A Model for Ocean Vertical Mixing Including Convection

CANUTO/LDEO提案编号：OCE-0241668该提案的主要目标是建立和测试一个统一的、非本地模式来描述海洋中的非对流和对流垂直混合过程。该模型旨在用于粗分辨率海洋模型。这项工作的第一部分已经开展(Canuto等人，2001、2002，c1-2)。建立了一个基于雷诺应力模型的湍流模型，该模型包括速度场和温度、盐度两个活跃标量场。相应地，该模型提供了动量、热量和盐度的涡旋扩散系数。该模型还提供了被动标量的涡流扩散系数。所有的扩散系数都被解析地表示为OGCM可计算的大尺度场。根据北大西洋示踪剂释放实验(NATRE)的数据，特别是热、盐和浓度扩散系数随深度的变化，对该模型进行了直接检验。数据的重现性相当令人满意，包括盐与热扩散系数的比率。最近在巴巴多斯测量的数据(由世界卫生组织的R.W.施密特在2002年2月夏威夷海洋科学会议上报告)显示，巴巴多斯的混合程度比NatRE大得多，这与该模型的预测也是一致的。当在粗分辨率OGCM中使用时，该模式比以前的模式更接近莱维图斯数据(例如，北冰洋T-z剖面的更好的一致性)。然而，该模式是局地的，不能解释关键的动力学特征，如卷吸过程，对流翻转或深对流的前兆DC。后一过程是海洋的主要特征之一，因为它代表了世界海洋全球范围通风循环的初始阶段，也是北大西洋深水(NADW)产生的主要机制。在过去的十年里，许多研究强调了DC的主要特征，其中包括：空间局部性(例如，类似羽状物的行为)、非定域性(注入羽状物中的示踪剂不会混合到环境流体中，直到羽状物“褶皱缠绕”；因此，属性直接从表面传输到底部，这是一个非局部过程)；对旋转的依赖。与这些丰富的数据形成对比的是，DC丰富现象在OGCM中的表现仍然相当初级。最常用的是1969年提出的对流平差。然而，最近的研究突显了许多不足之处，其中最突出的可能是对流程度过高，最终导致对冰数据的描述不佳(冰太少)。一个更有前景的模型是根据G.I.泰勒最初设计的羽流模型的海洋改编而成的。该模型产生了更多可接受的结果。因此，目前垂直混合过程只能用一种对流过程模式和另一种非对流(日)混合模式来处理。在这里，我们建议对这两个过程进行统一处理。如上所述，我们已经研究了非对流混合部分，现在我们计划将以前的局部模式扩展为非局部模式，以涵盖对流过程。我们不仅使用这样的模型来研究其他类型的对流，而且最近我们还表明，非局部湍流模型重现了羽流模型，该模型假定羽流占据对流斑块的一小部分，这一假设仅在初始阶段适用。我们的混合模型将首先根据LES数据进行测试，然后在HYCOM(等轴坐标下的OGCM)中实现。如果我们的项目成功，不仅对流和非对流过程将成为同一混合模式的一部分，而且我们将避免刚才讨论的羽流模式的局限性。

ABSTRACTPI/Institution: Canuto / LDEO Proposal No: OCE-0241668The main goal of this proposal is to build and test a unified, non-local model to describe non-convective and convective vertical mixing processes in the ocean. The model is intended for use in coarse resolution ocean models. The first part of this work has already been carried out (Canuto et al., 2001, 2002, C1-2). A turbulence model based on the Reynolds Stress Model was constructed which included the velocity field and two active scalars fields, temperature and salinity. Correspondingly, the model provided the eddy diffusivities of momentum, heat and salinity. The model also provided the eddy diffusivities for a passive scalar. All the diffusivities were expressed analytically in terms of the large-scale fields calculable form the OGCMs. The model was tested directly against the data from NATRE (North Atlantic Tracer Release Experiment) specifically, the heat, salt and concentration diffusivities vs. depth. The data were reproduced quite satisfactory including the ratio of salt to heat diffusivity. Recent but yet unpublished data from measurements at Barbados (reported by R.W. Schmitt of WHOI at the Hawaii Ocean Sciences Meeting, Feb. 2002) of a much larger mixing at Barbados than at NATRE, are also in agreement with the predictions of the model. When used in a coarse resolution OGCM, the model reproduced data closer to Levitus data than previous models (e.g., a better agreement of the T vs.z profile in the Arctic Ocean). The model is however local and cannot account for key dynamical feature such as the entrainment process, a precursor of convective overturning or Deep Convection, DC. The latter process is one of the ocean's major features since it represents the initial stage of the global-scale ventilation loops of the world ocean and is also the predominant mechanism involved in the production of NADW (North Atlantic Deep Water). Many studies in the last ten years have highlighted major features of DC among which: spatial localization (e.g., plumes-like behavior), non-locality (a tracer injected in a plume will not get mixed into the ambient fluid until the plume "creases to entrain"; a property is thus transported directly from the surface to the bottom, a non-local process); dependence on rotation. In contrast to this wealth of data, the representation in OGCMs of the rich phenomenon of DC is still rather rudimentary. The most commonly used is the Convective Adjustment proposed in 1969. Many shortcomings have, however, been highlighted by recent studies the most prominent being perhaps the excessive level of convection that ultimately yields a poor representation of the ice data (too little ice). A more promising model is based on an oceanic adaptation of a Plume Model originally devised by G.I. Taylor. The model has produced considerably more acceptable results. Thus, at present, vertical mixing processes can only be dealt with by using one model for convective process and another model for non-convective (diapycnal) mixing. Here, we propose a unified treatment of both processes. As stated above, the non-convective mixing part has already been studied and we now plan to extent the previous local model to become non-local so as to encompass the convective processes. Not only have we used such a model to study other types of convection but we have recently shown that the non-local turbulence model reproduces the Plume Model which assumes that Plumes occupy a small fraction of the convective patch which holds true only in the initial stages. Our mixing model will be first tested against LES data and then implemented in HYCOM (an OGCM in isopycnal coordinates). If our project is successful, not only will the convective and non-convective processes be part of the same mixing model but we shall avoid the limitation of the Plume Model just discussed.