Radio Acoustic Sounding in Urban Meteorological Observations

Transcription

1 AUGUST 2002 AKAI ET AL Radio Acoustic Sounding in Urban Meteorological Observations YUKIO AKAI, TAKAO KANZAKI, AKIRO SHIMOTA, AND YOICHI ICHIKAWA Atmospheric Science Department, Central Research Institute of Electric Power Industry, Komae-shi, Tokyo, Japan (Manuscript received 20 June 2001, in final form 14 December 2001) ABSTRACT Meteorological observations are carried out using a mobile Radio Acoustic Sounding System (RASS), a radiosonde system, and ground-based meteorological measurement systems in order to understand urban meteorological conditions. The newly mobile RASS was operated in the one-shot acoustic pulse (OSAP) mode, allowing the measurement of atmospheric temperature up to about 200 m without causing sound pollution. Based on the temperature profiles obtained using the mobile RASS and radiosondes in the Tokyo metropolitan area during the end of February 2000, a marked decrease in temperature within a short period of time was observed. Rare meteorological phenomena such as this can be analyzed based on temperature profiles and ground-based meteorological data that are observed in Tokyo by government offices at a large number of the sites. As a result, temperatures decreased rapidly due to cold airflow into the measurement sites, and the occurrence of the inversion layer was recognized from RASS temperature profiles within a short time interval. 1. Introduction The relationship between atmospheric diffusion in urban areas and urban heat islands is an important issue that needs to be elucidated. A mobile Radio Acoustic Sounding System (RASS) was developed to measure atmospheric temperature profiles under urban heat island conditions. The propagation velocity of acoustic waves in the atmosphere varies mainly with the square root of the atmospheric temperature. The RASS measures the propagation velocity of vertically transmitted acoustic pulses using a Doppler radar and calculates the atmospheric temperature (Marshall et al. 1972). The Central Research Institute of Electric Power Industry (CRIEPI) RASS was designed to measure atmospheric temperature at high altitudes even under high wind speed conditions (Fukushima 1987). It is composed of 44 acoustic and electromagnetic antennas and is a large apparatus that cannot be truck-mounted for mobile observation. Because the urban heat island phenomenon often occurs under low wind speed conditions, it is possible to reduce the number of radio and acoustic Corresponding author address: Yukio Akai, Central Research Institute of Electric Power Industry, Atmospheric Science Dept., Iwado Kita, Komae-shi, Tokyo, Japan. akai@criepi.denken.or.jp antennas during observation under low wind speed conditions (Johnson et al. 1991). The mobile RASS is composed of a 1.1-m-diameter acoustic antenna with a 2-m-high cylindrical shield, two 1.8-m-diameter parabolic radar antennas, a continuous wave (CW) Doppler radar, (operated at MHz), a data processing unit, power supply, and other equipment. All pieces of the equipment are loaded on a 4-ton truck so that they can be used for observation after moving to a site. A new one-shot acoustic pulse (OSAP) method of acoustic transmission allows measurement of the vertical temperature profile up to about 200 m (excluding the range of m near ground level) using a single acoustical pulse without causing sound pollution (Akai and Kanzaki 1998, 1999). In recent years, the mobile RASS was used to obtain temperature profiles at many sites in the Tokyo metropolitan area. Also, temperature profiles were obtained using radiosondes at Komae City in parallel with RASS observations, which enabled the detection of the urban heat island phenomenon. In the winter of 2000, measurement of temperature profiles in the Tokyo metropolitan area using the mobile RASS and radiosondes was carried out in order to understand urban meteorological conditions. 2. Results Observations of the mobile RASS were conducted at Oi Wharf in Tokyo during the end of February American Meteorological Society

2 1194 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 FIG. 1. Temperature profiles obtained using the radiosonde system in Komae on Feb Table 1 shows the mobile RASS parameters. Radiosondes were also used to measure temperature, humidity, and atmospheric pressure in suburban Komae City, a suburb about 17 km away from the center of Tokyo, which is surrounded by residential areas. Government offices observe ground-level temperatures and winds at a large number of sites in Tokyo, including Shinkiba. TABLE 1. Mobile RASS parameters. FIG. 2. Temperature profiles obtained using the mobile RASS in Oi Wharf on Feb The ground-level temperature was measured using a thermometer mounted on a truck. Oi Wharf and Shinkiba are located near Tokyo Bay. (The observation sites are shown in Fig. 5.) The conditions of the mobile RASS observation sites were generally fine with weak wind on the night of February Based on the temperature profiles obtained using radiosondes in Komae shown in Fig. 1 and using the mobile RASS in Oi Wharf shown in Fig. 2, the following observations were made. CW Doppler radar Operating frequency Radiated power Antenna type Acoustic system Operating frequency Radiated power Pulse length Antenna type Shield Computer Type, CPU clock A D converter Sampling frequency Spectrum analyzer MHz 10 W 1.8-m-diameter parabola 950-Hz band 400 W (electric) ms 1.1-m-diameter parabola 2-m-height enclosure Notebook, 266 MHz 12 bit 5 10 khz FFT by software a. Temperature profiles 1) Temperature fell rapidly at both sites, Komae and Oi Wharf, from 2200 to 0200 LST. In Komae, a decrease of about 3 C was observed from near the ground to an altitude of approximately 100 m in 2 h. Based on the temperature profile distribution obtained using the mobile RASS in Oi Wharf, a decrease of about 3.2 C was observed over a 1-h period from 0100 to 0200 LST. 2) The observed variation of temperature was small over the 4-h period after a rapid decrease in temperature. The decrease in temperature at an altitude of m was small, with a decrease of 1.5 C in Oi Wharf. 3) In Komae, the decrease in temperature was about

3 AUGUST 2002 AKAI ET AL FIG. 4. Frequency distribution of the hourly ground temperature difference of Feb 2000 at Shinkiba. FIG. 3. Meteorological observation results from AMeDAS in Shinkiba on Feb C over the 4-h period after temperature fell at an altitude of m. The temperature fell greatly near the ground in the suburbs of Komae during the early morning, and a ground-base inversion layer that depended on nocturnal radiation cooling was found. b. Rare meteorological phenomenon 1) Figure 3 shows the data of time series for ground temperature, wind speed, and wind direction in Shinkiba near Oi Wharf on February from the Japan Meteorological Agency s Automated Meteorological Data Acquisition System (AMeDAS). It was shown that there was a decrease in temperature of 2.3 C from 0100 to 0200 LST. 2) The frequency distribution of hourly ground temperature difference of February 2000 at Shinkiba is shown in Fig. 4. When ground temperature decreases with time, the temperature difference is negative. Figure 4 shows that a 2.3 Ch 1 difference is a meteorological phenomenon that rarely appears. 3) There was a northerly wind direction change this time. Because of this, cold air flowed into the observation sites, resulting in the rapid decrease in temperature. Based on the plot of 0140 LST shown in Fig. 2, temperature near the ground is low, but the upper temperature does not fall. c. Meteorological conditions in a wide area of Tokyo 1) Figure 5 shows surface wind directions and temperatures in a large region of the Tokyo metropolis on February The wind direction changed from southwest to west to northwest in this region from 2300 to 0100 LST. 2) This was a result of the change in atmospheric pressure patterns in Japan. Atmospheric pressure patterns with a 6-h interval are shown in Fig. 6. A low pressure condition occurred anew in northern Japan at 0300 LST on 24 February. Conceivably, the wind direction changed with the change in the isobaric lines of hpa near Tokyo. 3) It is clear from the observed results of temperatures at 2300 LST that an urban heat island was found in central Tokyo. The maximum temperature in the heat island exceeded 8 C. The temperature at an inland region of the Tokyo metropolis at the same time was less than 1 C. The wind direction changed within a short time, and cold air flowed into central Tokyo, resulting

4 1196 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 in a rapid decrease in temperature. The intensity of the heat island weakened at 0100 LST. 3. Conclusions The OSAP mode of the mobile RASS allowed rapid measurement of the temperature profiles at Oi Wharf near Tokyo Bay without causing sound pollution. Suburban Komae City, which has a radiosonde station, is about 17 km away from Oi Warf. In spite of the distance, a marked fall in temperature at a height of 100 m within a short period of time was observed in Oi Warf and Komae City almost simultaneously. On the other hand, a rapid change in ground temperature was observed at a ground-based meteorological station in Shinkiba. The proportion of ground temperature drops was large at the rate of 2.3 C h 1. When it snows suddenly because of the passage of low pressure, temperature falls rapidly, but the case of a marked fall in temperature within a short period of time in the case of clear atmosphere is rare. The wind direction changed from southerly to northerly in a large region of the Tokyo metropolitan area within 2 h. Because a low pressure region occurred anew in northern Japan, the atmospheric pressure pattern changed, and the wind direction changed in the Tokyo metropolitan area. As a result, it was found that rapid temperature changes occurred near the ground, based on the detailed observation results using the mobile RASS within short time interval, and a strong inversion layer was formed temporarily by cold air flowing into the atmosphere. Acknowledgments. I would like to express my deep appreciation to my colleagues A. Sato, K. Wada, and K. Nishizawa for their cooperation and support in conducting radiosonde observations and other activities during the research project. FIG. 5. Surface wind directions and temperatures obtained in a large region of the Tokyo metropolis on Feb The data were obtained from the Tokyo metropolitan government.

5 AUGUST 2002 AKAI ET AL FIG. 6. Atmospheric pressure patterns at (a) 2100 LST 23 Feb 2000 and (b) 0300 LST 24 Feb 2000.

6 1198 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 19 REFERENCES Akai, Y., and T. Kanzaki, 1998: The Application of a mobile RASS to observation of an urban heat island. Proc. Ninth Int. Symp. on Acoustic Remote Sensing, Vienna, Austria, ISARS, , and, 1999: Development and initial results of a mobile RASS. Meteor. Atmos. Phys., 71, Fukushima, M., 1987: Received signal characteristics of a Radio Acoustic Sounding System (RASS) Influence of horizontal winds for temperature measurement. Trans. IEICE, 70E, Johnson, G. T., T. R. Oke, T. J. Lyons, D. G. Steyn, I. D. Watson, and J. A. Voogt, 1991: Simulation of surface urban heat islands under ideal conditions at night. Part I: Theory and tests against field data. Bound.-Layer Meteor., 56, Marshall, J. M., A. M. Peterson, and A. A. Barnes Jr., 1972: Combined Radar Acoustic Sounding System. Appl. Opt., 11,