Collaborative Research: Radiatively Driven Convection in a deep freshwater lake

合作研究：淡水深湖中的辐射驱动对流

• 批准号: 1829924
• 负责人: Alberto Scotti
• 金额: $ 28.48万
• 依托单位: University of North Carolina at Chapel Hill
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-10-01 至 2023-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1829924&HistoricalAwards=false
• 关键词: Collaborative; Research; Radiatively; Driven; Convection

Due to the anomalous behavior of fresh water, which below 3.98C has a negative thermal expansion coefficient, convection occurs when the surface of a mid- to high latitude freshwater lake is heated in the spring. This implies that convection can be driven by shortwave radiation in the absence of wind on calm, sunny days. Observations over the last several years in Lake Superior, made as part of unrelated fieldwork, have provided an opportunity to start to explore the dynamics of this convective process from a number of different platforms, including moored and glider-based observations. Observations show parcels of water being warmed at the surface and transported to the bottom (O(200m)) on the order of hours, and suggest short (O(10m)) horizontal scales for convective chimneys, providing real challenges for observations and models alike. This project will combine observations, numerical modeling, and analytical work to determine and understand the spatial and temporal scales of the instability process. Lakes at mid-to high latitudes can spend a significant portion of the year in this state, during which these dynamics will dominate circulation and mixing. From a fundamental fluid dynamics perspective, this phenomenon provides an opportunity to study buoyancy-driven convection in the absence of other forcing mechanisms such as surface waves and wind-driven currents, in contrast to most significant examples of oceanic convection. This process may have significant impacts on primary productivity, since the strong convection, spanning the water column, will make it impossible for primary producers to maintain their position in the euphotic zone. In addition, parcels of water are being transported from the surface of the lake to the bottom on timescales of hours, and may have significant impacts to lake sediment biogeochemistry. A graduate student will be supported at each of the three institutions involved. An interactive one dimensional mixing model will be developed for classroom use, similar to tools already developed by one of the PIs. This work will foster collaboration between limnologists and oceanographers.The centerpiece of the field work will consist of a "horizontal mooring" equipped with fast thermistors as well as ADCPs capable of measuring vertical velocities, and an auxiliary mooring to measure surface heat fluxes. Glider transects and velocity microstructure surveys will be taken during both calm and windy conditions. Modeling will consist of an exploration of parameter space using direct numerical simulation and large eddy simulations. Analytical methods will be used to constrain the parameter space. Unlike well-studied Rayleigh-Benard convection, the process is driven by injection of Available Potential Energy directly into the bulk of the fluid. The researchers intend to study not just the turbulent properties of the phenomenon, as in most of the existing literature, but also approach it from the novel perspective of an instability problem.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

由于淡水的异常行为（低于3.98c）的热膨胀系数为负，因此当春季中较高至高纬度淡水湖的表面加热时会发生对流。这意味着在没有风的阳光时，可以通过短波辐射来驱动对流。在过去的几年中，苏必利尔湖（Lake Superior）的观察结果是无关现场工作的一部分，它提供了一个机会，开始从许多不同平台（包括基于系泊和滑翔机观测值）探索这种对流过程的动态。观察结果表明，水在地面上被加热，并在小时的顺序上运输到底部（O（200m）），并建议对流烟囱的短（O（10m））水平尺度（O（10m））水平尺度，为观察和模型提供了真正的挑战。该项目将结合观察结果，数值建模和分析工作，以确定和理解不稳定过程的空间和时间尺度。中高纬度地区的湖泊可以在这一州的一年中大部分时间花费，在此期间这些动态将主导循环和混合。从基本的流体动力学角度来看，这种现象提供了一个机会，在没有其他强迫机制（例如表面波和风驱动电流）的情况下，研究浮力驱动对流，与海洋对流的最重要例子相比。这个过程可能会对一级生产率产生重大影响，因为跨越水柱的强对流将使主要生产者无法保持其在舒适区域的位置。此外，在小时的时间尺度上，水包裹从湖面运输到底部，可能会对湖泊沉积物生物地球化学产生重大影响。研究生将在相关的三个机构中的每个机构中得到支持。将开发一种交互式的一维混合模型供课堂使用，类似于其中一个PIS开发的工具。这项工作将促进林木学家和海洋学家之间的协作。现场工作的核心将由配备快速热敏电阻的“水平系泊”以及能够测量垂直速度的ADCP，以及用于测量表面热量的辅助系泊型。在平静和大风条件下，将进行滑翔机样带和速度微观结构调查。建模将包括使用直接数值模拟和大型涡流模拟对参数空间的探索。分析方法将用于约束参数空间。与良好的雷利 - 贝纳德对流不同，该过程是由直接将可用势能注入到大部分流体中的驱动。研究人员不仅要像大多数现有文献一样研究现象的动荡特性，而且还从不稳定问题的新角度来研究它。该奖项反映了NSF的法定任务，并被认为是值得通过基金会的知识分子优点和更广泛影响的审查标准来通过评估来支持的。

项目成果

Characteristic scales during the onset of radiatively driven convection: linear analysis and simulations

辐射驱动对流开始时的特征尺度：线性分析和模拟

• DOI: 10.1017/jfm.2023.702
• 发表时间: 2023
• 期刊: Journal of Fluid Mechanics
• 影响因子: 3.7
• 作者: Chang, Yun;Scotti, Alberto
• 通讯作者: Scotti, Alberto