[AAS15-01] Preliminary analysis of the nighttime boundary layer temperature structure in the area from western suburbs and central Tokyo
★Invited Papers
Keywords:Urban climate, Boundary layer, Vertical temperature profile, Inversion layer, Temperature profiler, Tokyo
Introduction
Observations aimed at understanding the boundary layer structure in and around central Tokyo are scarce, especially at night, and only short-term during the day. Therefore, the difference between rural/suburban and urban areas was not clarified, which essential to understanding the heat island phenomenon of Tokyo. Furthermore, the variability and diversity of the boundary layer structure, including the effect of local circulation such as land and sea breezes, is not sufficiently understood, making the discussion about the urban atmosphere uncertain. In this study, the characteristics of temperature inversion focused on the nighttime are presented based on data from temperature profilers installed in central Tokyo wards area (Chiyoda-ku; hereafter, central Tokyo) and western part of Tokyo wards area (Suginami-ku; hereafter, Tokyo West) and temperature loggers set at the Tanashi Tower in the western suburbs of Tokyo (Nishi Tokyo City).
Outline of observation and analysis
In central Tokyo and Tokyo West, temperature profilers (MTP-5H, from Attex) are installed on the rooftops of buildings at heights of approximately 98 and 20 m above the ground (at altitudes of approximately 8 and 45 m), respectively. These obtain air temperatures every 10 min at 50 m intervals from the installation altitude to 1000 m (in central Tokyo) and to 600 m (in Tokyo West). In the western suburbs of Tokyo, measurements are performed using temperature loggers (LR5011 with temperature sensor LR9601 from Hioki) installed in natural ventilation-type shelters at heights of 10, 20, 40, 60, 90, and 125 m from the ground (altitude of approximately 64 m). Based on these, the vertical temperature gradients for the bottom 100 m for the three locations (the temperature (°C) is regarded as the potential temperature when the air mass is lowered to sea level by the dry adiabatic lapse rate). For Tokyo West and central Tokyo, when the inversion was recognized from the lowest layer, the top height of the inversion layer and the maximum value of the vertical potential temperature gradient and its height were obtained. The objective period was after March 22, 2019, when all observations were conducted. The hourly data from the Tokyo Metropolitan Air Monitoring Stations were also used in the related analysis.
Characteristics of temperature inversion
During night (from preceding night 18:00 to 9:00), the maximum vertical temperature gradient of the lowest layer becomes frequently large, in late October, with values of 3°C/100 m or more in the western suburbs of Tokyo and 2°C/100 m or more in the Tokyo West frequently becomes large. Even in central Tokyo, where gradients in summer used to be around 0.5°C/100 m, gradients exceeding 1.5°C/100 m are observed. The most frequent height of the top of the inversion layer in the Tokyo West appears approximately 250–300 m above the ground. As the vertical potential temperature gradient becomes greater, the higher inversion top cases increase. However, the vertical potential temperature gradient is highest in the lowermost layer (20–120 m) and accounts for more than 70%. The regression equation between the locations and the maximum value of vertical potential temperature gradient of the lowest layer every night for 48 nights from November to mid-December shows that the magnitude of the vertical potential temperature gradient is approximately 2/3 in the Tokyo West against that in the western suburbs and 1/4 in central Tokyo against that in the western suburbs (the correlation coefficient is significantly large, exceeding 0.8). Specifically, the degree of inversion in the suburbs is also strongly reflected in the lower atmosphere near the city center, suggesting that the vertical mixing of the lower atmosphere near the city center has not sufficiently progressed.
Observations aimed at understanding the boundary layer structure in and around central Tokyo are scarce, especially at night, and only short-term during the day. Therefore, the difference between rural/suburban and urban areas was not clarified, which essential to understanding the heat island phenomenon of Tokyo. Furthermore, the variability and diversity of the boundary layer structure, including the effect of local circulation such as land and sea breezes, is not sufficiently understood, making the discussion about the urban atmosphere uncertain. In this study, the characteristics of temperature inversion focused on the nighttime are presented based on data from temperature profilers installed in central Tokyo wards area (Chiyoda-ku; hereafter, central Tokyo) and western part of Tokyo wards area (Suginami-ku; hereafter, Tokyo West) and temperature loggers set at the Tanashi Tower in the western suburbs of Tokyo (Nishi Tokyo City).
Outline of observation and analysis
In central Tokyo and Tokyo West, temperature profilers (MTP-5H, from Attex) are installed on the rooftops of buildings at heights of approximately 98 and 20 m above the ground (at altitudes of approximately 8 and 45 m), respectively. These obtain air temperatures every 10 min at 50 m intervals from the installation altitude to 1000 m (in central Tokyo) and to 600 m (in Tokyo West). In the western suburbs of Tokyo, measurements are performed using temperature loggers (LR5011 with temperature sensor LR9601 from Hioki) installed in natural ventilation-type shelters at heights of 10, 20, 40, 60, 90, and 125 m from the ground (altitude of approximately 64 m). Based on these, the vertical temperature gradients for the bottom 100 m for the three locations (the temperature (°C) is regarded as the potential temperature when the air mass is lowered to sea level by the dry adiabatic lapse rate). For Tokyo West and central Tokyo, when the inversion was recognized from the lowest layer, the top height of the inversion layer and the maximum value of the vertical potential temperature gradient and its height were obtained. The objective period was after March 22, 2019, when all observations were conducted. The hourly data from the Tokyo Metropolitan Air Monitoring Stations were also used in the related analysis.
Characteristics of temperature inversion
During night (from preceding night 18:00 to 9:00), the maximum vertical temperature gradient of the lowest layer becomes frequently large, in late October, with values of 3°C/100 m or more in the western suburbs of Tokyo and 2°C/100 m or more in the Tokyo West frequently becomes large. Even in central Tokyo, where gradients in summer used to be around 0.5°C/100 m, gradients exceeding 1.5°C/100 m are observed. The most frequent height of the top of the inversion layer in the Tokyo West appears approximately 250–300 m above the ground. As the vertical potential temperature gradient becomes greater, the higher inversion top cases increase. However, the vertical potential temperature gradient is highest in the lowermost layer (20–120 m) and accounts for more than 70%. The regression equation between the locations and the maximum value of vertical potential temperature gradient of the lowest layer every night for 48 nights from November to mid-December shows that the magnitude of the vertical potential temperature gradient is approximately 2/3 in the Tokyo West against that in the western suburbs and 1/4 in central Tokyo against that in the western suburbs (the correlation coefficient is significantly large, exceeding 0.8). Specifically, the degree of inversion in the suburbs is also strongly reflected in the lower atmosphere near the city center, suggesting that the vertical mixing of the lower atmosphere near the city center has not sufficiently progressed.