Water Vapor and Heat Exchanges over Lakes

Abstract

Quantifying the interaction of the atmosphere and water surfaces is of great importance for water resources management, climate studies of ocean-atmosphere exchange and regional climate in coastal areas. Atmospheric dynamics over water surfaces have generally received less attention than land-atmosphere interactions due to difficulties in operating field studies. In this research we are trying to improve the physical parameterizations of lake-atmosphere processes by integrating measurements and modeling studies. The Lake-Atmosphere Turbulent EXchanges (LATEX) field measurement campaign is analyzed to understand air-water interactions over Lac Léman (Geneva). Large eddy simulations are used to study sensible and latent heat fluxes over heterogeneous wet surfaces. We present parameters of interest for land-surface modeling, i.e. the surface energy budget and the roughness lengths for momentum, heat and water vapor that are embedded in the Monin-Obukhov similarity theory used in atmospheric models. The storage of energy in the lake is a very important term, yet methods used to quantify it, relying on temperature profile measurements from a fiber optic and the theory of conduction of heat, are not successful. We revisit classical wet surface evaporation estimation methods that include this challenging term and derive an evaporation formulation based on sensible heat flux measurements. We then focus on numerical (large eddy) simulations of the flow above a water surface. Small-scale turbulence (the so-called subgrid scales, SGS) over the lake that cannot be captured in large eddy simulations is investigated. The measurements of LATEX allowed, for the first time, the study of subgrid-scale turbulent transport of water vapor over a lake, which reveals itself well correlated with the transport of heat. Results from an a priori analysis of subgrid-scale fluxes and dissipations indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces. We use the EPFL-LES code, with its scale-dependent Lagrangian dynamic SGS model, to simulate flows over transition from dry land to wet surfaces. We observe the fetch requirement for evaporation formulations and compare to results from simplified Lagrangian footprint models. We explore the limits of applicability of Monin-Obukhov similarity theory that are due to the finite fetch.