北海道・東北地方における冬期の早朝に最低気温が記録される時刻の地理的分布

Abstract

The object of this study was to obtain the geographical distribution of minimum temperatures in northeastern Japan during winter using ten-minute Automated Meteorological Data Acquisition System (AMeDAS) readings. The area under analysis included the Hokkaido and Tohoku districts, as well as north of 37 North Latitude in Niigata Prefecture. Within this area there are 332 observation sites from which data were collected. Data were obtained from AMe-DAS, which records data every ten minutes. The observation period covered the months of January and February from 1998 through 2003. There were a total of 355 days of readings. For each station, seventy-one of the days from this period were extracted for an analysis of lowest temperature, being a 20 percent representation of the data. In this paper, the author defines minimum temperature as not being a record low but rather above a rank of six from data taken between 00 : 10 and 09 : 00 Japan Standard Time.The time at which minimum temperatures were most frequently observed was 06 : 50, with a frequency of 4.49%. The time at which minimum temperatures were observed least was 01 : 40, with a frequency of 1.14%. From 07 : 20, the frequency decreases abruptly.Using a Correspondence Analysis, the author classified the data into four types according to time and geographical distribution.In type I, there is no prominent overall peak of frequency, but the frequency increases slightly after 07 : 30. This type I was distributed along the eastern coast of the Hokkaido and Tohoku districts, as well as within Aomori prefecture. The type most closely identified with sunrise time (06 : 20 07 : 30) was located in the basins of inland areas in Hokkaido, Iwate, and Fukushima prefectures (type II). Pre-sunrise times were defined as 04 : 40 to 06 : 10 (type III). This type was distributed along the eastern coast of Hokkaido and the Pacific Ocean coast. Data from before 04 : 30 are classified as type IV. Type IV has a small peak in temperature variation before 01 : 00. Type I has a small peak after 07 : 30. These two types can be seen along the coast of the Sea of Japan. Types II and III are related to sunrise time. In these areas there is frequent radiative cooling and a good probability of a clear sky throughout the night. Types I and IV have less relation to sunrise time.