Observations of ice-free radiative convection and turbulent mixing in Lake Michigan

Session: Physical Processes in Lakes (2)

David Cannon, Purdue University, [email protected]
Cary Troy, Purdue University, [email protected]
Harvey Bootsma, University of Wisconsin-Milwaukee, [email protected]
Qian Liao, University of Wisconsin-Milwaukee, [email protected]
Rae-Ann MacLellan-Hurd, University of Wisconsin-Milwaukee, [email protected]
Tong Jin, University of Wisconsin-Milwaukee, [email protected]

Abstract

While radiative convection is a potentially important mixing mechanism in the Laurentian Great Lakes, little work has been done to explore its effect on over-winter vertical transport or turbulence enhancement, especially in the absence of surface ice. In this work, we analyze data collected during a two-week ADCP/thermistor chain deployment and microstructure campaign in order to investigate the potential impact of radiative convection during the “isothermal” winter in Lake Michigan.  High-resolution thermistors deployed over the experiment showed strong diurnal variability, with water column temperatures increasing across all depths (~55m) during daylight hours despite weak wind-driven mixing and a photic depth that extended less than 10m below the water surface. Direct observations of turbulent temperature and velocity fluctuations were largely cyclical, with turbulence parameters increasing at all depths during the day. Weakly convective temperature instabilities were directly correlated with elevated turbulent kinetic energy dissipation, increasing depth-averaged dissipation estimates by almost two orders of magnitude (10^-9 - 10^-7 W/kg). Turbulent scalar diffusivities showed similar multi-order daytime enhancements, suggesting that radiative convection plays a major role in driving vertical mixing throughout the water column during the “isothermal” winter.

Twitter handle of presenter
@TroyLab_Purdue