Using Evaporation Stable Water Isotopologues to determine Great Lakes Influence on Atmospheric Moisture

The Great Lakes region provides energy and moisture to fuel seasonal convective storms and lake-effect snow, but these weather effects are difficult to model due to simplifications of the physical processes of evaporation, turbulent mixing, and atmospheric transport (Mason et al. 2016; Fujisaki-Manome, Ayumi et al. 2017; Sharma et al. 2018). These processes are hard to measure directly, leading to lack of ground-truth data to validate models (Sharma et al. 2018). Stable water isotopes can act as a tracer of humid air masses in order to parse out lake moisture influence and can be used as validation in modern and paleo-lake system models.

To track water vapor originating from the lakes using isotopes, we must know the isotopic value of evaporation (δ_E) of the source. This is currently estimated from the Craig and Gordon isotopic evaporation model (1965) using mostly unmeasured atmospheric isotopic values and a debated kinetic fractionation factor (Horita et al. 2008; Xiao et al. 2017). The Craig and Gordon Model was created to describe the marine boundary layer and the ocean as a source of evaporation (Craig and Gordon 1965; Jasechko et al. 2014; Galewsky et al. 2016), but it is currently unknown how well the model applies to large lakes which may exhibit horizontal and vertical moisture gradients due to continental advection (Horita et al. 2008; Feng et al. 2016).

To better constrain δ_E, determine Great Lake influence on the above-lake and downwind atmosphere, and test the Craig and Gordon model in large lake environments, direct flight measurements of atmospheric water vapor isotopes around Lake Michigan were made in October 2018. We see differences between the upwind and downwind sides of the lake in both water concentration and isotopic values as increasing amounts of lake evaporation are mixed into the lower boundary layer. We estimate the isotopic composition of Lake Michigan evaporation using these direct measurements of atmospheric water vapor isotopes and Keeling plot analysis on 3 of the 5 flight days with suitable observations. These observations could be useful for testing the evaporation rate and vertical mixing in isotope-enabled atmospheric models.