Collaborative Research: Radiatively Driven Convection in a deep freshwater lake

由于淡水的反常行为，在3.98℃以下具有负的热膨胀系数，在春季中高纬度淡水湖表面加热时会发生对流。这意味着，在风平浪静、阳光明媚的日子里，在没有风的情况下，对流可以由短波辐射驱动。过去几年在苏必利尔湖进行的观测是无关实地工作的一部分，为开始从若干不同平台探索这一对流过程的动态提供了机会，包括系泊和滑翔机观测。观测显示，水块在表面被加热，并在数小时内被输送到底部(O(200米))，并建议对流烟囱的水平尺度较短(O(10米))，这给观测和模型带来了真正的挑战。该项目将结合观测、数值模拟和分析工作，以确定和了解不稳定过程的空间和时间尺度。中高纬度的湖泊可能会在一年中的很大一部分时间里处于这种状态，在此期间，这些动态将主导环流和混合。从基本流体动力学的角度来看，这一现象提供了一个在没有其他强迫机制的情况下研究浮力驱动的对流的机会，与大多数重要的海洋对流例子形成对比。这一过程可能会对初级生产力产生重大影响，因为横跨水柱的强对流将使初级生产者无法维持其在真光区的地位。此外，成片的水正在以几个小时的时间从湖面输送到湖底，并可能对湖泊沉积物的生物地球化学产生重大影响。所涉及的三所院校将各资助一名研究生。将开发一种用于课堂使用的交互式一维混合模型，类似于其中一个PI已经开发的工具。这项工作将促进湖沼学家和海洋学家之间的合作。现场工作的中心将包括一个配备快速热敏电阻和能够测量垂直速度的ADCP的水平系泊设备，以及一个测量表面热通量的辅助系泊设备。在风平浪静的条件下，都将进行滑翔机横断面和速度显微结构的调查。建模将包括使用直接数值模拟和大涡模拟对参数空间的探索。将使用解析方法来约束参数空间。与研究得很好的瑞利-贝纳德对流不同，这个过程是通过将可用势能直接注入到流体主体中来驱动的。研究人员不仅打算像大多数现有文献那样研究这一现象的动荡属性，而且还打算从一个不稳定问题的新角度来研究它。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.