PhD Proposal Defense: Yun Chang

The PhD Proposal Defense of Yun Chang is presented by the University of North Carolina at Chapel Hill’s Earth, Marine, and Environmental Sciences Department. This event will be held on Monday, December 13th at 10:00 am. This event will be held in person and will also be streamed online. The physical location of the event is on UNC-CH campus in room G201, on the ground floor  of Venable/Murray Hall. To watch online join via Zoom Meeting ID 316 690 8951.

Title: Radiatively Driven Convection and Langmuir Turbulence in a Large Freshwater Lake

Abstract: The water column of mid- to high-latitude lakes at the end of winter is usually between 0C and 4C. In this temperature range, the density of fresh water increases with increasing temperature. As solar radiation penetrates a lake, the near-surface water becomes warmer, denser, and sinks (radiatively driven convection, or RDC), a relatively new area for geophysical convection research. This talk focuses on the scaling and flow structure of RDC using large eddy simulation (LES), which is compared to field observations. First, formulating the sinking velocity scale W as a simple function of water temperature, solar radiation, photic zone extent, and lake depth, we simulate the process with the Stratified Ocean Model with Adaptive Refinement in LES mode (SOMAR-LES) to verify the proposed W. With constant solar radiation in time, the horizontal scale of convection cells, suitably normalized, grows linearly with normalized time. This relation is then used to infer the horizontal scale of convection cells subject to diurnal cycle of solar radiation. Next, W is adopted to estimate the Rossby number, which for a 180 m deep lake is around 3. Numerical simulations with typical Coriolis parameter fz alone reveal that rotation does have a dynamical effect, reducing the horizontal scale of convection cells. Further, scaling analysis suggests that the latitudinal component of Coriolis parameter fy, which deflects flow to the left on a longitudinal-vertical plane, should be considered. Indeed, fy and fz together generate horizontal mean flow in both longitudinal and latitudinal directions, inducing a large-scale circulation. Finally, RDC subject to wind-induced Langmuir turbulence is briefly introduced. This project allows us to scale up to submesoscale dynamics, investigating how small-scale and submesoscale dynamics intertwine.