Numerical Simulations of Radiatively Driven Convection in a Freshwater Lake During Spring and Early Summer

Alberto D Scotti, University of North Carolina at Chapel Hill, Marine Sciences, Chapel Hill, NC, United States, Zachary Taebel, University of North Carolina at Chapel Hill, Chapel Hill, United States, Jay A Austin, Univ Minnesota Duluth, Duluth, MN, United States and Stefan G Llewellyn Smith, University of California San Diego, La Jolla, CA, United States
Many of the oceanic processes that contribute to vertical transport are absent in lakes, and, conversely, some important drivers
of vertical exchange in lakes are not generally active in the ocean. In this talk we consider vertical exchange
in a relatively deep freshwater lake driven by incoming solar shortwave radiation applied to the surface during
the late spring and early summer months. Below ~ 4C, freshwater has a a negative thermal expansion coefficient,
so that in a situation where
a significant fraction of the water column is below 4C, heating applied at or near the
surface drives convection (Radiatively Driven Convection, or RDC). We consider this problem numerically,
using SOMAR (the Stratified Ocean Model with Adaptive Refinement) to analyze the dynamics of RDC
in an idealized geometry (a rectangular column of water 100 m X 100 m X 185 m) subject to different
forcing regimes. In RDC, the forcing occurs over the extent of the photic zone, decreasing exponentially with an e-folding
scale O(10 m), unlike the more familiar Rayleigh-Benard convection, where forcing is applied only at the surface.
The simulations reveal that the convective motion follows a cyclical pattern: a few hours after sunrise, instabilities
generate column-like motion that spread downward, as they become more turbulent as the day progresses.
By sunset, convection spans the entire length of the water column. During the night, the energy injected during the previous
day is dissipated, and the water column returns to an unstratified condition, having warmed by an amount that depends on the
total radiation received, and the cycle is ready to repeat itself.