## Gravity wave

 prefLabel Gravity Wave definition A wave disturbance in which buoyancy acts as the restoring force on parcels displaced from hydrostatic equilibrium. inScheme http://gcmdservices.gsfc.nasa.gov/kms/concept/sciencekeywords/gcmd_c47f6052-634e-40ef-a5ac-13f69f6f4c2a broader Abstract from DBPedia In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere and the ocean, which gives rise to wind waves. A gravity wave results when fluid is displaced from a position of equilibrium. The restoration of the fluid to equilibrium will produce a movement of the fluid back and forth, called a wave orbit. Gravity waves on an air–sea interface of the ocean are called surface gravity waves or surface waves, while gravity waves that are within the body of the water (such as between parts of different densities) are called internal waves. Wind-generated waves on the water surface are examples of gravity waves, as are tsunamis and ocean tides. Wind-generated gravity waves on the free surface of the Earth's ponds, lakes, seas and oceans have a period of between 0.3 and 30 seconds (frequency between 3.3 Hz and 33 mHz). Shorter waves are also affected by surface tension and are called gravity–capillary waves and (if hardly influenced by gravity) capillary waves. Alternatively, so-called infragravity waves, which are due to subharmonic nonlinear wave interaction with the wind waves, have periods longer than the accompanying wind-generated waves.流体力学における重力波（じゅうりょくは、英:gravity wave）とは、重力の作用によって力学的平衡状態にある媒質が、異なる密度の媒質中に変位したとき、重力を復元力として再び元の平衡状態に戻ろうとする過程で、媒質の界面で発生して界面に沿って進む波動のこと。よく見られる現象として、水面に生じる波浪が上げられる。これは界面である明瞭な表面を進む波となるため表面波とも呼ばれる。これらの波動は撹乱となって成長することがあり、これはケルビン・ヘルムホルツ不安定性として知られる。なお、一般相対性理論によって記述される、時空の変位が光速で伝播する現象については重力波（相対論）を参照。両者は英語ではそれぞれgravity wave, gravitational waveと区別されているが、日本語では同じ重力波と呼ばれる。 このほか、海洋で湖沼で生じる水温躍層中に急激に温度差が生じると、浮力を復元力として波動が発生する。また大気中においても同様に波動が発生し、大気重力波と呼ばれる。波状雲はこのような機構で生じる身近で観察できる例である。いずれも同じ媒体の内部で伝播する波という意味でこちらは内部波とも呼ばれる。 重力波の伝播速度 c は、生じた波の波長λと、媒質の（重力に対して）鉛直方向の高さ（深さ）h を用いて次式で決まる： ここで g は重力加速度である。上式は波の伝播速度c が波長λに依存する分散性を示す。 媒質の深さh に比べて波長λが短い場合は、 となる。逆に媒質の深さh に比べて波長λが長い場合は、伝播速度c は波長λに依存せず、次の式で与えられる： 海洋で生じる津波は、波長が数10 ㎞以上に及ぶ極めて長い重力波であり、深い海底を伝わるとき伝播速度は非常に速くなる。 なお、実際にはこれに加えて界面付近の物質の表面張力を復元力とする波動（表面張力波）も加わる。無重力下ではこちらの波動が優性となった表面波が現れる。 data publication(s) found by GCMD Science Keywords) Japanese 25-year Reanalysis ProjectJapanese 25-year Reanalysis Project, Monthly MeansERA-Interim ProjectERA-Interim Project, Monthly MeansERA-Interim Project, Single Parameter 6-Hourly Surface Analysis and Surface Forecast Time SeriesJRA-55: Japanese 55-year Reanalysis, Daily 3-Hourly and 6-Hourly DataJRA-55: Japanese 55-year Reanalysis, Monthly Means and VariancesJRA-55C: The Japanese 55-year Reanalysis Using Conventional Data OnlyJRA-55C: Monthly Means and VariancesJRA-55AMIP: The Japanese 55-year Reanalysis AMIP-type SimulationJRA-55AMIP: Monthly Means and Variances Including Diurnal StatisticsWCRP and WWRP THORPEX YOTC (Year of Tropical Convection) ProjectWCRP and WWRP THORPEX YOTC (Year of Tropical Convection) Project, Single Parameter 6-Hourly Surface Analysis and Surface Forecast Time Series, Transformed to a Regular 1600 by 800 (N400) Gaussian GridWCRP and WWRP THORPEX YOTC (Year of Tropical Convection) Project, Single Parameter 3-Hourly Surface Forecast Time Series, Transformed to a Regular 1600 by 800 (N400) Gaussian GridERA5 ReanalysisERA5 Reanalysis (0.25 Degree Latitude-Longitude Grid)NCEP GDAS/FNL 0.25 Degree Global Tropospheric Analyses and Forecast GridsNCEP GFS 0.25 Degree Global Forecast Grids Historical ArchiveNCEP GDAS/FNL Global Surface Flux GridsNMC MRF Global Flux Fields, daily 1990Mar-1999FebNCEP/NCAR Global Reanalysis Products, 1948-continuingNCEP Climate Forecast System Reanalysis (CFSR) 6-hourly Products, January 1979 to December 2010NCEP Climate Forecast System Reanalysis (CFSR) Monthly Products, January 1979 to December 2010NCEP Climate Forecast System Version 2 (CFSv2) 6-hourly ProductsECMWF TOGA Global Supplementary Fields, 1985-2010ECMWF TOGA Global Extension Fields, 1991-2010ECMWF's Operational Model Analysis, starting in 2011ECMWF IFS CY41r2 High-Resolution Operational ForecastsERA-15 Supplementary Flux Accumulation FieldsERA-15 Model Resolution Supplementary Flux Accumulation FieldsERA-40 Global Surface and Flux DataERA-40 Surface and Flux Forecast FieldsERA-40 Monthly Means of Surface and Flux Forecast DataERA40 T85 6-hourly Surface Analysis and Surface Forecast Fields, created at NCARERA40 T85 Monthly Mean Surface Analysis and Surface Forecast Fields, created at NCARERA40 T106 6-hourly Surface Analysis and Surface Forecast Fields, created at NCARNOAA CIRES Twentieth Century Global Reanalysis Version 1 (1908-1958)NOAA CIRES Twentieth Century Global Reanalysis Version 2NOAA/CIRES Twentieth Century Global Reanalysis Version 2cHistorical Unidata Internet Data Distribution (IDD) Gridded Model Data