The Synoptic-Climatological Characteristics of the Most Frequent Heavy-Rainfall Area along the Pacific in Tohoku District

Abstract

The areas where heavy-rainfalls most frequently occur are the coastal regions of Fukushima and Iwate Prefectures in the Pacific Ocean side of Tohoku District.
In these areas, the following two synoptic-climatological characteristics are clarified:
1) The points where the maximum rainfall amount is frequently observed do not correspond to the topographic gradient when the easterly air current is most prevailing.
2) Under the northeasterly prevailing air current, the heavier rainfall in the coastal region is at the colder temperature of the current.
About the heavy rainfall in the coastal region, hitherto, such ideas were proposed as the secondary orographic rainfall, the orographic rainfall or the effect of coast on precipitation. These ideas were based upon the heavy rainfalls or convective clouds generated in coastal region. However, the author would like to propose the following idea as the foundation of the coastal heavy rainfall. The comparison between the air temperature at the coast and the sea surface temperatures at the coast and off the coast under the northeasterly prevailing air current show that the cool air mass drifted by the northeasterly air current is heated from the sea and therefore convective clouds are generated in the air mass. On the other hand, as the heat supply is cut off on land, convective clouds are dissipated in the coastal region. Then the most rainfall amount on the land is concentrated along the coastal line.