Collaborative Proposal: Joint US/Norwegian Studies of Ice/Ocean Interaction in Frozen Fjords

冰/海界面的热量和盐分交换在海冰生长和消融的年度循环中起着关键作用。最近观察到北极冰盖的范围和厚度发生了重大变化，人们把注意力集中在控制物质平衡的因素上，在数值模式中处理冰/海洋交换的方法正变得越来越复杂。对浮冰下湍流热通量的直接测量(用协方差技术)表明，它与摩擦速度(运动雷诺应力的平方根)和冰点以上混合层温度升高的乘积几乎成正比，但交换系数比相应的动量交换系数小一个数量级。这意味着，与动量流不同，界面上的热流受到与表面相邻的薄子层中分子过程的速率限制。在低温下，盐在海水中的分子扩散系数仅为热扩散系数的0.6%左右，因此存在双重扩散效应的可能性，其传递速率最终由盐扩散控制。根据对边缘冰区相对温暖的水域的测量，有证据表明，当冰正在融化时，这一点是成立的。Yaglom和Kader发展了一个模型，用于描述水力粗糙表面上的热和质量传输的实验室研究，该模型考虑了标量性质之间的扩散系数的差异，并已在几个冰/海洋数值模式中得到应用。这些模型没有区分融化和冻结(尽管YK的研究没有考虑固/液界面上的晶体生长)，因此，在冰生长过程中，热量通过界面扩散的速度快于盐的喷射速度。因此，这些模型预测了薄冰下海洋热通量的增强，并产生了大量的过冷水，通常认为过冷水会以某种方式分布在海洋边界层中，形成晶核。观察结果并不支持模拟的反应：几乎没有证据表明，在北极冰层不断增长的情况下，冰片的广泛生产，也没有来自薄冰下的有限数据表明更大的热流。这项拟议的工作旨在通过与挪威和美国科学家合作，对斯瓦尔巴特群岛潮汐活跃的峡湾中的冰/海洋相互作用进行详细测量，以了解冰/海洋的热量和质量交换，特别是在冰冻条件下。快速冰将作为一个实验室，进行仔细控制的实验，测量冰盖和海洋边界层中的交换。本文提出的工作主要涉及后者，包括测量冰下表面附近边界层中的动量、热量和盐度通量。一个具体的目标是改进数值模式的冰/海洋热量和质量传输的参数化。

Heat and salt exchanges at the ice/ocean interface play a key role in the annual cycle of sea-ice growth and ablation. Recent observations of significant change in the extent and thickness of the Arctic ice cover have focused attention on factors that control the mass balance, and treatment of ice/ocean exchanges in numerical models is becoming increasingly sophisticated. Direct measurements of turbulent heat flux under pack ice (by covariance techniques) have shown that it is nearly proportional to the product of friction velocity (square root of the kinematic Reynolds stress) and the elevation of mixed layer temperature above freezing, but that the exchange coefficient is an order of magnitude smaller than the corresponding exchange coefficient for momentum. This implies that unlike momentum flux, heat flux at the interface is rate-limited by molecular processes in thin sublayers adjacent to the surface. At low temperature, the molecular diffusivity of salt in seawater is only about 0.6% of that for heat, thus the possibility exists for double diffusive effects, where the transfer rate is ultimately controlled by salt diffusion. There is evidence that this holds when ice is melting, according to measurements in relatively warm water in the marginal ice zone. A model developed by Yaglom and Kader to describe laboratory studies of heat and mass transfer over hydraulically rough surfaces considers the difference in diffusivities between scalar properties, and has been applied in several numerical ice/ocean models. The models do not distinguish between melting and freezing (although the YK studies did not consider crystalline growth at the solid/liquid interface), consequently, heat is diffused through the interface faster than salt is ejected during ice growth. The models thus predict enhanced ocean heat flux under thin ice, and produce a significant amount of supercooled water, which is generally assumed to nucleate as frazil crystals distributed in some fashion throughout the ocean boundary layer. Observations do not support the modeled response: there is little evidence of extensive frazil production under growing ice in the Arctic, nor do the limited data from under thin ice indicate larger heat flux. The proposed work aims to understand ice/ocean heat and mass exchange, particularly during freezing conditions, by collaborating with Norwegian and US scientists in making detailed measurements of ice/ocean interaction in tidally active fjords in Svalbard. Fast ice will serve as a laboratory for carefully controlled experiments measuring exchanges both in the ice cover and in the ocean boundary layer. Work proposed here deals mainly with the latter, including measuring momentum, heat, and salinity flux in the boundary layer near the ice undersurface. A specific goal is to improve parameterization of ice/ocean heat and mass transfer for numerical models.