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Examination of Harmonic Analysis Methods Using Semi-regular Sampling Data from an ADCP Installed on a Regular Ferry : Evaluation of Tidal and Residual Currents in the Eastern Mouth of the Tsugaru Strait

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  • 定期船ADCPによる semi-regular sampling データを用いた調和解析手法の検討 : 津軽海峡東口における潮流と残差流の評価
  • テイキセン ADCP ニ ヨル semi regular sampling データ オ モチイタ チョウワ カイセキ シュホウ ノ ケントウ ツガル カイキョウ ヒガシグチ ニ オケル チョウリュウ ト ザンサリュウ ノ ヒョウカ

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Abstract

津軽海峡東口周辺における実際の定期船ADCP流速データを用いて,黒田・磯田(2004)が提案した年周斯変動成分に関する調和解析上の注意点に基づき,どれだけ確からしい潮流と残差流(季節変動)が見積もれるかを検討した。日単位のデータサンプリングに依存した調和解析上の最大の問題点は,K_1とP_1分潮からSa分潮と同じ周期のエイリアシングが生じるため, K_1,P_1,Saの3分潮を同時に分離することが不可能なことである。ここにおいて, Sa,K_1の2分潮を選択し, O_1,M_2,Q_1,N_2の4分潮を加えて調和解析を行なった結果,構造的で卓越したK_1分潮流を見積もることができた。しかし,残差流に含まれる可能性があると推定されたP_1分潮の最大振幅は,残差流の年周期変動の振幅に対して無視できず,P_1分潮を調和解析によって分離する必要がある。そこで,津軽海峡東口周辺の検潮所におけるP_1とK_1分潮の潮位の振幅比と位相差の関係を潮流に適用した調和解析を再び行ない,過去に行われた短期の係留流速観測結果と比較して,両者の共通点と相違点を整理した。更に,このような方法で見積もられた潮流成分を除去することで,津軽海峡内における流れの季節変動を提示することが初めて可能となった。Some harmonic analysis methods using actual current data measured by an ADCP installed on a regular ferryboat were examined based on the suggestions made by Kuroda and Isoda (2004) regarding the harmonic analysis of annual variations. Hence the tidal and residual currents (seasonal variation) were estimated around the eastern mouth of the Tsugaru Strait. The biggest problem in the harmonic analysis is that the K_1, P_1, and Sa constituents cannot be separated simultaneously since a sun-synchronous infrequent semi-regular sampling causes K_1 and P_1 aliasing, the period of which is the same as that of the Sa constituent, i.e. 365.24 (d). First, harmonic analysis of the Sa, K_1, O_1, M_2, Q_1, and N_2 constituents could spatially estimate the systematic and dominant K_1 tidal current. However, the guessed maximum P_1 amplitude that may have been included in the residual current was not sufficiently less than the amplitude of the seasonal variation. Therefore, it was impossible to discuss the seasonal variations unless the P_1 constituent was estimated, based on assumptions and subsequently excluded. Another harmonic analysis of tidal currents was carried out using the amplitude ratio and phase difference of the P_1 tide relative to the K_1 tide. The harmonic constants were compared with those estimated from the mooring current measurements made in past years, and the points of commonality and difference between them were clarified. Thus, for the first time, the seasonal variations in the surface currents at the Tsugaru Strait were demonstrated by excluding the estimated tidal currents.

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