ABSTRACT

This study uses statistical evaluation by correlation analysis to examine the effects of thermal environment on the frequency of convective precipitation in the Greater Tokyo Area between 12:00 and 18:00 on summer days from 1997 to 2006. To extract the frequency of convective precipitation we used Automated Meteorological Data Acquisition System radar data to obtain detailed rainfall distribution maps, and to extract the urban thermal environment we used surface temperature data from a National Oceanic and Atmospheric Administration weather satellite. Results were a coefficient of determination of 0.01, indicating no clear relation between surface temperature and convective rain frequency in the study area. Examining the convective rain frequency distribution map in conjunction with an elevation map of the area indicates that higher elevation is a better predictor of increased frequency of convective rainfall than is surface temperature. Because this indicates that orographic precipitation has a large influence in the study area, we used an elevation map to exclude hilly and mountainous regions, regions bordering flat areas (under the assumption that wind could easily move orographic precipitation to such areas), and regions containing marine areas. Doing so resulted in a coefficient of determination of 0.38, a clear signal that differences in the thermal environment in the Greater Tokyo Area have an effect on the frequency of convective precipitation. We next focused on metropolitan Tokyo, the most developed part of the region and the part experiencing the most frequent occurrences of convective precipitation, and we performed correlation analysis considering parameters related to buildings. Results indicate that orographic precipitation has a strong influence in metropolitan Tokyo as well, so we excluded those areas that were excluded from the Greater Tokyo Area analysis and again performed correlation analysis. However, we found no clear relation of convective precipitation frequency with surface temperature or building parameters.

1. Introduction

Cities are spaces dedicated to human activity, and their unchecked growth has resulted in numerous problems. One such problem is the development of the urban heat islands (UHIs). The UHI effect is a phenomenon where the air temperature is higher in an urban environment than in the surrounding area, resulting in a direct impact on residents. There are also suggestions that the UHI effect influences mesoscale circulation in the city, potentially inducing convective precipitation.

There have been various discussions of the effects of factors other than differences in synoptic place occurring in urban environments, such as complex cityscapes [1-3] and urban aerosols [4-7]. An often-cited characteristic of such precipitation is increased precipitation amounts leeward of the city center [8-15]. There have also been reports of increased urban rainfall in Japan [16,17], and many reports indicating UHIs as a contributing factor [18-22].

Japanese megacities such as Tokyo already demonstrate the UHI effect, yet continue to expand to the point of assimilating smaller nearby towns. One serious consequence is that the increase in impervious surfaces can cause stormwater runoff to exceed wastewater treatment capacity, resulting in flooding. Convective precipitation in particular is difficult to predict and can result in heavy rainfall over a short time, so knowing what areas frequently experience convective precipitation and understanding the relationship between precipitation and UHIs is a first step in planning measures to address flood prevention and damage control measures.

The present study examines the urban and suburban areas of Tokyo (Figure 1), using statistical evaluation by correlation analysis to examine the effects of thermal environment on convective precipitation frequency. We also perform statistical analysis that focuses on the many convective precipitation events that occur in metropolitan Tokyo in the hope of finding a relation between differences in heat environment and event frequency. An example of convective precipitation in Tokyo is the 21 July 1999 event occurring at around 15:00, which brought rainfall of 129 mm/h.

From the definition of UHI, it is preferable to use temperature as the basis for evaluating differences in thermal environment. However, it is difficult to create a detailed temperature distribution map based on actual observations over an area as large as the one examined in this study, and ensuring observations under similar atmospheric conditions would be difficult and expensive. This study therefore uses satellite imaging to extract a surface temperature distribution, and for convenience, that distribution was used as the UHI in the study area.

As has been stated in previous studies, daytime surface temperatures obtained from satellites do not necessarily give an accurate depiction of UHIs [23], but satellite data have one significant advantage in that surface temperatures can be extracted for specified times and dates. The present study examines summer (July and August) surface temperatures in the daytime (12:00 to 18:00) from 1997 to 2006.

2. Overview of Study Area

Tokyo Metropolis contains the 23 wards that include the seat of the national government in Japan, and also covers the Tama District, the Izu Islands, and the Ogasawara Islands. The Greater Tokyo Area, which covers the urban areas of Tokyo Metropolis and the surrounding prefectures of Kanagawa, Chiba, Saitama, Gunma, Tochigi, and Ibaraki, has a population of approximately 37 million people, making it the world’s most populous metropolistan area [24].

With high concentrations of people, goods, money, and information, such large cities offer conveniences over smaller ones. However, such excessive concentrations can worsen urban environments. One example is the UHI effect, in which cities become warmer than do their surrounding areas; while the average temperature of Japan as a whole rose 1˚C in the 20th century, Japan Meteorological Agency (JMA) data show that Tokyo’s average temperature has risen approximately 3˚C.

Approximately 70% of the world’s population is expected to live in urban regions by 2050 [25], and environmental problems similar to those of existing cities are expected to arise in newly developing cities as well. Using Tokyo as a case study of evident indirect problems such as increased levels of convective rain due to a worsened thermal environment can therefore provide vital data for consideration when developing cities planning measures against similar problems.

3. Data and Analysis

3.1. Frequency of Convective Precipitation

We used data from the Automated Meteorological Data Acquisition System (AMeDAS) to extract information for measuring the frequency of convective precipitation. AMeDAS consists of radar stations operated by JMA and the River Bureau and the Road Bureau of the Ministry of

Land, Infrastructure, and Transport, as well as approximately 1300 automated weather stations. This system collects data from ground-level rain gauges, which can be used to collect detailed information about rainfall.

AMeDAS rainfall data from 1988 through March 2001 used a mesh with 5 km resolution, but that was increased to 2.5 km resolution in April 2001, and again to 1 km in January 2006. For consistency, this paper uses data over a 5 km × 5 km mesh. Note also that starting in May 2003 the coordinate system was changed from Japan datums to geodetic datums, but this study uses geodetic datums throughout for consistency. The time resolution of the data is 1 h.

Because the present study aims at extracting those convection precipitation events considered to be the result of factors related to the UHI effect, it is necessary, to the extent possible, to remove from consideration effects that are due to differences in synoptic place. We therefore omitted data for days of weather phenomena such as fronts, low pressure systems, and typhoons, and used the following four criteria for classifying a day as one experiencing convective precipitation.

• There was a gradual change in pressure gradient from the Sea of Japan (Niigata City) to the Pacific Ocean (Tokyo), and there was good sea breeze circulation.

• No precipitation systems had moved over land from the sea.

• Total rainfall was less than 0.5 mm/h at the Tokyo District Meteorological Observatory.

• There was on average 50 min/h sunlight from sunrise until 12:00 at the Tokyo District Meteorological Observatory.

The convective rain rate was calculated as the total number of times that rainfall began within 1 h during the observation period for days meeting the above conditions.

3.2. Calculation of Surface Temperature

To extract surface temperatures, we visually selected photographs featuring low cloud cover which were acquired by the Advanced Very High Resolution Radiometer (AVHRR) sensors on the National Oceanic and Atmospheric Administration (NOAA) 12 and 14 satellites to create composite images. Spatial resolution of the AVHRR sensors is 1.1 km from orbit. We used the split window method (Equation (1) [26]) to calculate surface temperatures from the difference in brightness temperature between AVHRR Channels 4 and 5:

(1)

The above method gives the temperature in kelvins, so values were converted to degrees Celsius before use in analysis.

4. Results and Discussion

4.1. Convective Precipitation and Surface Temperature Distributions

Figure 2 shows the distribution of convective precipitation frequency in the study area, and indicates that frequency increases in a WNW direction from the city center. This is similar to previously reported phenomena [27, 28], a pattern in which easterly wind from the Kashimanada coast and southerly wind from the Sagami Bay coast converge in the Tokyo area.

There is also a tendency for more frequent convective precipitation in Chiba Prefecture, to the north and west of the study area. Comparison with a topographic map created with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model (GDEM) shows that these areas have relatively high altitude (Figure 3), suggesting that rainfall in these areas is not convective precipitation due to an urban thermal environment, but rather orographic precipitation due to geographically induced updrafts.

Figure 4 shows a distribution of surface temperatures in the study area. While NOAA/AVHRR data are from directly below orbit and provided at a 1.1 km resolution, we used a 5 km × 5 km mesh of averaged values to allow correlation analysis. Results show a tendency for central city areas from Yokohama to Chiba City to have significantly higher temperatures, which decrease toward the suburbs. Higher temperatures are also seen in prefectural capitals such as Maebashi, Tochigi, and Mito, and in the cities of Kamisu and Kashima, site of the Kashima Coastal Industrial Zone, the largest industrial agglomeration in Ibaraki Prefecture. Comparing these areas with the elevations in Figure 3, areas at higher elevations have relatively low surface temperatures.