Geophysical Fluid Dynamics: Stratified Flows

The daily cycle of solar heating leads to the development of a diurnal warm layer (DWL) directly beneath the surface of a body of water. Nighttime cooling causes the DWL to become unstable to convective motions, which mix the warm water downwards. In shallow bodies of water, the DWL may account for a significant portion of the overall depth, and its breakdown each night affects the turbulence and stratification of the entire water column. Motivated by observations from the Stockton Deepwater Shipping Channel, which is prone to high levels of primary productivity depending on the dynamics of the DWL, we present 3D large eddy simulations of a shallow freshwater channel forced by surface winds and cyclic surface heating and cooling. The presence of surface wind further adds the complexity of stratified shear instabilities arising near the water surface. We explore the formation and breakdown of the DWL over the course of a day, focusing on the transition between daytime stratification with shear mixing and nighttime turbulent convection. In order to understand the interaction between shear and convective turbulence in this scenario, we vary the relative strength of the surface wind forcing and heat flux. Both parameters impact the behavior of the flow, with only certain conditions resulting in mixing to the full depth of the channel. Additionally, we explore using a bulk Richardson number criterion to characterize the flow.

Funding acknowledgement

Support from the NSF Graduate Research Fellowship Program and the California State Water Resources Control Board is gratefully acknowledged.