Three-Dimensional Hydrothermal Model of Onondaga Lake, New York

Abstract

A three-dimensional time-dependent hydrodynamic model of Onondaga Lake, an inland lake in central New York, emphasizing the simulation of dynamics and thermal structure has been developed. The model is based on the ECOM family of models; this version, called ECOMsiz, employs a semi-implicit time splitting algorithm and a z-level vertical coordinate system. Proper assignment of boundary conditions, especially surface heat fluxes, has been found crucial in simulating the lake's hydrothermal dynamics. Formulas for atmospheric radiation and sensible and latent heat fluxes are introduced, which have been found most appropriate for evaluating the heat budget for this midlatitudinal urban lake. The ECOMsiz model has been calibrated and validated against data for two years, 1985 and 1989, representing a wide spectrum of atmospheric and hydrographic conditions in the lake. These two years, marked by significantly different freshwater inputs from tributary inflows, ionic waste loadings, wind forcing, and atmospheric heating and cooling, form a firm basis for evaluating the robustness of the hydrodynamic model. The simulation period chosen for both years, April through October, spans the entire range of lake physical processes as it covers the well-mixed spring condition, the summer period marked by strong vertical stratification, and the well-mixed fall period. Significant differences in thermal structure have been observed in 1985 and 1989 as a result of different meteorological conditions. The mixed layer depth in 1985 is about 3 m deeper (about 9 m) than that in 1989 (about 6 m), consistent with a stronger prevailing wind in 1985. The model has successfully predicted the mixed layer depth for both the years. The model computed total heat storage for both years is in good agreement with the observed conditions.