Improving ocean tides by constraining the dynamic HAMTIDE model with altimetry an GRACE data

通过同化测高和GRACE数据获得的潮汐信息来改进全球海洋潮汐模型的拟议努力是SPP 1257“Massentransporte and Massenveilungen im system Erde”的核心问题，也是改进GRACE和GOCE重力场模拟(以避免混叠)所必需的。同时，这一努力将提高我们对潮汐在海洋一般环流中的作用的认识。全球海洋潮汐模型描述了海洋质量的短期变化，量化了陆地站点的负荷效应，解释了在地球自转过程中观察到的部分变化，并且对于改正测高海面高度以及GRACE和GOCE重力场模型(去混叠)是必需的。同样重要的是，潮汐摩擦，主要是在浅海，有助于混合海洋，并对角动量产生影响，从而影响一天的长度。此外，正压潮流与地形相互作用产生的内波阻力导致深海内潮的产生。机械能到内潮的这种转换对于深海混合以维持一般海洋循环(接近稳定的过程)是重要的。潮汐的另一个重要特征是它们影响固体地球的变形(加载和自吸引效应)，并影响重力场的变化，从而影响卫星轨道的变化。因此，提高我们对海洋潮汐的理解，扩大我们对潮流、潮汐能量转移或耗散的认识，对于物理海洋学和大地测量界来说都是一个O(1)问题。除了我们可以从经验潮汐模型中了解到的以外，通过用观测到的潮汐信息来约束动力潮汐模型，可以获得对海洋潮汐的改进估计。在该项目中使用的数据同化模式HAMTIDE已被证明是有价值的，能够在DAROTA的头两个资助期内模拟海洋潮汐。通过一种有效的方法来获得高质量的动力解，该方法使模型受到表面潮汐信息(由测高确定的水位以及推断潮流的数据)的约束。结果可以用来探测表层到内部潮汐转换的速率和发生地点，寻找潮汐引起的垂直混合的区域，从而对气候产生影响，即从卫星信号到气候变化的重要联系。在下一阶段，重力测量(GRACE、GOCE)和测高(T/P、ICESat、CRYOSAT2)数据将同时用作同化过程中的约束。这将改善潮汐估算，不仅在可以进行测高的中低纬度地区，而且在高纬度地区，那里需要更好的潮汐模型来改进重力场反演。这种资料同化的第一个结果(分辨率低，重力资料少)是非常有希望的。结果将用于更好地理解潮汐消散(另见标题页上的插图和所附文件草稿田口、斯塔默和扎赫勒，2010年)。作为拟议工作的一部分，结果将同样与所有最先进的全球潮汐模型进行相互比较。

The proposed effort of improving global ocean tide models through the assimilation of tidal information available from altimetry and GRACE data is of central concern for the SPP 1257 “Massentransporte und Massenverteilungen im System Erde” and is necessary to improve GRACE and GOCE gravity field modeling (to avoid aliasing). At the same time the effort will improve our knowledge about the role of tide in the general circulation of the ocean. Global ocean tide models describe short term ocean mass variations, quantify loading effects for stations on land, explain parts of the variations observed in Earth rotation, and are required for correcting altimetric sea surface heights and for GRACE and GOCE gravity field modeling (dealiasing). Equally important, tidal friction, mainly in the shallow seas, contribute to mixing the ocean, and exerts influence on the angular momentum and hence on the length of the day. Moreover, the internal wave drag caused by interaction of barotropic tidal currents with topography induces generation of the internal tides in the deep ocean. Such conversion of the mechanical energy into the internal tides is important for the abyssal ocean mixing to maintain the general ocean circulation (nearly steady-state process). Other important features of the tides are their influence on the solid Earth deformation (loading and self-attraction effect) and on the variation of the gravitational fields, consequently on the variation of the satellite orbits. Improving our understanding of ocean tides and expanding our insight into tidal currents, tidal energy transfer or dissipation is therefore an O(1)-problem to physical oceanography and the geodetic community alike. Improved estimates of ocean tides, beyond what we can learn from empirical tide models, can be obtained by constraining dynamical tide models by observed tidal information. The dataassimilative model HAMTIDE, which is used in this project, has already proven valuable and capable of modelling ocean tides during the first two funding periods of DAROTA. The high quality of the dynamical solution is obtained through an effective method by which the model is being constrained by surface tide information (water elevation defined by the altimetry as well as data inferring tidal currents). Results can be used for detecting rates and generation sites of the surface-to-internal tide conversion, finding regions of the tidally induced vertical mixing, and consequently influence on the climate, i.e., an important link from satellite signals to climate variations. In the next phase, the gravimetry (GRACE, GOCE) and altimetry (T/P, ICESat, CRYOSAT2) data will be used simultaneously as constraints in the assimilation process. This will improve tidal estimates not only in low and mid-latitudes where altimetry is available, but also in high latitudes where better tide models are needed to improve gravity field retrievals. The first result of this kind of data assimilation (with coarse resolution and few gravimetry data) is highly promising. Results will be used for an improved understanding of tidal dissipation (see also the figure on the title page and the attached draft paper Taguchi, Stammer and Zahel, 2010). Results will likewise be intercompared with all state of the art global tide models as part of the proposed effort.