Subgrid-scale dynamics of water vapor, heat, and momentum over a lake

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. The Lake-Atmosphere Turbulent EXchanges (LATEX) field measurement was designed to address the issues of air-water interactions over Lac Léman (Geneva) making use of a 10 meter high tower, situated 100 meters from the northern shore of the lake near Lausanne. Data were collected from mid August till late October of 2006.

The main instrumentation consisted of four sonic anemometers (Campbell Scientific CSAT 3) and four open path gas analyzers (Licor-7500) measuring wind speed, temperature, and humidity at 20 Hz. The four pairs were setup as a vertical array. Additional supporting measurements included net radiation, water surface temperature, relative humidity and temperature of air, wave height and speed, as well as several point-measurements of air and water temperature.

The measurements allowed, for the first time, the study of subgrid-scale turbulent transport of water vapor over a lake, which is found to be well correlated with the transport of heat. This result suggests that the subgrid-scale modeling of the two scalars could be coupled to save computational time in LES. Results from a-priori analysis of subgrid-scale fluxes and dissipations (model coefficient dependence on stability and filter size, correlations of modeled and measured statistics, …) indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces, mainly due to the small effect of the lake waves on surface layer turbulence. The findings are then combined with large eddy simulations to investigate the influence of lakes on atmospheric dynamics and their parameterization in mesoscale and global atmospheric models.