Spatial Analysis of Typhoon Rain in Japan
John C． KIMuRA
INTRODUCTION
Typhoons with their attendant tidal waves，1andslides， floods and wind lead the list of causes of natural disasters in the Japanese Islands． Japan lies in the path of one of the world s most active typhoon belts． This is true especially in September． Saito（1959）writes that although this chain of islands is noted for the extremes of the violence of nature， not even earthquakes cause as much loss of life and property as do the typhoons． Sekiguti
（1965）notes that heaviest rainfall during short periods is related to thunderstorm showers，
but that the heaviest amounts of daily rainfall are caused by typhoons in Tokyo． Ito（1959）
states that precipitation in Japan is highly influenced by topography， and typhoon rain is not an exception．
The total number of typhoons， the total amount of typhoon rain and the average amount of rain per typhoon vary with different regions in Japan and are significant because of the disasters that may be associated with them． In this connection， the number of typhoons， the total amount of typhoon associated rain， average rainfall per typhoon， the percentage of typhoons with various amounts per storm，the percentage of typhoon rain by catagories of amount to the total typhoon rain， and typhoons with exceptionally heavy rain will be examined．
DATA
Daily、 precipitation data were analyzed， and the rain related to typhoon causes was summed and values were determined for 25 stations as in a previous report（Kimura，1970）。
Data were obtained from the Japan Meteorological Agency． In the previous study，averages were computed on a variety of base periods， depending 6n the year that the respective stations were established． In this report， however， all values are based on data from 1930 through l 963．
Usually， heavy rain falls within several hundred kilometers from the center of a typhoon；however， in many cases low values are recorded in much of the remainder of the country， because these areas were affected by only the fringe of the storm． In this report， a storm front that is activated by a typhoon with resulting rain is also considered to be typhoon related． The amounts vary from low to very high・Sekiguti（1965）states that pre−typhoon frontal rain contributes a significantly high percentage of the total amount．
TOTAL NUMBER OF TYPHOONS AND TYPHOON−ASSOCIATED RAIN
Figure l・A shows the total number of typhoons that affected Japan from May 31
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through October 27，1930 through 1963． The number of typhoons used in this study represents the number of passages of typhoons with measurable precipitation caused by such storms． The values are highest in the southern Pacific coastal region， especially in the vicinity of the Kii Peninsula． The number of typhoons during this period decreased to the Japan Sea side， where the values are similar． to those in the southern portion of Tohoku． Further decreases in the number of typhoons are noted in northern Tohoku and in Hokkaido．
The frequency of typhoons is greatly affected by the directional trend of the islands．
In southwestern Japan， where the trend is northeast−southwest， the isolines depicting frequency tend to be parallel with the trend of the country． From central Japan， the directional trend is approximately north・south and the frequency of typhoons decreases uniformly northward． Similar tendencies appear with regard to total typhoon precipitation
（Figure l B）and average precipitation per typhoon（Figure 2）．
Figure 2
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Total typhoon−related rain during this period is portrayed in Figure IB（average annual typhoon rain is noted in Kimura，1970）． The region of maximum rainfall is along southern Shik・ku・whi・h is slightly・・uthw・・t・f th・・egi・n with th・g・eat・・t numb…ftyph・・n、．
The rainfall gradient is steep to the Inland Sea， indicating that the total amount of typhoon−associated rain decreased markedly． The Inland Sea region experiences about 85％
as many typhoons as the southern Pacific area， but only about 60％as much typhoon−related rain is received． Sim且arly， the southern Japan Sea side is affected by slightly more than 70％
of the storms as compared to the southern Pacific region， and comparatively less than 50％
・ftyph・・n−ass・・i・t・d・ain i・exp・・i・nced・T・h・ドu・v・・ages slightly less th・n 80％・f th・
number of storms that the southern Pacific region experiences， and has about 35％as much typhoon rain． The greatest disparity is in Hokkaido，where two・thirds as many storms occur，
but only 20％as much typhoon associated rain is recorded．
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・h・wing t・t・1 typh・・n p・ecipit・ti・n（Figu・e lB）． High・・t valu・・a・e f・und・1・ng th…uth・・n Pacific coastal district， where the rainfall amount per storm exceeds 75 mm at Kochi． A
steep gradient exists to the Inland Sea and North Kyushu，where about 45 mm per typhoon is the average． A continuous decrease in the rainfall amount per typhoon is evident along the Pacific， especially north of the Boso Peninsula， but the values are the lowest on the Japan Sea side of Tohoku and in Hokkaido， where below 20 mm of rain per typhoon is the average・
While warm−season precipitation curves in Japan do not show a decreasing pattern to the Japan Sea from the Pacific side， typhoon・related rain does． The principal consideration is undoubtedly related to the general paths of the typhoons． Those storms affecting Japan form the south and southeast must traverse the mountains， and， thus， are greatly weakened．
In such regions as the Inland Sea Basin， typhoons not only must cross the mountains of Kyushu and／or Shikoku， but a rainshadow effect undoubtedly also prevails． Moreover，
typhoons that parallel the southern Pacific coast may affect only the Pacific side from the Kanto Plain southward with copius rain， while the remainder of the country may only be fringed by the storm． Therefore， while the variation in the number of storms in various sections of Japan is great， the variation in the amount of typhoon−related rain is infinitely greater， and is reflected in the average amount of rain per typhoon．
PERCENTAGE OF TYPHOONS BY PRECIPITATION AMOUNTS
Figure 3 shows the percentage of typhoons in 6 categories of rainfall amount． The greatest percentage of typhoon occurence is associated with the lowest categories of precipitation． Figure 3A indicates that an exceedingly high percentage of typhoons occurs within the category of O−10 mm of rain． In portions of the Japan Sea side of Hokuriku，
Tohoku and northern Hokkaido， about one−half of all of the typhonns was associated with less than l O mm of rainfall． Understandably， percentages in this category decrease to the south・east facing Pacific coast， where more severe storms are experienced and where the amount of typhoon rain is the greatest。 A high percentage of typhoons is associated with low rainfall amounts， because only the fringe of storms affects most of the country， while the core area of storms may progress in relatively narrow paths． Moreover， the Japan Sea side represents the leeward side with respect to most typhoons． Accordingly， the storms are greatly weakened， and a high percentage of typhoons with low precipitation should result． In the precipitation categories of lO−−25 mm and 25−50 mm， there is a decline in the percentages of storms from slightly below l 5 to about 25． Most typhoons within these categories are along the Japan Sea side rather than the Pacific coast・
Amajor change in pattern exists with the rainfall category of 50・75 r寧m． A higher percentage of typhoons in this category is now experienced on the Pacific side， though in central Japan both the Japan Sea and the Pacific sides have similar values． Lowest percentages are found in the northwestern part of the country， and in the Inland Sea to the Japan Sea adjacent to Kyushu and the southwestern portion of Honshu． In this category the values range from above 5 to less than 15％・In the rainfall category of 75−100 mm・a complete reversal of the pattern from the lower categories is noted． The entire Japan Sea side has typhoon percentages that are lower than those of the Pacific side． The highest figures are found in the extreme southern tips of Kyushu， Shikoku and the Kii Peninsula，
where the values slightly exceed lO％． The greatest range in the percentages of typhoons is
associated with the precipitation category of over l OO mm（Figure 3F）， where the values are
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from less than 5 to slightly over 30％． The lowest figures are found in Hokkaido and on the Japan Sea side of Tohoku， while the highest are in the Pacific side of southwest Japan， where the values are about equal to those in the O−10mm category．
PERCENT OF TYPHOON RAIN BY CATEGORIES
TO TOTAL TYPHOON RAINFALL
Figure 4 illustrates the percentages of rain in 6 categories based on amount compared to total typhoon rainfall． It was noted above that the greatest percentage of typhoons is associated with the O−10 mm category． However， the percentage of rain in thls group as compared to the total typhoon rain shows values ranging from slightly over l to 8％． The values are understandably low despite the great number of storms， because rainfall amount per typhoon is negligible． The lowest figures are found along the Pacific from the Kanto Plain southwestward including the Inland Sea region and all of Kyushu，while higher values
are for the most part on the Japan Sea side from Hokuriku to Hokkaido． This general pattern prevails for the three lower categories． In the 50−75 mm group， while the overall pattern，is similar to that of the lower categories， the extreme southwestern portion of the country has the lowest values（about 10！of the total typhoon rain）． The remainder of the country has figures ranging for the most part between l O to over 20％．
The pattern of greater values on the Japan Sea side， and lower figures on the Pacific side breaks down from the 75−100 mm category． In the over lOO mm grouping， the pattern has decidedly reversed from the Japan Sea to the Pacific sides． Over 70％of all typhoon rain in southeastern Kyushu and in southern Shikoku is in this category， while it was noted earlier that in this region about 25 to 30％of the storms has rain amounting to 100mm or more． The values decrease to the Japan Sea side and northward to Hokkaido，
where the amount of typhoon rain in this category is well below 10％as compared to the total typhoon rainfall， while the percentage of storms with over l OO mm of rain is from 2 to
3％．TYPHOONS WITH OVER lOOmm OF RAIN
Figure IAshows the total number of typhoons that affected Japan during the study period． Figure 5A depicts tlle number of typhoons with over lOO mm of rain and Figure 3F 丑lustrates the percentage of typhoons with over lOO mm of rain during the same period．
Typhoons with more than lOO mm of rainfall are especially numerous in the southern
Pacific area。 The frequency of storms decrease to the Inland Sea， the Japan Sea side and
finally northward to Hokkaido． Similar patterns are descernable in all of the heavy rainfall
categories as shown in Figure 5B to 5F． Typhoons with increasingly heavy rains diminish
from the north． Figure 5C illustrates typhoons with 150 to 200 mm of rain． It is noted that
these storms are not found in northwestern Tohoku and most of Hokkaido． In the
succeedillg groups， the boundaries of unaffected areas move progressively southward， and
regions where typhoons with excessively high amounts are for the most part found in the
southern Pacific district．
CONCLUSION
Floods and landslides resulting from typhoon rain are the paramount natural disasters in Japan． Even in a controlled experiment in Kawasaki in November，1971，aslope that was being sprinkled gave way， and a slide occurred which claimed a dozen lives． Numerous natural major disasters can be directly attributed to typhoons． To alleviate the impact of such phenomena， rainfall characteristics of typhoons should be studied in detai1． Such investigations perhaps might commence with the regionality of typhoons in terms of frequency， average rain per typhoon， and heavy rainfall values and their probability・
With regard to frequency of typhoons， total typhoon precipitation， average rain per storm， and the frequency of typhoons with extremely heavy rain， the southern Pacific coastal region has the highest values． Accordingly， natural disasters related to typhoons are the most frequent in that portion of the country． Though the frequency of disasters decreases to the Inland Sea， the Japan Sea side，Tohoku，and Hokkaido， the intensity of the disasters may not necessarily decrease at the same rate．
Atemporal and migrational analysis of typhoon rain may shed additional light on alleviating the effects of disasters． With this in mind， the migration of typhoon rain based on 10−day periods is currently being studied．
Refelences
Central Meteorologica10bservatory（Japan Meteorological Agency）
Taifuηi To〃lonau Oa〃ze， Tokyo，1950（in Japanese），
Taifu ni Tomonauκひuリノo Yoso Sh iiツo， Tokyo，1950，（in Japanese）．
Oame｝ oso Shiryo， Tokyo， Annual（in Japanese）．
Ito， H．，1959： The Characteristic Feature of Rainfall Distribution Caused by a Typhoon， Vol．13，
Meteorology， Pacific S「cience（〕ongアθ∬P｝ oceedings， Bangkok， pp．202−204．
