open access

https://doi.org/10.11501/3084722

https://hdl.handle.net/2115/49649

冬期北海道西岸に発生する収束帯状雲の観測的、解析的研究

英語

1991-03-25

Hokkaido University

131p.

It is well known that heavy snowfalls in Japan are brought by convective cloud lines． On the west coast of Hokkaido， the more larger band cloud is frequently observed under the winter monsoonsynoptic situation． This larger band cloud having 200 to 300 km in length and several tens of kilometer in width is formed along the west coast of Hokkaido and is called as the convergence band clouds （CBCs）． It is important to understand therefore that why the CBC is maintained for a relatively long time along the west coast of Hokkaido． The purpose of this study is to clarify the life cycle，mesoscale features and mechanisms of the long－lasting CBC． From the analyses of synoptic situations and satellite data，the long lasting CBCs were divided into two types； Type A and Type B. ln this study, especially, we focused our attention tothe CBCs of type A． The CBCs of type A are originated under the following situations． The main cyclones are located far to the east of Hokkaido and cold air covers over Hokkaido where the cloud streaks are recognized over the Japan Sea and the Okhotsk Sea． The initial stage of CBCs is characterized by the disturbances which generate to the west of the Soya Strait according to the low TBB area under the convergence of the northwesterly monsoon wind and the northeasterly cold wind． The mature stage is characterized by the organized CBC for 200 to 300km along the west coast of Hokkaido under the convergence of the monsoon wind and the southeasterly land breeze． After that, the CBC decays at the same place （the dissipating stage）. Considering the pressure field around the west coast, the low pressure area is formed over the Japan Sea in the initial stage and the high pressure area is formed inland in the mature stage．Therefore, the relative low pressure field is maintained along the west coast of Hokkaido through the lifetime． During the special radar observations at Haboro, echo structures of the mesocyclone generated in CBC and the merging process of CBC were observed． The mesocyclones have about 30 kmin diameter which is one order of magnitude smaller than CBC. The distinct wind field, the pressure drop and the thermal structure of the warm core of mesocyclones are the remarkable features． The mesocyclones are formed at the south edge of CBC near the west coast and it is considered that the mesocyclones are generated at the surface convergence zone where the cyclonic three different winds are prevailed and the surface temperature gradient is steep between the relative warm airmass of the monsoon wind and the cold airmass of the land breeze. Considering the merging and separation processes of echo bands, it is recognized that the CBC as seen from the satellite is constructed of several echo bands which merge under the difference of the propagation speed． The theoretical explanation of the maintenance process of CBC is performed through the two-dimensional dynamical model． ln the model, the effects of diabatic heating from the sea surface are calculated． As a result, non-geostrophic wind circulations are formed over the coast line which is characterized by the convergence zone over the sea． The solutions correspond well with the analytical field. We conclude therefore that the CBCs are possible to consider as a coastal front．

1991-03-25

甲第2856号

博士(理学)

博士論文

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