AVAILABLE POTENTIAL-ENERGY FOR MODE EDDIES
AVAILABLE POTENTIAL-ENERGY FOR MODE EDDIES
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DOI：
10.1175/1520-0485(1981)011
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发表时间：
1981-01-01
影响因子：
3.5
通讯作者：
FOFONOFF, NP
中科院分区：
文献类型：
--
作者：
BRAY, NA;FOFONOFF, NP
可用势能（APE）被定义为重力场中流体的总势能加上内能与相应参考场之间的差，在所述参考场中流体被重新分配（水平化）以沿着重力面具有恒定的稳定分层密度。势能变化是由流体质量相对于位势表面的局部变化引起的，这些局部变化伴随着焓和内能的局部变化以及不改变焓或内能的质量的全局变化（因为流体元素的体积不守恒）。通过计算单位质量的有效重力势能（GPE）和单位面积的总重力势能（TGPE），分别考察了势能的变化;通过引入参考密度场（或等效比容异常场），建立了一种估算海洋重力势能的方法。计算方法采用全经验海水状态方程。估算的准确性受到数据和采样的限制，而不受算法本身的限制，算法本身可以达到所需的精确度。为海洋区域局部定义的参考密度场允许重新定义相对于参考场的动态高度ΔD（每单位质量的势能）。单位面积的TGPE简单地成为所考虑区域内动态高度在深度上积分的水平平均值。参考密度面提供了一个精确的近似物质表面跟踪保守变量，如盐度和位温，并估计表面之间的涡拉伸。该程序适用于1973年收集的MODE密度数据。对于五个2周时间窗内的每组台站（指定为A-E组），将估计的GPE与基于Boussinesq近似的净APE以及从系泊浮标测量的低频动能进行比较。从一个时间窗到下一个时间窗的参考场的位能变化与每个时间窗内的GPE相比是大的，这表明存在比台站网格更大的尺度。
Available, potential energy (APE) is defined as the difference between total potential plus internal energy of a fluid in a gravity field and a corresponding reference field in which the fluid is redistributed (leveled) adiabatically to have constant stably-stratified densities along geopotential surfaces. Potential energy changes result from local shifts of fluid mass relative to geopotential surfaces that are accompanied by local changes of enthalpy and internal energy and global shifts of mass (because volumes of fluid elements are not conserved) that do not change enthalpy or internal energy. The potential energy changes are examined separately by computing available gravitational potential energy (GPE) per unit mass and total GPE (TGPE) per unit area.A technique for estimating GPF, in the ocean is developed by introducing a reference density field (or an equivalent specific volume anomaly field) that is a function of pressure only and is connected to the observed field by adiabatic vertical displacements. The full empirical equation of state for seawater is used in the computational algorithm. The accuracy of the estimate is limited by the data and sampling and not by the algorithm itself, which can be made as precise as desired.The reference density field defined locally for an ocean region allows redefinition of dynamic height ΔD (potential energy per unit mass) relative to the reference field. TGPE per unit area becomes simply the horizontal average of dynamic height integrated over depth in the region considered. The reference density surfaces provide a precise approximation to material surfaces for tracing conservative variables such as salinity and potential temperature and for estimating vortex stretching between surfaces.The procedure is applied to the MODE density data collected in 1973. For each group of stations within five 2-week time windows (designated Groups A–E) the estimated GPE is compared with the net APE based on the Boussinesq approximation and to the low-frequency kinetic energy measured from moored buoys. Changes of potential energy of the reference field from one time window to the next are large compared with the GPE within each window, indicating the presence of scales larger than the station grid.An analysis of errors has been made to show the sensitivity of the estimates to data accuracy and sampling frequency.