EFFECTS OF LARGE CITIES ON WIND SYSTEMS OVER THE KANTO PLAIN WHEN HEAVY RAINFALL OCCURRED IN TOKYO

Tomohiko Inamura*, Takeki Izumi*, Hiroshi Matsuyama* *Tokyo Metropolitan University, Tokyo, Japan

Abstract

The purpose of this study was to investigate the effects of large cities on wind systems over the Kanto plain when heavy rainfall occurred in Tokyo, by numerical simulation using a meso-scale atmospheric model. Two types of land-use distribution were prepared for the numerical simulation. One was actual land-use; the other was virtual land-use, where cities were converted to forests. The results of these simulations indicated that no large difference appeared in the wind systems of the Kanto plain even if cities were converted to forests; thus the influence of cities on wind systems of this scale was small. However, wind convergence changed around the urban area; therefore cities changed wind around the urban area and developed wind convergence.

Key words: heavy rainfall, sea breeze, numerical simulation

1. INTRODUCTION

In recent years, short-time heavy rainfall has occurred in summer afternoons in Tokyo, causing an inundation under the ground and severe flood above/under the floor. On July 21, 1999, a heavy rainfall event exceeding 100 mm/h occurred at Nerima, and one person died in the inundation under the ground. The tendency of heavy rainfall in Tokyo has increased in recent years (e.g., Sato and Takahashi, 2000). Fujibe et al. (2002) suggested that a relationship existed between heavy rainfall in Tokyo and the heat-island phenomena, but details were not clarified.

Many studies of short-time heavy rainfall in Tokyo have analyzed observation data of the Automated Meteorological Data Acquisition System (AMeDAS). Many have focused specifically on the wind system over the Kanto plain. Fujibe et al. (2002) demonstrated that when heavy rainfall occurred in the 23 wards of Tokyo, an E-S wind system (convergence of easterly wind from the Kashima Sea and southerly wind from the Sagami Bay in Tokyo) often appeared; then heavy rainfall occurred in this convergence area. Sawada and Takahashi (2007) analyzed heavy rainfall events when the E-S type of wind and “not the E-S” type appeared. This study demonstrated that rainfall Intensity with the E-S type was stronger than that of other types.

However, the effects of the sea breeze were strong for the distribution of temperature in the summer afternoon. Therefore, it was not clear how the city affected a wind system (Fujibe et. al., 2002). Actually, Sawada and Takahashi (2007) suggested that the effects of a local low pressure over Chubu Mountains were more important in the occurrence of intense rainfall than the high temperature area of the urban region.

The purpose of the present study is to investigate the effects of large cities on wind systems over the Kanto plain when heavy rainfall occurred in Tokyo, by numerical simulation using a meso-scale atmospheric model. We used the Regional Atmospheric Modeling System (RAMS, Pielke et al., 1992) Ver. 4.4 coupled with a single-layer urban canopy model (Zhang et al., 2008). We performed simulations using actual and virtual land-use where cities were converted to forest and investigated the effects of cities by comparing the results of these simulations.

2. METHODS2.1 Study flow

First, we performed a case study simulation of the short-time heavy rainfall that occurred on 18 July 2001. Simulations using actual and virtual land-uses were performed; the effects of cities were then investigated by comparing the results of these simulations.

Second, a generalized heavy rainfall event reproduced by upper air observation of Tateno

Corresponding author’s address: Tomohiko Inamura, Department of Geography, Tokyo Metropolitan University 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan E-mail: inamura-tomohiko@ed.tmu.ac.jp

Basic equations Non-hydrostatic time-split compressible Grid structure Arakawa-C staggered grid Horizontal coordinate Rotated polar-stereographic transformation Vertical coordinate Terrain-following height coordinate Lower boundary Soil / vegetation / snow parameterization – Leaf-2Cloud microphysics Walko et al.(1995) Convective Modified Kuo (only Grid1) Radiation Chen and Cotton(1983) Turbulence closure Horizontal: Smagorinsky deformation

Vertical: Mellor-Yamada level 2.5 Initial value and assimilated data

Four-dimensional analysis nudging using MANAL(produced by JMA)

Table 1 Model settings

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was simulated. In this step, simulations were performed using actual and virtual land-use; the effects of cities were then investigated by comparing the results of these simulations.

2.2 Model configuration and land-use data

RAMS has various options. We can choose appropriate options that match to reproduce a phenomenon. Table 1 presents the options we chose in this study.

The numerical simulations were performed in the two domains indicated in Fig.1. The horizontal resolution of the outer domain (Grid 1) was 10 km, and that of the inner domain (Grid 2) was 2 km.

Figure 2 depicts land-use distribution used in the simulations. Figure 2(a) indicates actual land-use created from the National Land Information Land-use Data. Figure 2(b) indicates virtual land-use where the cities in Fig. 2(a) were converted to forest. In the case study simulation, the simulation using actual land-use was Control (CTL), and the simulation using virtual land-use was Urban to Forest (U2F). In the generalized simulation, the simulation using actual land-use was CTL-g, and the simulation using virtual land-use was U2F-g.

2.3 Overview of the case study simulation and initial values used for the generalized simulation

In the case study simulation, the heavy rainfall event of 18 July 2001 was reproduced. On this day, the observed maximum temperature exceeded 30 degrees at all AMeDAS stations in the 23 wards of Tokyo. Rainfall was observed from 2120 JST at Nerima, and precipitation was 53 mm/h from 2100 to 2200 JST. The simulation was performed for 18 hours, starting at 0900 JST on 18 July 2001. Initial values and assimilated data were taken from the Meso Analysis (MANAL) produced by the Japan Meteorological Agency.

The initial values of the generalized simulation, which were produced from the upper air observation of Tateno, were horizontally homogeneous. These initial values were averages of 13 cases of E-S type from 1994 to 2006. The simulation was conducted for 24 hours, starting at 0300 JST on 2 August.

3. RESULTS

Figure 3 indicates the average simulated wind and temperature from 1800 to 2100 JST in CTL and U2F. Also, Fig. 4 indicates the average convergence at the surface from 1800 to 2100 JST in CTL and U2F. In both cases, E-S type occurred, and almost no difference was observed between the wind system of CTL and that of U2F (Fig. 3). A strong convergence

Fig.1 Calculation area

Fig. 2 Distribution of land-use on Grid2 (a) CTL, (b) U2F

Fig.3 Average simulated wind andtemperature from 1800 to 2100 JST on 18July 2001. (a) CTL, (b) U2F.

(℃)

(a)

(b) (m/s)

(m/s)

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area appeared in almost the same area in both cases (Fig. 4); however, convergence of CTL in this area was stronger than that of U2F.

Figure 5 indicates the average simulated wind and temperature from 1500 to 2100 JST in CTL-g and U2F-g. Also, Fig. 6 presents the average convergence at the surface from 1500 to 2100 JST in CTL-g and U2G-g. In both cases, southerly wind dominated; however, almost no difference was observed between the wind system of CTL-g and that of U2F-g (Fig. 5). Convergence of CTL-g at Saitama was stronger than that of U2F-g. These results were the same as those of the case study simulation. In addition, the strong convergence area at Saitama in CTL-g moved to Tokyo in U2F-g.

These results indicated that the effects of large cities on the wind system over the Kanto plain were small. However, large cities enhanced convergence around the cities.

4. DISCUSSION OF EASTERLY WIND FROM KASHIMA SEA

In the generalized simulation, no easterly wind form the Kashima Sea appeared (Fig. 5). Therefore, we analyzed MANAL and performed a simulation on 18 July 2001 without data assimilation. This simulation was called Nudging Off Run (NOR).

Figure 7 depicts the sea surface pressure and wind field of MANAL at 1500 JST on 18 July 2001. A low pressure area was found in the Pacific. The northeasterly wind from this low pressure and the southwesterly wind from the local low pressure over the Chubu Mountains converged at the Kashima Sea. In other words, an easterly wind from the Kashima Sea easily occurred in this situation. In addition, a northeasterly easily occurred on the eastern coast of the Kanto and Tohoku

Fig. 4 Average convergence at the surface from 1800 to 2100 JST on 18 July 2001. (a) CTL, (b) U2F

(a)

(b)

CTL

U2F

0.06 0.05 0.04 0.03 0.02 0.01

0

(/s)

Fig. 5 Average simulated wind andtemperature from 1500 to 2100 JST on18 July 2001. (a) CTL-g, (b) U2F-g

(℃)

(a)

(b) (m/s)

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Fig. 6 Average convergence at the surfacefrom 1500 to 2100 JST on 18 July 2001.(a) CTL-g, (b) U2F-g

0.10 0.08 0.06 0.04 0.02

0

(/s)

(a)

(b)

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regions because a high pressure area was found northeast of Hokkaido. Therefore, an easterly wind from the Kashima Sea occurred by a synoptic pressure field. The effect of the pressure field on the Pacific seemed to be especially strong.

Figure 8 indicates the average simulated wind and temperature from 1800 to 2100 JST in NOR. In this simulation, a southeasterly wind from the Kashima Sea appeared. The northeasterly wind from the Kashima Sea that occurred in CTL did not occur. Figure 9 indicates the sea surface pressure and wind of MANAL, CTL and NOR at 1500 JST on 18 July 2001. The high pressure area of the northeast that was reproduced in CTL did not appear in NOR. Therefore, no northeasterly wind from the Kashima Sea occurred in NOR.

In conclusion, a northeasterly wind from the Kashima Sea blew because the synoptic condition was effectively produced in the data assimilation. Therefore, the effect of synoptic condition was strong and important for the outbreak of northeasterly wind from the Kashima Sea. We conclude that the northeasterly wind from the Kashima Sea is not a sea breeze.

References Chen, C., Cotton, W.R., 1983, A one-dimensional simulation of the stratocumulus-capped mixed layer, Bound.-Layer Meteor., 25, 289–321. Fujibe, F., Sakagami., K., Chubachi, K., Yamashita, K., 2002. Surface wind patterns preceding short-time heavy rainfall in Tokyo in the afternoon of midsummer days, Tenki, 49, 395-405. (in Japanese with English abstract) Pielke, R.A., Cotton, W.R., Walko, R.L., Tremback, C.J., Lyons, W.A., Grasso, L.D., Nicholls, M.E., Moran, M.D., Wesley, D.A., Lee, T.J., Copland, J.H., 1992, A comprehensive meteorological modelling system–RAMS, Meteor. Atmos. Phys., 49, 69-91. Sato, N., Takahashi, M., 2000. Long-term changes in the properties of summer precipitation in the Tokyo area, Tenki, 47, 643-648. (in Japanese with English abstract) Sawada, Y., Takahashi, H., 2007. Relationship between the intensity of convective rainfall in central Tokyo and the distribution of surface temperature and wind systems over the Kanto plain in summer, Geographic Review of Japan, 80, 70-85. (in Japanese with English abstract)Walko, R.L., Cotton, W.R., Meyers, M.P., Harrington, J.Y., 1995, New RAMS cloud microphysics parameterization part I: the single-moment scheme, Atmos. Res., 38, 29–62. Zhang, H., Hanaki, K., Sato, N., Izumi, T., Aramaki, T. 2008. Modified RAMS-Urban canopy model for heat island simulation in Chongqing, China. J. Appl. Meteor. Climatol, 509-524

Fig. 9 Sea surface pressure and wind fieldat 2100 JST on 18 July 2001. (a) MANAL,(b) CTL, (c) NOR.

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(b)

(c)

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Fig. 7 Sea surface pressure and wind field of MANAL at 1500 JST on 18 July 2001.

Fig. 8 Average simulated wind andtemperature from 1800 to 2100 JST in NOR.

(m/s)

(hPa) (℃)

(m/s)

(hPa)

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