (a) Diurnal variation of differences in the simulated (Fig. 1b). surface air temperature for cases SMOIS_WET and SMOIS_DRY Figure 18a shows the diurnal variation of surface air from 0500 to 1900 JST in Kumagaya. (b) Horizontal distribution temperature for the cases SMOIS_DRY and SMOIS_WET of the temperature difference between cases SMOIS_DRY and in Kumagaya. A temperature difference of over 10.58C SMOIS_WET (case SMOIS_DRY minus SMOIS_WET). The thick solid line represents the 200-m height. appears after around 1330 JST. The horizontal distribution of the temperature difference between cases SMOIS_DRY and SMOIS_WET at 1500 JST is shown Fig. 18b. Areas 5. Discussion with temperature differences of over 10.58Careseennot only in the Chubu Mountains but also in the inland part As described in section 2, the sunshine duration on of the Tokyo metropolitan area. This indicates that the 16 August 2007 in Kumagaya was 10.8 h. This is the 72nd- temperature difference over the inland area is also caused longest sunshine duration in July and August from 1998 by the difference in sensible heat flux from the ground to 2008 (Fig. 19a; Table 5). According to the numerical surface in the Chubu Mountains. The horizontal distri- simulation in section 3, surface heat flux associated with bution of the temperature difference inland is in good sufficient solar radiation contributes to only 49% of the agreement with that of the tracer concentration (Fig. 18b temperature rise in the mixed layer from 0500 to 1500 JST versus Fig. 17a). This result suggests that the air masses on the 16th. These results imply that the sunshine dura- are heated by subgrid-scale turbulent diffusion and sen- tion on 16 August 2007 is a necessary but not a sufficient sible heat flux from the ground surface during the time condition to cause the extreme high surface air temper- when the air masses move over the Chubu Mountains. ature event of 40.98C. Those results support the theory of the type 4 foehnlike Figure 19b shows a scatter diagram of the temperature wind (Fig. 16). around 1500-m height at 0900 JST in Tsukuba versus the

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FIG. 19. Scatter diagram of (a) the sunshine duration vs the daily maximum surface air temperature, (b) the temperature around 1500 m vs the daily maximum surface air temperature, and (c) the number of previous consecutive clear-sky days vs the daily maximum surface air temperature in July and August from 1998 to 2008. The black solid circle indicates 16 Aug 2007.

daily maximum surface air temperature in Kumagaya in were seven consecutive clear-sky days preceding the 16th, July and August from 1998 to 2008. As the figure shows, which is the 12th longest in the study period (Table 5). the temperature around 1500-m height at 0900 JST on 16 There is a possibility that the ground surface in Kumagaya August 2007 was 21.68C, which is the 30th-highest tem- on 16 August 2007 was dry because of these persistent perature at that period (Table 5). This temperature is clear-sky days. Fujibe (1996) shows that the dryness of the high; however, it is not a sufficient condition to cause the ground surface was significant in causing the extreme extreme high temperature event, although such a high events during the summer of 1994 in Japan. Thus, the temperature in the upper level would be a potential factor persistent clear-sky days are thought to have contributed in the extreme event. to the extreme high temperature event. This idea is qual- Figure 19c shows a scatter diagram of the number of itatively consistent with the results from the sensitivity previous consecutive clear-sky days versus the daily experiments of cases SMOIS_DRY and SMOIS_WET, maximum surface air temperature in Kumagaya in July described in section 4. and August from 1998 to 2008. Here, a clear-sky day is The numerical simulation in section 4 shows that the defined as a day with sunshine duration of more than 6 h, dynamic foehn wind along the course high lines, and the and without precipitation on the previous day. There foehnlike wind along the course low lines blew during

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TABLE 5. For the period July and August from 1998 to 2008, the According to the numerical simulation, surface statistical rankings of the sunshine duration in Kumagaya, tem- sensible heat flux associated with sufficient solar perature around 1500-m height in Tsukuba, and previous consec- radiation contributes to only 49% of the tempera- utive clear-sky days in Kumagaya on 16 Aug 2007. ture rise in the mixed layer from 0500 to 1500 JST. Obs site Value Ranking Thus, this sunshine duration is a necessary but not Sunshine duration (h) Kumagaya 10.8 72 a sufficient condition to cause the extreme high Temperature around Tsukuba 21.6 30 temperature event of 40.98C. 1500-m height (8C) (ii) A high temperature of 21.68C around 1500-m height Previous consecutive Kumagaya 7 12 was observed at 0900 JST 16 August at the Tsukuba clear-sky days station, which is the 30th-highest temperature for that time in July and August from 1998 to 2008. This high temperature is not a sufficient condition to the daytime on 16 August 2007. The backward trajec- cause the extreme high temperature event, although tories of the air parcels along the course high lines oc- such a high temperature in the upper level would be cupy about 30% of the entire air parcel trajectories a potential factor in the extreme high temperature reached at Kumagaya at 1500 JST. On the other hand, event. the backward trajectories of the air parcels along the (iii) A dynamic foehn wind was found in the numerical course low lines occupy about 57% of the total, almost simulation. Backward trajectories of the air parcels double that along the course high lines. In other words, along the course high lines account for about 30% of the air parcels reached at Kumagaya flew mainly along all the air parcel trajectories reached at Kumagaya the course low lines. In addition, the mean temperature at 1500 JST. At that time, the mean temperature of of the air parcels along the course low lines is about the air parcels was about 36.18C. Consequently, the 38.18C at 1500 JST, which is higher than that along the dynamic foehn wind is not a dominant factor in the course high lines. One reason for this temperature dif- extreme high temperature event. This idea differs ference is that the air parcels along the course low lines from the conclusions of the previous studies. move in the mixed layer and have been heated by subgrid- (iv) The persistent sunshine and lack of precipitation dur- scale turbulent diffusion and sensible heat flux from the ing the seven consecutive days preceding 16 August in ground surface for 5 h, whereas the air parcels move in Kumagaya must be a significant factor. This was the the mixed layer for only 2.5 h in the course high lines. 12th-longest duration of consecutive clear-sky days In other words, a fetch in the mixed layer of course low for July and August from 1998 to 2008. The persistent lines is longer than those of course high lines. From these clear-sky days can dry out the ground surface, and this findings, the foehnlike wind along the course low lines dryness can contribute to the extreme event. This is contributes substantially to the extreme high temperature qualitatively supported by the sensitivity experiments event compared to the dynamic foehn wind along the of soil moisture on surface air temperature. course high lines. (v) A foehnlike wind with diabatic heating from the ground surface is an important factor. The trajectories 6. Conclusions of the air parcels along the course low lines occupy about 57% of all trajectories. This is almost double A record-breaking high surface air temperature of of those backward trajectories of the dynamic foehn 40.98C was observed at 1442 JST 16 August 2007 in wind taking the course high lines. The mean temper- Kumagaya, located 60 km northwest of central Tokyo. ature of the air parcels along the course low lines is Around this time, a high surface air temperature region of about 38.18C at 1500 JST, which is close to the sim- over 388C extended over cities of Kumagaya and Saitama; ulated maximum surface air temperature of 39.08Cin Kumagaya is in the north of the convergence zone be- Kumagaya. It can be concluded that the foehnlike tween north-northwesterly wind zone and southerly sea- wind along the course low lines contributed substan- breeze zone. In this study, we quantitatively investigated tially to the extreme high temperature event com- the formation mechanisms of the extreme high temper- pared to the dynamic foehn wind along the course ature event, considering the contribution of foehn and high lines. foehnlike winds associated with the north-northwesterly (vi) North-northwesterly winds blowing over the Chubu wind. Our findings are summarized as follows: Mountains prevent the sea breeze from penetrating (i) A sunshine duration of 10.8 h was observed on 16 the cooler air mass, thus maintaining the high sur- August in Kumagaya, which is the 72nd-longest face air temperature region in the northern part of duration for July and August from 1998 to 2008. the Tokyo metropolitan area.

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