Long term trend of heavy precipitation around Japan
Nobuo Yamazaki (1)
(1) Climate Research Department, Meteorological Reserach Institute
Using daily and hourly precipitation data in Japan, and ERA40 monthly 500 hPa geopotential
height (z500), long term trend of heavy precipitation around Japan and the associated large-
scale trend are analyzed.
Frequencies of both three hourly and daily heavy precipitation averaged over 7 stations in
Japan at the 1990s are higher than those at and before the 1950s. More increasing trend 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
precipitation in the late summer has more increasing trend than those in the early summer
and in the whole summer.
Those trends are compared with z500 from ERA40 in the period from 1958 to 2001.
Correlation of time series of the June to September heavy precipitation with z500 shows
negative area in Korean Peninsula sandwiched with two positive areas, the North Sea of
Okhotsk and the North Western Pacific (~150E25N). On the other hand trend of z500 in June
to September has positive areas in north and south of Japan but no clear negative area. The
above two patterns are similar to each other in positive areas but the trend pattern of z500
has no negative center. Some resemblance between the two correlation patterns implies
favorable large-scale condition for heavy precipitation increases recently to some degree.
Similar 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.