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风致水流深度和海岸线

在大海的速度目前发达海洋可以估计3%风速10米。

为什么海洋表面速度大约3%的风速?

有一个类似的估计当前的深度吗?

直觉上,表面电流不能继续朝着海岸线,没有足够的空间在底部电流补偿表面电流,这些电流会影响。还应该有一些水的混合层在海岸附近的碎波,这将导致取消表面电流。

风致表面电流指向海岸线,接近岸边如何等当前的继续,根据海底的陡度,风能和波浪条件?

回答

首先,背景,navier - stokes方程和稳态假设条件后,我们可以获得一个简化的平衡被称为埃克曼平衡边界层的上层海洋(订单几百米)。在海洋的这一部分水平梯度垂直梯度相比非常小。在这种情况下,科氏力之间有一个平衡,摩擦(有时压力梯度力需要也被认为是在平衡)。由此产生的理论流与减少垂直结构大小与深度和流动方向逐渐向右旋转在南半球(左)深度增加。这就是所谓的[埃克曼螺旋][1]。[![埃克曼螺旋][2]][2]来源:http://offshoreengineering.com在这些条件下,在开放海域(实际上假设无限深海)、埃克曼的运输层是垂直于风向。埃克曼层的深度,或者埃克曼深度,只是依赖于科里奥利和粘度:$ $ D_e = \ sqrt {2 K_m \ / f} $ $ K_m美元在哪里运动湍流粘度(假定常数)和f是科氏参数美元(中纬度地区的典型值是10美元^ {4}$ $ s ^ {1} $)。典型的埃克曼的深度是订单10到30米。典型值K_m约0.1美元美元$ $ m ^ 2美元/ s(不要混淆分子粘度大约是10美元^ {6}$ $ m ^ 2美元/ s),但可以通过几个数量级不同,导致一系列D_e美元10美元至150美元。 A good explanation of Ekman balance and much more information can be found in [Cushman-Roisin & Beckers][3] ([Chapter 8][4]). While all that is useful in a theoretical setting, the depths obtained from observations of the Ekman depth can be approximated by the expression: $$D_e = 0.4\sqrt{\tau\over f^2 \rho_0 }$$ where $\tau$ is the wind stress magnitude and $\rho_0$ is water density. The factor $0.4$ has been derived empirically and can varied slightly under different oceanic conditions. All that was describing the surface boundary layer, but in reality a bottom boundary layer also forms. The solution to the bottom layer can also be approximated as an Ekman layer for which the forcing is not the wind, but the bottom friction. The resulting boundary layer also has an Ekman depth approximated with the expression above. In an sloping bottom, the boundary layer follows the bottom slope and the thickness (depth above bottom) of the layer (measured perpendicular to the bottom) is scaled by the cosine of the bottom slope. Taking 10 meters (one of the smallest values in mid-latitudes) for the surface and bottom boundary layer thicknesses, that implies that in water depths shallower than 20 m the two boundary layers overlap. In water depths shallower than that, the transport in the surface layer is no longer perpendicular to the wind direction. The momentum input into the water by the wind (wind stress) is affected by the presence of the bottom and it is directly dissipated by bottom friction. The mixing by the wind (and wave breaking near the surf zone) results in additional mixing through the water column and facilitating well-mixed water columns. Under these conditions the wind-induced currents extend all the way to the bottom with the direction of the flow being a function of bottom depth, wind direction, and bottom slope. The presence of wind-induced currents does not preclude the occurrence of flow in the opposite direction of the wind. A fantastic article summarizing the different flow conditions under different wind and wave fields in shallow water depths is given in [Lentz and Fewings (2012).][5] [(Reprint)][6] Additional factors to consider are: - stratification by temperature and salinity that reduces mixing and helps maintain a separation between surface and bottom layers (some effects described [here][7]). - what happens when the wind is changing speed and direction (non-steady state)? - what happens at the Equator where the Coriolis parameter is zero? [1]: https://en.wikipedia.org/wiki/Ekman_layer [2]: https://i.stack.imgur.com/w5qGS.jpg [3]: http://ta.twi.tudelft.nl/dv/users/schuttel/gfd2.pdf [4]: http://thayer.dartmouth.edu/~d30345d/books/EFM/GFDchap8.pdf [5]: http://dx.doi.org/10.1146/annurev-marine-120709-142745 [6]: http://www.ccpo.odu.edu/~klinck/Reprints/PDF/lentzAnnRevMS12.pdf [7]: http://rachelhorwitz.weebly.com/inner-shelf.html

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\ begingroup美元根据Lentz和Fewings标准差的潮下的泥沙cross-shelf电流要小得多(0.01 - 0.05 m s−1),增加与离岸的距离接近零值附近的海岸。”So basically my assumption was correct - there is no cross-shelf current in the surf zone. \ endgroup美元
- - - - - -乔治Polevoy
2017年8月8日15:13
\ begingroup美元答案是否定的!他们正在谈论泥沙cross-shelf电流。根据定义的深度平均速度必须是零的海岸。问题是,你还会有电流附近的表面和底部集成“接近于零”靠近海岸。也有一个三维组件的问题海岸附近的共同点和不同点。一个很好的例子是激流。 \ endgroup美元
- - - - - -arkaia
2017年8月8日多

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