Mesoscale circulation induced by the coupled effects of urban heat ...

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concerned with the summertime condition over the Osaka urban area which ... Osaka Bay, the control experiment well captures the mesoscale features of wind ...
Mesoscale circulation induced by the coupled effects of urban heat island and land-sea contrast and its response to land use change Tetsuya Takemi a, Tsuyoshi Arimitsu b, Masahiro Tamai c a b

Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama, Kanagawa, Japan The Kansai Power Electric Co.Inc., 3-11-20 Nakoji, Amagasaki, Hyogo, Japan c Osaka University, 2-1 Yamadaoka, Suita, Osaka, Japan

ABSTRACT: This study investigates numerically the mesoscale circulation affected by both urban heat island and land-sea contrast by using a mesoscale meteorological model, MM5. The purpose of the study is to demonstrate the response to possible changes in urban land use and to predict a possible future change in mesoscale circulation over an urban area. We are specifically concerned with the summertime condition over the Osaka urban area which sprawls over the entire area of the Osaka Plain and is surrounded by the Osaka Bay and by mountainous terrain. We conduct a series of sensitivity experiments, in addition to a control experiment, by changing land-use parameters in the urban category to examine the effects of the urban area. Through the comparison with the conventional surface meteorological observations in the Osaka area and a specially coordinated observation of boundary-layer winds by a Doppler sodar at the coast of the Osaka Bay, the control experiment well captures the mesoscale features of wind circulation associated with the heat-island and sea-breeze effects. The sensitivity experiments indicate that the urban effects are significantly seen in the northeastern part of the Osaka Plains and the reduction in the roughness length in the urban area leads to a temperature decrease only near the coast. KEYWORDS: Urban meteorology, Urban heat island, Sea/land breeze, Local circulation. 1 INTRODUCTION The effects of urban heat island on local wind and temperature have been a critical environmental issue in the urban regions, especially in big cities worldwide, and therefore the techniques and the measures for the mitigation of urban heat island have been investigated and implemented. The Japanese Government has started a project for promoting measures against heat island within the framework of the Urban Renaissance Project. One of the proposed approaches to mitigate the effects of the urban heat island is to increase the penetration of sea breeze into inland urban centers and thereby to decrease temperatures in the urban centers. This approach seems to be plausible, because in Japan big cities are located in coastal areas. However, considering that the urban regions have a large roughness length, the internal boundary layer may develop from at the coast line after the penetration of sea breeze, which will negate the mitigation effects of that approach. In addition, local wind fields during the development of sea breeze are influenced by the size of the urban area and the surrounding terrain features. Thus, the understanding of the temporal and spatial variation of mesoscale circulation in urban areas is necessary for planning effective approaches against the urban heat island. In this study, numerical simulations of mesoscale circulation induced by the effects of urban heat island and land-sea contrast are conducted with a mesoscale meteorological model and the

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results are compared with observations. The responses to land-use change in the urban area are also investigated. We examine the situations under summertime fair-weather conditions in the Osaka region which is the second largest metropolitan area in Japan. 2 OBSERVED METEOROLOGICAL FIELDS 2.1 Observational data The observational data used in the present analyses are hourly surface meteorological parameters measured at the Automated Meteorological Data Acquisition System (AMeDAS) stations of the Japan Meteorological Agency (JMA) in the Osaka region. The data include air temperature, relative humidity, wind direction and speed, precipitation amount, and daylight hours. In addition to the use of the conventional observation, we conducted a special observation profiling boundarylayer winds with a Doppler sodar at a site in the Osaka Nanko Power Plant located at the coast of the Osaka Bay during 09 JST (Japan Standard Time, UTC+9 hours) 22 August 2003 to 09 JST 10 September 2003. We focus our observational and numerical analyses on this period. The wind data were acquired at 19 levels from the 50-m height to the 800-me height with the resolutions of 25 m below the 200-m height and 50 m above. The time interval of the data is 10 minutes. 2.2 Surface fields Several synoptic-scale disturbances passed over the Osaka region during the analysis period; otherwise, a summertime subtropical high dominates. From the weather maps and the daylighthour data, we determine 22, 23, and 24 August and 1, 2, 8, and 9 September as a fair-weather day among the days with sodar data available. On these days sea breeze of about 5—7 m/s at the coast penetrated into the inland Osaka Plains, while land breeze was basically very weak over the plains except near the mountains. The wind hodograph at the Osaka City site demonstrated a counter-clockwise diurnal change, which indicates that the wind fields are affected not only by local sea/land contrast but also by wider-scale thermally induced circulation (Ito [1]). The temperature fields showed that the high temperature area spread over the whole plains; the daytime maximum occurred at the Hirakata site, in the northeastern part of the Osaka Plains. 2.3 Vertical structure The diurnal variation of the vertical distribution of streamwise wind speed averaged over the fair-weather days from the Doppler sodar data shows that sea breeze starts at 10 JST and reaches its maximum intensity during 14-18 JST (Fig. 1). The sea breeze has a depth of at least 400 m.

Figure 1. The observed diurnal variation of the vertical profile of streamwise wind speed averaged over the fairweather days.

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3 NUMERICAL SIMULATION MODEL The mesoscale meteorological model used here is the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5 [2]). The MM5 is initialized with the 10-km mesh JMA mesoscale objective analysis data from 09 JST 22 August 2003 in two nested, two-way-interacting, computational domains covering the whole Kinki region down to the Osaka area. The variables at the lateral boundaries of the larger domain are given from the 6-hourly JMA mesoscale objective analysis data. The grid resolutions of the two domains are 3 and 1 km with the sizes of 110 × 110 and 88 × 88, respectively. All the grids are spaced at 27 levels in the vertical. The simulated boundary-layer winds can be sensitive to the choice of the planetary boundary layer (PBL) mixing scheme [3]. Therefore, we employ two types of PBL schemes (i.e., local versus non-local schemes) to examine the sensitivity and to choose a better scheme for our further simulations. The schemes examined here are the Mello-Yamada-Janjic scheme (a local scheme, hereafter referred to as ETA) [4] and the Medium-Range Forecast Model (MRF) scheme (a nonlocal scheme) [5]. The cloud microphysics processes are parameterized with the Reisner scheme [6], and no cumulus parameterization is used. The surface energy budget is computed with a multi-layer soil model [7]. The United States Geological Survey 30-sec terrain dataset is used for setting the elevation and land use. 4 SIMULATION RESULTS 4.1 Control experiment First we examined the response of the simulated results to PBL scheme by comparing the results with the surface and sounding observations and found that the ETA scheme overall gave better correlation coefficients than the MRF scheme. Therefore, the ETA scheme is used hereafter. As a reference simulation (referred to as control), default land-use parameters in MM5 are used. The roughness length in the urban category is 50 cm in this run. Figure 2 shows the fineday composite of the wind and temperature fields at the surface at 14 JST in the control simulation. It is seen that a higher temperature area extends over the northeastern part of the Osaka Plains. This result is consistent with the surface observation.

Figure 2. The wind and temperature fields at the surface at 14 JST averaged over the fair-weather days.

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4.2 Sensitivity experiments In order to investigate the sensitivity to land-use change, we perform a series of sensitivity experiments by changing the land-use parameters as follows: 1) S1, the grids with the urban category are reset to the cropland/grassland mosaic category; 2) S2, the urban grids are extended to the whole Osaka Plains; and 3) S3, the roughness length in the urban category is changed to 14 cm (which is the same as in the cropland/grassland mosaic category). Figure 3 shows the surface wind and temperature fields at 14 JST for these sensitivity simulations. In S2, temperatures decrease by 1-2 degrees; however, the highest temperature area is still found in the northeastern part of the plains. The extension of the urban area (S2) results in just a little wider area of high temperatures. The decrease of the roughness length (S3) results in a cooler area extending near the coast. The wind fields indicate that in S2 the wind speed in the northeastern part is reduced, while in S1 the wind speed is generally stronger than in the other experiments.

Figure 3. The same as Figure 2, except for the cases of S1 (left panel), S2 (center), and S3 (right).

5 CONCLUSIONS We have performed a series of mesoscale simulations of the local circulation over the Osaka region in order to examine the urban effects on the local meteorological fields. From the numerical simulations, it is found that the urban effects are significantly seen in the northeastern part of the Osaka Plains and the reduction in the roughness length in the urban area leads to a temperature decrease only near the coastal region of the Osaka Plains. 6 REFERENCES 1 H. Ito, Numerical experiments on the extended sea breeze in the Kinki District, Tenki, 42 (1995) 17-27. 2 J. Dudhia, A nonhydrostatic version of the Penn State/NCAR mesoscale model: Validation tests and simulation of an Atlantic cyclone and cold front, Mon. Wea. Rev., 121 (1993) 1493-1513. 3 D. Zhang and W. Zheng, Diurnal cycles of surface winds and temperatures as simulated by five boundary layer parameterizations, J. Appl. Meteor., 43 (2004) 157-169. 4 Z. I. Janjic, The step-mountain coordinate: physical package, Mon. Wea. Rev., 121 (1990) 1429-1443. 5 S. -Y. Hong and H. -L Pan, Nonlocal boundary layer vertical diffusion in a medium-range forecast model, Mon. Wea. Rev., 124 (1996) 2322-2339. 6 J. Reisner, R. J. Rasmussen, and R. T. Bruintjes, Explicit forecasting of supercooled liquid water in winter storms using the MM5 mesoscale model. Quart. J. Roy. Meteor. Soc., 124B (1998) 1071-1107. 7 J. Dudhia, A multi-layer soil temperature model for MM5, Proc. 6th PSU/NCAR Mesoscale Model User’s Workshop, Boulder, CO, pp. 49-50.

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