Mobile Observation of Air Temperature and Humidity Distributions under Summer Sea Breezes in the Central Area of Osaka City

Atsumasa Yoshida,Shinichi Kinoshita,Ryusuke Yasuda

doi:10.3390/atmos11111234

Abstract

Thermal environment of urban areas in the summertime has become harmful to human health due to global warming and the urban heat island (UHI) effect. Mobile observations enable us to obtain the distribution of air temperature at microscale, such as urban blocks, which cannot be captured by the coarse network of meteorological sites. A series of mobile measurements was executed in the central area of Osaka City in Japan, around the Nakanoshima district which lies between two rivers, to investigate the air temperature and humidity distributions in a built-up area under sea breeze conditions. Upper wind and surface temperature of the rivers were also observed using pilot balloons and infrared thermography camera, respectively. The mean air temperature in Nakanoshima was generally lower than that of the surrounding area. Urban geometries such as building density and building height seem to affect the mean air temperature by changing the ventilation efficiency. Humidity was inversely correlated with air temperature distribution but was higher at the confluence of rivers and green parks. The depth of the sea breeze layer was found to be about 1 km. Sea breezes close to the ground surface penetrated the city along the rivers, sandwiching the Nakanoshima district. During the daytime, the surface temperatures of the rivers were lower than the air temperature observed at the nearest stationary observation point, and the difference reached approximately 2 °C.

Highlights

The most notable feature of the urban heat island (UHI) effect is an increase in nocturnal temperature (e.g., Oke, 1987 [1])
Since the thermal environment of urban areas in the summertime is becoming harmful to human health due to the UHI effects in addition to global warming [7,8,9], it is becoming increasingly important to pay attention to diurnal temperatures
The IQR in each subarea was relatively large (0.8–1.1 ◦ C at 1400 and 0.5–0.7 ◦ C at 1800), considering that the range of difference in median value of the temperature between the subareas was around 1 ◦ C (1.0 ◦ C at 1400 and 1.2 ◦ C at 1800). These results suggest that the geometric characteristics of the urban block in a subarea affect the mean temperature to some extent

Summary

Introduction

The most notable feature of the urban heat island (UHI) effect is an increase in nocturnal temperature (e.g., Oke, 1987 [1]). Since the thermal environment of urban areas in the summertime is becoming harmful to human health due to the UHI effects in addition to global warming [7,8,9], it is becoming increasingly important to pay attention to diurnal temperatures. In 2007, the Japan Meteorological Agency (JMA) defined a new statistical index on air temperature, “extremely hot day” (EHD) (daily maximum temperature ≥35 ◦ C), in addition to previously defined indices of “hot day” (daily maximum temperature ≥30 ◦ C) and “hot night” (daily minimum temperature ≥25 ◦ C).

Methods

Results

Conclusion