Long term trend of heavy precipitation around Japan* Nobuo Yamazaki(Climate Research Department, Meteorological Reserach Institute)* 1-1 Nagamine, Tsukuba, Ibaraki, Japan.
e-mail: nyamazak@mri-jma.go.jp
Abstract
Using daily and hourly precipitation data in Japan, and ERA40 monthly 500 hPa geopotential height (z500), long term trendof heavy precipitation around Japan and the associated large-scale trend are analyzed. Frequencies of three hourly heavy
precipitation averaged over 7 stations in Japan at the 1990s are higher than those at and before the 1950s. More increasingtrend in early morning heavy precipitation is seen. These heavy precipitation events occur mainly in June to September.
When the early summer (June-July) and the late summer (October-September) are separately analyzed, heavy precipitationin the late summer has more increasing trend than those in the early summer and in the whole summer. Those trends arecompared with z500 from ERA40 in the period from 1958 to 2002. Some resemblance between the two correlation patterns
implies favorable large-scale condition for heavy precipitation increases recently. Analysis for the early and late summers
reveals that in the late summer correlation pattern of the heavy precipitation with z500 has strong resemblance to trend
pattern of z500; negative center in the East China Sea surrounded by positive circular belt region. This result is consistent
with the above observation about the different trends in the early and late summer.
Keyword: climate extreme, heavy precipitation.

1. Introduction
Precipitation is one of the key factor of the monsoon
activity and heavy precipitation events have a great social
influence. We analyze trend of heavy precipitation in Japanand a possible link to large-scale field around Japan.

2. Data and method
Data used are daily precipitation data in Japan from 1951
to 2002 compiled by Japan Meteorological Agency (JMA).
Total number of stations are 130 from North Japan to theSouthwestern Islands. Also hourly precipitation data for
1901-1960 and 1990s at 7 stations (Sapporo, Tokyo,Nagoya, Osaka, Fukuoka, Nagasaki, and Kumamoto) areused.
Monthly mean geopotential height at 500 hPa (z500)
from ECMWF ReAnalysis-40 (ERA40). from 1958 to2002 is used for study of large-scale field around Japan.

precipitation averaged over 7 stations in Japan at the 1990s
are higher than those at and before the 1950s. Heavy
precipitation centered in the early morning prominently
increases.
Fig.2 shows time series of frequencies of days with morethan 100 mm/day in summer (June to September), in earlysummer (June to July) and in late summer (August toSeptember) in Japan. Although none of them has any clear
increasing or decreasing trend, frequency in the latesummer tends to have a little increasing trend.
Horizontal distribution of heavy precipitation trend isshown in Fig. 3. Trend patterns are clearly different

3. Results
Fig. 1 illustrates change of occurrences of 3 hourly
heavy precipitation for each local time averaged over 7stations in Japan. Frequencies of three hourly heavy
3 hourly largest 50th 7 stations nochk
90
8070
1895

60

1905
1915

50

1925
1935
1945

40

1955
1995

30

1900's20
1000-3hr

3-6hr

6-9hr

9-12hr12-15hr

15-18hr

18-21hr

21-24hrFig. 1: Change of occurrences of 3 hourly heavy
precipitation for each local time averaged over 7 stationsin Japan. Legend is shown in the right hand.

Fig. 2: Time series of frequencies of heavy precipitation
days with more than 100 mm/day in June to September(Yellow line), in June to July (Black line) and in Augustto September (Green line) in Japan. Thin (thick solid)lines denote original (5 year running mean) timeseries.
between in early summer and in late summer. Hereafteranalysis is made separately for the early summer and thelate summer.
Upper figures in Figs. 4 and 5 are linear trends in z500
from ERA-40 for June to July and August to September,respectively. In June to July there are positive trends in thenorth and south of Japan and negative trends in Japan

Fig. 3: Linear trend of heavy precipitation days withmore than 100 mm/day in June to July (left figure) andin August to September (right figure) from 1951 to 2002
in Japan.
extending northwestwards and northeastwards. The
northern positive trend corresponds to recent tendency ofstrengthening of the Okhotsk high over the Sea of Okhotsk.
Trend pattern in August to September is similar to that inJune to July, but negative trend from the Yangtze River, theEast Sea and through South Japan is more prominent andpositive trend surrounds the negative trend. Center.
Lower figures in Figs. 4 and 5 are horizontal distributionsof correlation coefficient between the z500 and heavy

Fig. 4: (a) Linear trend of ERA-40 geopotential
height at 500 hPa (z500) during June to July in theperiod of 1958-2002. (b) Horizontal map ofcorrelation coefficient between z500 and time seriesof heavy precipitation during June to July in Japan.

precipitation in the early summer and the late summer,respectively. These figures are considered to representpatterns favorable for occurrence of heavy precipitation. If
the z500 trend pattern (upper figure) and the pattern of thecorrelation coefficients (lower figure) in Figs. 4 and 5
resembles to each other around Japan, more increasing
tendency of the heavy precipitation over Japan is expected.
in the early summer (From Fig. 4) some qualitative
similarity is seen as negative around Japan and positive inthe south of Japan. No positive pattern corresponding to
the strengthening of the Okhotsk high, however, isrecognized in the lower figure. On the other hand in thelate summer (Fig. 5), correlation pattern of the heavy
precipitation with z500 has strong resemblance to trend
pattern of z500; negative center in the East China Sea
surrounded by positive circular belt region.
Above result suggests that change of large-scale fields is
consistent with more increasing trend of heavy
precipitation in August to September than in June to July.

4. Summary
Frequencies of three hourly heavy precipitation averaged
over 7 stations in Japan at the 1990s are higher than thoseat and before the 1950s. More increasing trend in earlymorning heavy precipitation is seen. When the earlysummer (June-July) and the late summer (AgustSeptember) are separately analyzed, heavy precipitation in
the late summer has more increasing trend than those in theearly summer and in the whole summer. Some resemblance
between the two correlation patterns implies favorable
large-scale condition for heavy precipitation increases
recently. Analysis for the early and late summers reveals
that in the late summer correlation pattern of the heavy
precipitation with z500 has strong resemblance to trend
pattern of z500. These results imply that change of largescale field in the late summer around Japan brings aboutfavorable condition for heavy precipitation in Japan.

Fig. 5: The same as Fig. 4 except for August toSeptember.