Supercooled cloud tunnel studies on the shape variety of branched planar snow crystals grown while falling freely in the atmosphere

2018 Fiscal Year Final Research Report

• Project/Area Number: 16K05551
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Meteorology/Physical oceanography/Hydrology
• Research Institution: Hokkaido University of Education
• Principal Investigator: Takahashi Tsuneya 北海道教育大学, 教育学部, 教授 (60236297)
• Project Period (FY): 2016-04-01 – 2019-03-31
• Keywords: 樹枝状雪結晶 / 自由落下成長 / 鉛直過冷却雲風洞 / 降雪粒子の成長 / 雲微物理過程 / 降水機構

Outline of Final Research Achievements

Under what conditions do the branched snow crystals grow in the atmosphere? To address this issue, we carried out experiments between -12° and -16.5°C using a vertical supercooled cloud tunnel that can simulate the process by which a snow crystal continuously grows while falling in a cloud. The crystal habits were mainly temperature dependent, with habit regimes roughly symmetric about the fern regime near -14.2°C. Away from this temperature were dendrites, stellars, broad branches, and then sectors. When growth time shifts from 5 min, to 10min, to 20min, a regime of a higher density of side branches spread out. Even below water saturation, crystals having fern type, dendrite type, stellar type, and broad-branch type branches were grown. The crystal habits were dependent not only of temperature but also of excess vapor density with respect to ice.

Free Research Field

大気物理学、特に雲微物理学　気象教育

Academic Significance and Societal Importance of the Research Achievements

雪結晶研究の先駆者である故中谷宇吉郎博士は「雪の結晶は天から送られた手紙」という人口に膾炙している言葉に続けて、「その中の文句は結晶の形及び模様という暗号で書かれている」と記している。本研究により暗号の一端が読み解かれ、雪結晶の代表的な形で、羊歯六花や樹枝六花、星六花など多様な形態を有する樹枝状雪結晶の雲内における成長条件が明らかになった。降水機構及び関連大気諸過程等を理解する上で重要な基礎的知見が得られたばかりでなく、結晶の形態や質量、大きさ、落下速度等のデータは雲のモデリングや気象制御、水収支などでも基礎データとなる。