Effects of biogenic surfactants on the spatial variability of surface water temperature under low-wind conditions

Natural slicks are manifested by changing reflective properties of water surfaces through inhibition of wave-growth and damping short gravity-capillary waves. Such influence on near-surface hydrodynamics can in turn affects the air-water exchange of momentum, heat, and gas. Here, we present observations from a field campaign carried out in Lake Geneva in Spring 2019, which show the response of Lake Surface Water Temperature (LSWT) to transient light winds. For the purpose of this research, we used an autonomous catamaran equipped with near-surface water temperature sensors, a weather station, and an RGB camera taking slanted view images of the water surface. The second imagery package includes a LWIR camera carried by a balloon, providing thermal images with resolution of O(1 m) from altitude of about 500 m. As our result demonstrates, following the onset of low winds (U10 < 3 m/s) roughness patterns appeared on the surface that resembles slick/non-slick regions commonly observed on the water bodies. Such patterns have an exact correspondence with surface temperature contrasts of up to 2℃. To associate smooth and choppy surface conditions with the effects of biogenic surfactants, enrichment factor of Fluorescence Dissolved Organic Matter is measured as a proxy from samples collected at different field campaigns. The results point to the spatial variability of LSWT at the sub-pixel satellite scale. It is shown that such warm and cool patches can persist on the water surface after the surface roughness patterns disappeared following a drop in wind speed (U_10 < 2 m/s). Hence, such interaction between transient wind and random-like distribution of natural slicks leads to heterogeneous surface mixing. This can in turn play a role in the generation of intermittency of air-water fluxes, particularly during strong lake stratification and low-wind conditions.