DIURNAL VARIATION OF THE LOCAL AIR-PRESSURE SYSTEMIN THE URBAN TOKYOJunji Nishina*, Takehiko Mikami**

*Meijigakuin University, Tokyo, Japan; **Teikyo University, Tokyo, Japan

Abstract

We discuss the diurnal variation of the air-pressure system in the urban Tokyo. In summer, the pressure drop in themorning is larger in the suburban Tokyo, and in the afternoon is larger in the downtown Tokyo. After sunset, firstlythe pressure rise is larger in the downtown Tokyo, thereafter is larger in the suburban Tokyo. In winter, the diurnalvariation is almost opposite. Commonly, while the area-averaged air-pressure drops, the pressure drop is firstlylarger in the downtown; while rises, the pressure rise is firstly larger in the suburban. The development of localcyclones/anticyclones and semi-diurnal atmospheric tides also contribute to these diurnal variations.

Key words: air-pressure, diurnal variation, Tokyo

1. INTRODUCTION

The distribution of air-pressure in the urban Tokyo has not been discussed except by Kita (1939), mainly becausethere was not an appropriate observation network. In 2002, METROS (Metropolitan Environmental Temperatureand Rainfall Observation System) was set up, so both station and sea-level air-pressure data was able to beenobtained every 10 minutes at 20 stations in the urban Tokyo (Fig.1). In this paper, we show the distribution ofair-pressure and diurnal variation of the local air-pressure system in the urban Tokyo.

2. DATA AND METHODS

Figure 1 shows the location of the stations used in this study. The data is not sometimes obtained in Nos.8 and 14stations, so we used the data set of air-pressure except the data at these stations and added the data obtainedhourly at Japan Meteorological Agency (No.21). The days for analysis were selected as follows. In summer, thedays when “south-high” synoptic pressure pattern with weak pressure gradient was assumed to last almost duringat least 24 hours were selected as “calm summer days”. In winter, the days when “west-high and east-low”synoptic pressure pattern with cloud amount below 4 was assumed to last during at least 24 hours were selectedas “fine winter days”. That is;calm summer days: Jul. 7, 2004/ Jul. 8, 2004/ Jul. 20, 2004/ Jul. 24, 2004/ Aug. 1, 2004/ Aug. 2, 2004/

Aug. 8, 2004/ Aug. 13, 2004 (8 days in total)fine winter days: Dec. 11 , 2002/ Dec. 26, 2002/ Jan. 25, 2003/ Jan. 30, 2003/ Dec. 21, 2003/ Dec. 22, 2003/

Dec. 23, 2003/ Jan. 10, 2005/ Jan. 21, 2005 (9 days in total)We discuss firstly the distribution of sea-level pressure every one hour in the urban Tokyo. However, it cannot beperfectly denied that some barometers might have an instrumental error. So we discuss secondly the spatialdistribution of the mean station air-pressure difference from that of the previous hour.

3. THE DISTRIBUTION OF SEA-LEVEL PRESSURE IN THE URBAN TOKYO

Figure 2 shows the spatial anomalies of the mean sea-level pressure on calm summer days. The sea-levelpressure is low in the northern and southern sides of the urban Tokyo, and high in the western and eastern sidesthroughout almost one day. The sea-level pressure difference inside the urban Tokyo is assumed about 0.8hPa～1.3hPa. Figure 3 shows the spatial anomalies of the mean sea-level pressure on fine winter days. The sea-levelpressure difference inside the urban Tokyo is assumed about 1.2hPa～1.4hPa, a little larger on calm summer days.In both cases, the sea-level pressure is low in the northern and southern sides of the urban Tokyo, and high in thewestern and eastern sides throughout almost one day.

4. DIURNAL VARIATION OF THE LOCAL AIR-PRESSURE SYSTEM IN THE URBAN TOKYO

Figure 4 shows the diurnal variation of the area-averaged sea-level pressure on fine winter days. Similar variationis observed on calm summer days (not shown). Judging from two peaks a day at 09 JST and 21 JST, a deeptrough at 14JST, a shallow trough at 04 JST, we find diurnal and semi-diurnal atmospheric tides in the air-pressuresystem in the urban Tokyo, which are also seen in the air-pressure variation in the Central Mountainous Area of

Corresponding author:Junji Nishina, Meijigakuin University, 1-2-37, Shirokanedai, Minato-ku, Tokyo 108-8636, Japan

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Fig.1 Location of the stations used in this study1-20: METROS stations

21: Japan Meteorological Agency (Otemachi)

Fig.3 Spatial anomalies of the mean sea-level pressure (SLP)on fine winter days(A) at 14 JST (area-averaged SLP:1014.22hPa)(B) at 22 JST (area-averaged SLP:1016.93hPa)(C) at 05 JST on the following day

(area-averaged SLP:1016.02hPa)(D) at 09 JST on the following day

(area-averaged SLP:1016.95hPa)unit: hPasolid line: above the area-averaged SLPdashed line: below the area-averaged SLP

Fig.4 The diurnal variation of the area-averaged sea-levelpressure (SLP) on fine winter daysThe area-averaged SLP is calculated by using the dataset at 17 stations.

Fig.2 Spatial anomalies of the mean sea-level pressure (SLP)on calm summer days(A) at 14 JST (area-averaged SLP: 1008.63hPa)(B) at 22 JST (area-averaged SLP: 1009.64hPa)(C) at 03 JST on the following day

(area-averaged SLP: 1009.33hPa)(D) at 07 JST on the following day

(area-averaged SLP: 1010.29hPa)unit: hPasolid line: above the area-averaged SLPdashed line: below the area-averaged SLP

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Fig.5 The mean station air-pressure difference from that of theprevious hour on calm summer days(A) at 09 JST (B) at 11 JST (C) at 14 JST(D) at 17 JST (E) at 20 JST (F) at 22 JST(G) at 01 JST on the following day(H) at 03 JST on the following day(I) at 05 JST on the following day(J) at 07 JST on the following dayunit: hPasolid line: surface air-pressure risedashed line: surface air-pressure drop

Fig.6 The mean station air-pressure difference from that of theprevious hour on fine winter days(A) at 11 JST (B) at 13 JST (C) at 17 JST(D) at 20 JST (E) at 22 JST(F) at 02 JST on the following day(G) at 03 JST on the following day(H) at 05 JST on the following day(I) at 08 JST on the following day(J) at 09 JST on the following dayunit: hPasolid line: surface air-pressure risedashed line: surface air-pressure drop

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Japan (Iwai and Miyashita, 2005).Figure 5 shows the diurnal variation of the local air-pressure system in the urban Tokyo on calm summer days. At09 JST, the pressure drop in the suburban Tokyo, especially at No.3 station, is larger than that in the downtownTokyo. At 11 JST, the pressure drop is larger especially at Nos.2 and 7 stations. This reflects the development ofthe local cyclone generated in the Central Mountainous Area of Japan (Nishina, 2000). At 14 JST, the pressuredrop is larger in the downtown Tokyo. At 17 JST, the air-pressure is almost the lowest throughout one day and thepressure rise begins. At 20 JST, the pressure rise is larger in the downtown Tokyo, thereafter at 22 JST is larger inthe suburban Tokyo. At 01 JST on the following day, the pressure drop is larger in the downtown/coastal Tokyothan in the suburban Tokyo. On the other hand, at 03 JST and at 05 JST on the following day the pressure rise islarger in the suburban Tokyo, which reflects the development of the local anticyclone generated in the CentralMountainous Area of Japan (Nishina, 2000). At 07 JST on the following day, the pressure rise is larger in the areafrom No.2 southward to No.20 stations.Figure 6 shows the diurnal variation of the local air-pressure system in the urban Tokyo on fine winter days. At 11JST, the pressure drop in the suburban Tokyo is larger than that in the downtown Tokyo (except at No.21 station),on the other hand, at 13 JST in the suburban Tokyo is larger. At 17 JST, the pressure rise begins and is larger in thesuburban Tokyo and at 20 JST in the downtown/southern Tokyo. At 22 JST, in the downtown Tokyo the pressurerise continues, but in the suburban Tokyo the pressure drop begins. On the contrary, at 02 JST on the following dayin the downtown Tokyo the pressure drop continues, but in the suburban Tokyo the pressure rise begins. At 03 JSTon the following day, the pressure drop is larger in the suburban Tokyo. At 05 JST on the following day, in thedowntown Tokyo the pressure drop continues and in the suburban Tokyo the pressure rise begins. At 08 JST onthe following day, the pressure rise is larger in the suburban Tokyo, but at 09 JST on the following day the pressurerise is larger in the downtown Tokyo.

5. CONCLUDING REMARKS

In this paper, we discuss the diurnal variation of the air-pressure system in the urban Tokyo on calm summer daysand on fine winter days by using the METROS (Metropolitan Environmental Temperature and Rainfall ObservationSystem) data set. In both cases, the sea-level pressure is low in the northern and southern sides of the urbanTokyo, and high in the western and eastern sides throughout almost one day. The sea-level pressure differenceinside the urban Tokyo is assumed about 0.8～1.4hPa, a little larger on calm summer days. On calm summer days,during the daytime the pressure drop in the morning is larger in the suburban Tokyo, and in the afternoon is largerin the downtown Tokyo. After sunset, firstly the pressure rise is larger in the downtown Tokyo, thereafter is larger inthe suburban Tokyo. On the other hand, on fine winter days, during the daytime the pressure drop in the morning islarger in the downtown Tokyo, in the afternoon is larger in the suburban Tokyo. After sunset, firstly the pressurerise is larger in the suburban Tokyo, thereafter in the downtown Tokyo. It is common in both cases that while thearea-averaged sea-level air-pressure in the urban Tokyo drops, the pressure drop is firstly larger in the downtownTokyo; while area-averaged sea-level air-pressure rises, the pressure rise is firstly larger in the suburban Tokyo.These results show the fact that not only the artificial heat in the urban Tokyo, but also the development of localcyclones/anticyclones and semi-diurnal atmospheric tides contribute to the diurnal variation of the air-pressuresystem in the urban Tokyo.

References

Iwai, K., Miyashita, E., 2005, Diurnal, semi-diurnal and ter-diurnal pressure variations at the stations of CentralMountain Area of Japan, Tenki, 52, 831-836. (in Japanese)

Kita, T., 1939, Town climate of Tokyo, Journal of the Meteorological Society of Japan, 17, 155-161. (in Japanese)Mikami, T., 2004, Progress of urban heat island research and its relation to the city government policy. Tenki, 51,

119-122. (in Japanese)Nishina, J., 2000, Diurnal variations of local sea-level pressure system in Central Japan under summer pressure

patterns, Quarterly Journal of Geography, 52, 131-140. (in Japanese with English abstract)

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