Collaborative Research: Radiatively Driven Convection in a deep freshwater lake

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.