著者

TOCHIMOTO Eigo KAWANO Tetsuya

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2017-005, (Released:2017-01-13)

被引用文献数

10

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This work investigates development processes of Baiu frontal depressions (BFDs) using a numerical model. To investigate the effects of upper-level disturbances, latent heating, and baroclinicity on the development of BFDs, case-study numerical simulations are performed. In the present study, two typical cases were selected from BFDs that appeared in June and July, 2000-2007: a BFD that developed in the western part of the Baiu frontal zone (W-BFD) from 26 to 27 June 2003 and a BFD that had formed in the eastern part of the Baiu frontal zone (E-BFD) from 1 to 3 July 2003. An available potential energy (APE) diagnosis shows that the effect of latent heating is dominant during the W-BFD development, while baroclinicity as well as latent heating is important to the E-BFD development. A sensitivity experiment excluding upper-level potential vorticity (PV) anomalies shows that upper-level disturbances are important contributors to the development of E-BFDs. The low-level PV and its production associated with latent heating suggest that the W-BFD has a development mechanism driven by latent heating. In the early developmental stage, PV near the W-BFD center is enhanced. This feature is consistent with the nonlinear conditional instability of the second kind mechanism. In the later developmental stage, PV is produced in front of the W-BFD center, in which low-level baroclinicity is large. This process is consistent with a diabatic Rossby vortex. In contrast, the E-BFD develops through a baroclinic instability-like mechanism in the moist atmosphere.

著者

Akio KITOH

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.95, no.1, pp.7-33, 2017 (Released:2017-03-01)

参考文献数

220

被引用文献数

69

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This study provides an overview of the Asian monsoon and its change as simulated by atmosphere–ocean coupled general circulation models and high-resolution atmospheric general circulation models, focusing on the seasonal mean circulation and precipitation climatology. After reviewing the drivers of and the elements that affect the monsoon, the ability of those climate models to reproduce the Asian monsoon is assessed. The Asian monsoon is better reproduced in the Coupled Modeling Intercomparison Project phase 5 (CMIP5) models than in the CMIP3 models, although biases remain. Projected future changes in the Asian monsoon at the end of the 21st century are then reviewed. Overall projections are similar for both CMIP3 and CMIP5 models with increases in precipitation, albeit with weakened circulation in the South Asian summer, enhanced circulation and increased precipitation in the East Asian summer, and latitude-dependent changes in the winter monsoon circulation in East Asia. However, differences exist in the projected local changes, leading to uncertainty in projections.

著者

Kunihiko KODERA Nawo EGUCHI Jae N. LEE Yuhji KURODA Seiji YUKIMOTO

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.89, no.3, pp.283-290, 2011-06-25 (Released:2011-06-30)

参考文献数

24

被引用文献数

7 18

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In mid-January 2009, sudden changes in circulation occurred in the tropical troposphere and stratosphere. Convective activity situated over the equatorial Maritime Continent showed an abrupt weakening, whereas that over the South American to African sectors became stronger. Changes also occurred in the latitudinal structure; convective activity in the Northern Hemisphere became weaker, whereas that in the Southern Hemisphere became stronger. The change in convective activity took place in association with a change in tropical circulation, from east–west to north–south type (i.e., from Walker- to Hadley-type circulation). Almost simultaneously with these events in the troposphere, a change in meridional circulation occurred in the stratosphere during a record-breaking stratospheric sudden warming event in January 2009. Stratospheric tropical temperature showed a decrease in response to a strengthening of the hemispherical meridional circulation. In the present study, we show how the stratospheric and tropospheric circulation changes are dynamically coupled.

著者

Hiroaki KAWASE Hidetaka SASAKI Akihiko MURATA Masaya NOSAKA Noriko N. ISHIZAKI

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.93, no.5, pp.571-580, 2015 (Released:2015-11-17)

参考文献数

33

被引用文献数

3 19

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We investigate future changes in winter precipitation around Japan and their uncertainties using the downscalings of a non-hydrostatic regional climate model (NHRCM) with 20-km grid spacing according to global climate projections. The global climate projections were conducted by the atmospheric general circulation model with three patterns of sea surface temperature changes in the Coupled Model Intercomparison Project Phase 5 under the Representative Concentration Pathway 8.5. Moreover, three cumulus convective parameterizations were applied in the present and future climate experiments. The ensemble mean of nine future NHRCM experiments shows decreases in the winter precipitation on the coast of the Sea of Japan and over the Pacific Ocean in the south of the Japanese archipelago. The former decrease in precipitation results from a weakened winter monsoon. The latter corresponds to changes in extratropical cyclone number around Japan, which have a large uncertainty. On the other hand, winter precipitation increases over the northernmost part of Japan (Hokkaido) and the northeastern Asian continent. The strengthened northwesterly around Hokkaido, which results from the reduction of sea ice in the Sea of Okhotsk, causes increased precipitation in the inland area of Hokkaido. In addition, moistening due to global warming relates to increased precipitation in extremely cold regions. These signals are common to most experiments.

著者

Yuhji KURODA

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.85, no.6, pp.889-898, 2007 (Released:2008-03-20)

参考文献数

22

被引用文献数

8 15

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The effect of the Quasi-Biennial Oscillation (QBO) and the El Niño Southern Oscillation (ENSO) on the 11-year solar cycle modulation of the winter-mean North Atlantic Oscillation (NAO) is examined through analysis of observational data from 1958 to 2000. It is found that the solar cycle modulation of the NAO is more strongly enhanced in the westerly phase of the 50-hPa QBO wind and the cold phase of ENSO, although separation of these effects is statistically difficult. On these phases, the signal of the winter-mean NAO extends more to the upper stratosphere and summer-AO reappears more strongly in high solar years, whereas the signal is weaker throughout in low solar years.

著者

Hiroyuki IWASAKI

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.94, no.5, pp.431-442, 2016 (Released:2016-10-31)

参考文献数

25

被引用文献数

7

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Lightning features over the Tibetan Plateau were studied in relation to topography using the World Wide Lightning Location Network (WWLLN) dataset obtained from April 2009 until December 2014. To describe the strength of lightning strokes, lightning strokes with energies above the 90th percentile (7666 J) were defined as strong lightning (S-lightning) strokes, and the ratio of S-lightning strokes to the overall number of lightning (O-lightning) strokes was defined as the strong ratio (S-ratio). O-lightning density over the Tibetan Plateau was found to be high in general, except over the western part of analysis region. Minimum-density zones were observed along the Himalayas approximately 6 km above sea level and in deep valleys within the Tibetan Plateau. The maximum- and minimum-density zones also exhibited maximum and minimum annual rainfall amounts, respectively. S-lightning strokes were also found to frequently occur over the Tibetan Plateau, and most S-ratios in the analysis units exceeded 30 %, which corresponds to three times the global mean. In particular, the S-ratios over the southern part of the Tibetan Plateau, including the Himalayas, were found to be high (50 %) and to correspond with the zone that had the minimum O-lightning density. The maximum O-lightning density was observed to occur during the summer at an elevation approximately 0.2-1.0 km higher than the plateau level. The O-lightning and S-lightning densities around the Nagqu sonde station were negatively correlated with the Showalter stability index (SSI) and the vertical wind shear. The S-ratio and average stroke energy were found to be negatively correlated with the vertical shear but not with the SSI.

著者

Sushma NAIR Govindrajan SRINIVASAN Ramkrishna NEMANI

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.87, no.6, pp.927-939, 2009 (Released:2010-02-04)

参考文献数

11

被引用文献数

53 109 59

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The Tropical Rainfall Measuring Mission (TRMM) blended rainfall product (3B42-V6 data set) has been used for inter-comparison with observations from a high density rain-gauge network over the western state of Maharashtra, India. The state of Maharashtra was chosen for the inter-comparison exercise primarily because of its diverse rainfall regime. The rain distribution over Maharashtra, characterized by a pronounced longitudinal gradient is closely linked to the north-south oriented orography of the region known as the Western Ghats. The validation exercise was carried out for 7 monsoon seasons from June to September from 1998 to 2004 at various space and time scales. The results are consistent at daily, monthly and seasonal timescales. The comparisons among stations and over geographically similar climatic zones demonstrated that the performance of 3B42-V6 product varies over the different climatic regimes. In terms of spatial reproductivity, the existence of the west-east rainfall gradient along the west coast is captured by the satellite product, but the orographic effect (rainfall maxima is over the Western Ghats as captured by the rain-gauge) is not reflected by 3B42-V6 product. The 3B42V6 product shows rainfall maxima at the coast. The satellite estimates of rainfall amounts over the state were found to be most accurate over regions of moderate rainfall and mainly inaccurate in regions of sharp rainfall gradient. In terms of magnitude of the rainfall amounts, over the windward side of the Western Ghats the 3B42V6 product was unable to resolve the heavy orographic rainfall amounts and over the leeward side the rainfall amounts in the immediate rain-shadow region were overestimated. One of the key results obtained from the daily rainfall intercomparison exercise is the ability of the 3B42-V6 estimates to detect the wet and dry phases of monsoon over most parts of the state (except the leeward side). Though the rainfall amounts estimated by the satellite product were sometimes under/over estimated, the timing of the rain events as estimated by the satellite product was generally coincident with the gauge observations over most of the regions except in the immediate rain-shadow region of the state. The TRMM 3B42-V6 estimates therefore could have tremendous potential to be used for intraseasonal studies over most regions of the state.

著者

平野 烈介

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.2, no.4, pp.116-121, 1924-09-15 (Released:2009-02-05)

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From the durations of the preliminary tremors observed at various stations, the author, in his previous paper, located the seismic origin of the Great Kwanto Earthquake at a point to the north of Mt. Fuzi. In the present paper, he found the following two equations representing the relation between the distance of each station from the above point and the time of arrival of the first shock at that station in which Δ is the distance in kilometer and t the time in second measured from its origin assumed at 11h 58m, Sept. 1st, 1923. The first and second equations are well satisfied by the observed values of t and Δ at stations respectively in western and eastern Japan, the greatest discrepancy between the culculated and observed t being 2.7 seconds, while the total value of t ranges between 39-86 seconds. If the above equations are correct, there must be a time difference of 7 seconds for the commencements of the earthquake shocks at the seismic origin for the two waves, one travelling to the east and the other to the west. The author considers a highly strained state of earth crust, in a finite area near the origin, existing before the great earthquake and supposes that the waves actually started from the both sides of the area, whose diameter is about 50km, so that the apparent difference of time is accounted for.

著者

Masakazu TAGUCHI

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.93, no.1, pp.99-115, 2015 (Released:2015-03-18)

参考文献数

36

被引用文献数

2 10

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This study investigates observed interannual changes in the Northern winter stratosphere with El Niño/Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO) using the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data for 56 years. We focus on changes in occurrence of major stratospheric sudden warmings (MSSWs) as well as in seasonal mean states.  Our results reveal complex changes in the MSSW probability with both ENSO and QBO as in the seasonal mean states. However, statistically significant changes at the 90 % confidence level are obtained only for some combinations of ENSO and QBO conditions reflecting the limitation of the data period. When the QBO is in a westerly phase, the MSSW probability increases with the ENSO sea-surface temperature condition in the eastern equatorial Pacific, i.e., from ENSO cold (La Niña), through neutral, to warm (El Niño) years. When the QBO is in an easterly phase, on the other hand, the probability significantly increases for La Niña years than for neutral years, whereas the probability is not significantly different between neutral and El Niño years. A characteristic feature is the high MSSW probability for the La Niña and QBO easterly winters, which is consistent with strengthened stationary wave with zonal wavenumber 1 compared to the climatology.  These results suggest the importance of taking into account both ENSO and QBO factors, when one examines the frequency of MSSWs in the Northern winter stratosphere.

著者

Fumio MARUYAMA Kenji KAI Hiroshi MORIMOTO

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.93, no.3, pp.331-341, 2015 (Released:2015-07-10)

参考文献数

23

被引用文献数

1 4

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A climatic regime shift is characterized by an abrupt transition from one quasi-steady climatic state to another. We attempted to explain the change of multifractal behavior of climate indices when a regime shift occurred. We used the wavelet - transform method to analyze the multifractal behaviors of the El Niño/Southern Oscillation (ENSO) index (Niño3.4 index), Pacific Decadal Oscillation (PDO) index, North Pacific Index (NPI), Pacific/North American pattern (PNA) index, and West Pacific pattern (WP) index. We showed the change of multifractality of these climate indices. When the wavelet coherences between the Niño3.4 index and NPI, NPI and PDO index, and Niño3.4 and PDO indices became strong, changes from multifractal to monofractal behavior were observed at climatic regime shifts. It may be possible to explain the background of the change of fractality by regarding climate change as the consequence of mutual interactions of various climatic elements. A fluctuation increase is observed in a coupled chaotic system just before chaos synchronization, which is when fractality and states change. We expect that a similar mechanism possibly exists for a climatic regime shift. When fluctuations became large and multifractality became strong, a climatic regime shift occurred and a change from multifractal to monofractal behavior was observed. The strong interaction of climatic phenomena, such as the ENSO, PDO, and Aleutian Low, caused a climatic regime shift. The fractality change of the PDO index almost corresponded to the regime shifts. In terms of multifractal analysis, we conclude that a climatic regime shift corresponds to a change from multifractality to monofractality of the PDO index.

著者

B. G. Hunt

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.60, no.1, pp.309-318, 1982 (Released:2007-10-19)

参考文献数

7

被引用文献数

20

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年平均状態を求める半球大循環モデルを用いて,黄道傾斜が,23.5°から0°,65°に変化することによって生じた結果を考察した。この様な傾斜の値が,過去の地質時代にとられていたといわれている。傾斜0°の気候は,現在の気候より変化に富み,高緯度の地表面は,より寒冷で,乾燥している。しかし,モデル対流圏は,全体的にやや温暖化している。傾斜65°に対し実験を2例行なった。2例とも高緯度で低アルベードだが,一方は低緯度で氷河のアルベードを与えた。年平均状態に対し,最初の実験例では,対流圏の緯度方向の温度傾度が実質上無かった。第2の実験例では赤道で地表温度が最低となったが,氷河状態になる程の低温に到らなかった。この場合は「反転したハドレー細胞」によって,熱帯東風ジェットが維持された。シミュレートされた水文学,エネルギー交換等を合わせて考慮すれば,ここで得られた結果はこの様に極端な傾斜における気候状態を考える材料を与えてくれる。地球の居住可能帯は,傾斜0°,65°双方に対して,減少すると結論される。以上の実験例は,色々提唱される仮説的気候状態を評価する上で,大気大循環モデルは,かなりの未開拓な可能性を持っていることを示している。

著者

吉崎 正憲

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.57, no.6, pp.548-559, 1979 (Released:2007-10-19)

参考文献数

13

被引用文献数

3

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振幅方程式を用いて,カーブした温度分布を持ち,しかもシアのある流れのある流体中に実現する対流セルの形状を調べた.温度分布の一次関数からのずれとシア流の強さ,および系のレーリー数の臨界値からのずれをパラメータとして,定常解を求めてその安定性を調べた.系のレーリー数のカーブした温度分布に対する臨界値からのずれを(ΔR)*とすると,次の事が明らかになった.(1)シアが弱い場合,2次元ロールは(ΔR)*が限られた領域でのみ不安定で存在しない.しかし,シアがある値以上強くなると,ロール解は(ΔR)*が正の領域で常に安定に存在する.(2)シアがない時に六角形セルに帰着する解は,シアが強まるにつれて,シアの方向に伸びたセルに変形してゆく.そしてシアがある値以上になると,この解は安定に存在しなくなる.

著者

折笠 成宏 村上 正隆

出版者

(公社)日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.75, no.6, pp.1033-1039, 1997-12-25 (Released:2009-09-15)

参考文献数

8

被引用文献数

4 22

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低濃度の氷晶からなる巻雲を測定するために新型の雲粒子ゾンデを開発した。このゾンデは倍率の異なる2台の小型ビデオカメラを搭載しており、粒径7μm-5mmの粒子の映像を1.6GHzのマイクロ波を通して地上に伝送する。従来の雲粒子ゾンデでは、サンプリング体積が小さいために、雲粒子の数濃度が低い雲の粒径分布が正確に評価できなかった。この点を克服するために、吸引用の小型ファンを付加することによって十分なサンプリング体積を確保した。また、周囲の気圧変化やゾンデの上昇速度によるサンプリング体積の変化を室内実験によって評価した。捕捉率の理論的計算から、10μm以上の粒子が全て捕捉されると考えられる。強制吸引式雲粒子ゾンデによる巻雲の観測例から、10μm以上の氷晶の粒径分布を250m間隔で精度良く決定できることがわかった。この雲粒子ゾンデの観測結果は、巻雲の生成や維持の機構を理解するのに有益な情報を与える。

著者

Esperanza O. CAYANAN Tsing-Chang CHEN Josefina C. ARGETE Ming-Cheng YEN Prisco D. NILO

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.89A, pp.123-139, 2011 (Released:2011-05-05)

参考文献数

14

被引用文献数

37 42

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Intense southwest monsoon (SWM) rainfall events causing massive landslides and flash floods along the western sections of the Philippines were studied. These rainfall events, are not directly coming from the tropical cyclones (TCs) for they are situated far north to northeast of Luzon Island. The heavy rainfall is hypothesized as caused by the interaction of strong westerlies with the mountain ranges along the west coast of Luzon that produces strong vertical motion and consequently generates heavy rainfall. Four of heavy SWM rainfall cases were examined to determine how the presence and position of tropical cyclones in the Philippine vicinity affect these SWM rainfall events; three cases with TC of varying positions within the Philippine area of responsibility (PAR) and the fourth case without TC. Using a spatial Fourier decomposition approach, the total streamfunction is decomposed into two flow regimes: monsoon basic flow (Waves 0-1) and tropical cyclone perturbation flow (Waves 2-23) over a domain of (20°E-140°W, 5°S-35°N). The purpose of this flow decomposition is to determine the latter’s effect on or contribution to the monsoon activity. The analysis utilized the NCEP Final (FNL) data with 1° long. × 1° lat. resolution. Results show that the tropical cyclones over the Pacific Ocean located northeast of Luzon generate strong southwesterly winds over the west coast of Luzon. These in addition to the southwesterlies from the basic flow strengthened the southwest winds that interact with the high Cordillera Mountain ranges

著者

Le DUC Yohei SAWADA

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.102, no.1, pp.35-47, 2024 (Released:2023-12-12)

参考文献数

31

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It is well-known in rainfall ensemble forecasts that ensemble means suffer substantially from the diffusion effect resulting from the averaging operator. Therefore, ensemble means are rarely used in practice. The use of the arithmetic average to compute ensemble means is equivalent to the definition of ensemble means as centers of mass or barycenters of all ensemble members where each ensemble member is considered as a point in a high-dimensional Euclidean space. This study uses the limitation of ensemble means as evidence to support the viewpoint that the geometry of rainfall distributions is not the familiar Euclidean space, but a different space. The rigorously mathematical theory underlying this space has already been developed in the theory of optimal transport (OT) with various applications in data science.In the theory of OT, all distributions are required to have the same total mass. This requirement is rarely satisfied in rainfall ensemble forecasts. We, therefore, develop the geometry of rainfall distributions from an extension of OT called unbalanced OT. This geometry is associated with the Gaussian-Hellinger (GH) distance, defined as the optimal cost to push a source distribution to a destination distribution with penalties on the mass discrepancy between mass transportation and original mass distributions. Applications of the new geometry of rainfall distributions in practice are enabled by the fast and scalable Sinkhorn-Knopp algorithms, in which GH distances or GH barycenters can be approximated in real-time. In the new geometry, ensemble means are identified with GH barycenters, and the diffusion effect, as in the case of arithmetic means, is avoided. New ensemble means being placed side-by-side with deterministic forecasts provide useful information for forecasters in decision-making.

著者

KOSAKA Yuki KOBAYASHI Shinya HARADA Yayoi KOBAYASHI Chiaki NAOE Hiroaki YOSHIMOTO Koichi HARADA Masashi GOTO Naochika CHIBA Jotaro MIYAOKA Kengo SEKIGUCHI Ryohei DEUSHI Makoto KAMAHORI Hirotaka NAKAEGAWA Tosiyuki TANAKA Taichu Y. TOKUHIRO Takayuki SATO Yoshiaki MATSUSHITA Yasuhiro ONOGI Kazutoshi

出版者

公益社団法人 日本気象学会

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

pp.2024-004, (Released:2023-11-02)

被引用文献数

2

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The Japan Meteorological Agency (JMA) has developed the third Japanese global atmospheric reanalysis, the Japanese Reanalysis for Three Quarters of a Century (JRA-3Q). The objective of JRA-3Q is to improve quality in terms of issues identified in the previous Japanese 55-year Reanalysis (JRA-55) and to extend the reanalysis period further into the past. JRA-3Q is based on the TL479 version of the JMA global Numerical Weather Prediction (NWP) system as of December 2018 and uses results of developments in the operational NWP system, boundary conditions, and forcing fields achieved at JMA since JRA-55. It covers the period from September 1947, when Typhoon Kathleen brought severe flood damage to Japan, and uses rescued historical observations to extend its analyses backwards in time about 10 years earlier than JRA-55. This paper describes the data assimilation system, forecast model, observations, boundary conditions, and forcing fields used to produce JRA-3Q as well as the basic characteristics of the JRA-3Q product. The initial quality evaluation revealed major improvements from JRA-55 in the global energy budget and representation of tropical cyclones (TCs). One of the major problems in JRA-55—global energy imbalance with excess upward net energy flux at the top of the atmosphere and at the surface—has been significantly reduced in JRA-3Q. Another problem—a trend of artificial weakening of TCs—has been resolved through the use of a method that generates TC bogus based on the JMA operational system. There remain several problems such that volcanic-induced stratospheric warming is smaller than expected. This paper discusses the causes of such problems and possible solutions in future reanalyses.

著者

新野 宏

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.60, no.5, pp.1001-1023, 1982 (Released:2007-10-19)

参考文献数

39

被引用文献数

12 11

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密度一様の回転流体中の非発散の水平シア流の安定性を弱非線形理論によって調べた。考えたシア流はyを流れる横切る方向の座標とするとき,U(y)=tank y と U(y)=sech2y の2通りである。コリオリ係数の緯度変化の効果(ベータ効果)及び底面でのエクスマン摩擦の効果を考慮してある。不安定波の振幅の時間及び空間変調を記述する一般化されたランダウ方程式の係数を様々なβ(≅dy/dy)の値に対して求めた。この論文で考えた範囲のβの値に対しては,どちらのシア流の不安定も超臨界型であることがわかった。従って,臨界値より大きなレイノルズ数に対しては,有限振幅の不安定波が変形された基本流との間に平衡状態を形成することが予測される。有限振幅の定常波の side-band 波に対する安定性も調べた。超臨界のレイノルズ数に対して,波数κ0の有限振幅波は,もしκ0がS(κ1-κc)<κ0-κc<S(κ2-κc)をみたすならば安定である。ここで,kcは臨界波数,k1とk2(k<k2)はそのレイノルズ数に対する2つの中立波の波数であり,Sは定数である。U(y)=tanh y に対しては,β=0のときSは1/√3に等しい。Sはβの絶対値が増すと共に減少する。従って,有限振幅波が安定な波数領域はβの絶対値が大きくなるにつれて減少する。U(y)=sech2 yに対しては,βの絶対値が小さいときには(無次元のβでβ=-1.9~0.5)Sの値は0に等しく,有限振幅波は不安定である。βがこの範囲外のときには,Sはβの絶対値が増すと共に大きくなるが,1/√3より大きくなることはない。

著者

Karumuri ASHOK Zhaoyong GUAN Toshio YAMAGATA

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.81, no.1, pp.41-56, 2003 (Released:2003-03-18)

参考文献数

31

被引用文献数

187 209

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Using observed sea surface temperature data from 1871-1998, and observed wind data from 1958-1998, it is confirmed that the recently discovered Indian Ocean Dipole (IOD) is a physical entity. Many IOD events are shown to occur independently of the El Niño. By estimating the contribution from an appropriate El Niño index based on sea surface temperature anomaly in the eastern Pacific, it is shown that the major fraction of the IOD Mode Index is due to the regional processes within the Indian Ocean. Our circulation analysis shows that the Walker circulation during the pure IOD events over the Indian/ Pacific Ocean is distinctly different from that during the El Niño events. Our power spectrum analysis, and wavelet power spectrum analysis show that the periodicities of El Niño and IOD events are different. The results from the wavelet coherence analysis show that, during the periods when strong and frequent IOD events occurred, the Indian Ocean Dipole Mode Index is significantly coherent with the equatorial zonal winds in the central Indian Ocean, suggesting that these events are well coupled. During the periods when there seems to be some relationship between the equatorial zonal winds in the central Indian Ocean and ENSO index, no significant coherence is seen between the Indian Ocean Dipole Mode Index and the equatorial zonal winds in the central Indian Ocean, except after 1995, suggesting that most of the IOD events are not related to ENSO.

著者

竹村 和人 向川 均

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.101, no.1, pp.5-19, 2023 (Released:2023-02-07)

参考文献数

45

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本研究では、夏季アジアジェット出口付近でロスビー波の砕波を伴うシルクロードパターンが、太平洋・日本(PJ)パターンを引き起こす割合を調査した。ここで、シルクロードパターン事例は、ユーラシア大陸上での対流圏上層の南北風の主成分分析に基づき、黄海及び日本付近が高気圧性偏差となるパターンで特徴づけられる第1、2主成分を用いて抽出した。さらに、抽出した事例を、砕波を伴う事例と伴わない事例に分類した。 砕波を伴うシルクロードパターン事例では、アジアジェット出口付近の上層での高気圧性偏差は、砕波を伴わない事例と比べて東西により広がった形状を持ち、振幅も大きい。この事例の合成図では、シルクロードパターンに伴う波列パターンがユーラシア大陸上に存在し、アジアジェット出口付近で砕波を伴っていた。砕波の発生は、砕波域でのアジアジェットの強い減速及び分流と関連する。また砕波は、上層の高渦位気塊の進入を通して、砕波域の南側で活発な対流活動を促し、PJパターンを形成する。合成図において出現する明瞭なPJパターンは、南側で低気圧性偏差、北側で高気圧性偏差を持つ双極子構造を示す。そして、砕波を伴うシルクロードパターン事例の約60~70%が、PJパターンを伴っていた。 一方、砕波を伴わないシルクロードパターン事例の合成図では、ユーラシア大陸上で波列パターンは存在するが、砕波域の南側で活発化した対流活動及びPJパターンは存在しない。そして、砕波を伴わないシルクロードパターン事例の約40~50%がPJパターンを伴っていた。したがって、砕波によって正のPJパターンの出現頻度は1.2~1.7倍に増加し、砕波はPJパターンの励起に重要な役割を果たしていることが明らかになった。

著者

Brant Liebmann Harry H. Hendon John D. Glick

出版者

Meteorological Society of Japan

雑誌

気象集誌. 第2輯 (ISSN:00261165)

巻号頁・発行日

vol.72, no.3, pp.401-412, 1994-06-25 (Released:2009-09-15)

参考文献数

18

被引用文献数

372 397

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当論文においては、西太平洋及びインド洋における熱帯低気圧とMadden-Julian振動(MJO)との関連を記述する。熱帯低気圧は振動の積雲対流活動活発期に生じ易いし、雲塊は下層の低気圧性渦度の周辺に存在し、発散場はMJOに伴う積雲対流活動の西方極側に現れる。熱帯低気圧や台風の絶対数は振動の積雲対流活動活発期に増大するが、弱い熱帯低気圧から転化する熱帯低気圧と台風の比率は、積雲対流活動活発期と乾燥期において同一である。積雲対流活動活発期においてより多くの熱帯低気圧や台風が存在するのは、当時期により多くの弱い熱帯低気圧が存在することによる。当研究の第三の結果は、積雲対流活動活発期の熱帯低気圧の活動度がMJOの活動度に限定されていない点である。事実、我々はMJOと独立かつ無作為に選ばれた積雲対流活動活発期において熱帯低気圧の活動度が同等に増大することを見いだした。結論として、MJOは熱帯低気圧に影響を及ぼす独自の機構を持つと言うより、むしろそれに伴う熱帯の変動度が大きな割合を占めるという点で重要である。