Trend of the Occurrence of Heavy Rains in AMeDAS Hourly Precipitation

The frequency of extreme weather may increase due to the climate change of the globe. Some experts predict that we would see more intense typhoons or hurricanes, and more severe heavy rain due to the so-called global warming. One method to validate this hypothesis is to study the long-term trend of the actual frequency of heavy rain. The study of transition is an important theme to think about the impact of climate change to our society.

On the other hand, the increase of the frequency of heavy rain is predicted in a scale much smaller than global warming. Namely, the hypothesis goes that the frequency of heavy rain would increase in city areas due to the intensification of heat island effect. This phenomena has different causes from the former, but should be considered in the same framework with the former in the context of studying the societal impact of climate change caused by human activities.

We therefore summarize the change of the occurrence of heavy rain by the statistics of AMeDAS hourly precipitation data. As the criteria of heavy rain, we use the hourly precipitation of more than 50mm, 80mm and 100mm, and compare the number of those cases for each year. The following points should be remembered when using these statistics.

  • The number of occurrence of heavy rain tends to be smaller in the first few years due to the smaller number of AMeDAS stations available at that moment.
  • The hourly precipitation is derived from AMeDAS "hourly observation" (*a), namely a one-hour precipitation between 00 minute to 59 minute before the exact hour. The number of occurrece is smaller than hourly precipitation calculated from AMeDAS "10-min observation" (*b). According to the paper (in Japanese), the number of occurrence of *a is about half of that of *b for the hourly precipitation of more than 100mm. Hence we should have in mind that the occurrence of heavy rain in these statistics is less than half of the true occurrence of heavy rain derived from any one-hour period.

Occurrence of Hourly Precipitation of +50mm

1 1976 1976=220
220
2 1977 1977=173
173
3 1978 1978=175
175
4 1979 1979=280
280
5 1980 1980=208
208
6 1981 1981=171
171
7 1982 1982=298
298
8 1983 1983=234
234
9 1984 1984=136
136
10 1985 1985=195
195
11 1986 1986=125
125
12 1987 1987=238
238
13 1988 1988=323
323
14 1989 1989=252
252
15 1990 1990=362
362
16 1991 1991=200
200
17 1992 1992=141
141
18 1993 1993=321
321
19 1994 1994=169
169
20 1995 1995=208
208
21 1996 1996=123
123
22 1997 1997=234
234
23 1998 1998=419
419
24 1999 1999=361
361
25 2000 2000=304
304
26 2001 2001=270
270
27 2002 2002=233
233
28 2003 2003=224
224
29 2004 2004=470
470
30 2005 2005=262
262
31 2006 2006=328
328
32 2007 2007=280
280
33 2008 2008=354
354
34 2009 2009=250
250
35 2010 2010=283
283
36 2011 2011=360
360
37 2012 2012=377
377
38 2013 2013=317
317
39 2014 2014=327
327
40 2015 2015=202
202

Occurrence of Hourly Precipitation of +80mm

1 1976 1976=18
18
2 1977 1977=15
15
3 1978 1978=6
6
4 1979 1979=14
14
5 1980 1980=15
15
6 1981 1981=8
8
7 1982 1982=19
19
8 1983 1983=13
13
9 1984 1984=6
6
10 1985 1985=11
11
11 1986 1986=10
10
12 1987 1987=20
20
13 1988 1988=35
35
14 1989 1989=19
19
15 1990 1990=14
14
16 1991 1991=14
14
17 1992 1992=7
7
18 1993 1993=14
14
19 1994 1994=7
7
20 1995 1995=13
13
21 1996 1996=12
12
22 1997 1997=11
11
23 1998 1998=35
35
24 1999 1999=43
43
25 2000 2000=11
11
26 2001 2001=26
26
27 2002 2002=16
16
28 2003 2003=19
19
29 2004 2004=30
30
30 2005 2005=12
12
31 2006 2006=31
31
32 2007 2007=20
20
33 2008 2008=25
25
34 2009 2009=18
18
35 2010 2010=21
21
36 2011 2011=27
27
37 2012 2012=22
22
38 2013 2013=34
34
39 2014 2014=21
21
40 2015 2015=19
19

Occurrence of Hourly Precipitation of +100mm

1 1976 1976=1
1
2 1977 1977=5
5
3 1978 1978=1
1
4 1979 1979=2
2
5 1980 1980=2
2
6 1981 1981=0
0
7 1982 1982=5
5
8 1983 1983=3
3
9 1984 1984=0
0
10 1985 1985=3
3
11 1986 1986=1
1
12 1987 1987=2
2
13 1988 1988=9
9
14 1989 1989=6
6
15 1990 1990=1
1
16 1991 1991=2
2
17 1992 1992=0
0
18 1993 1993=0
0
19 1994 1994=0
0
20 1995 1995=1
1
21 1996 1996=3
3
22 1997 1997=3
3
23 1998 1998=4
4
24 1999 1999=10
10
25 2000 2000=6
6
26 2001 2001=6
6
27 2002 2002=2
2
28 2003 2003=4
4
29 2004 2004=7
7
30 2005 2005=2
2
31 2006 2006=6
6
32 2007 2007=0
0
33 2008 2008=4
4
34 2009 2009=3
3
35 2010 2010=2
2
36 2011 2011=7
7
37 2012 2012=3
3
38 2013 2013=8
8
39 2014 2014=2
2
40 2015 2015=6
6