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
Occurrence of Hourly Precipitation of +80mm
Occurrence of Hourly Precipitation of +100mm