2016 Articles

Riverine skin temperature response to subsurface processes in low wind speeds

Brumer, Sophia Eleonora; Zappa, Christopher J.; Anderson, Steven P.; Dugan, John P.

Both surface and subsurface processes modulate the surface thermal skin and as such the skin temperature may serve as an indicator for coastal, estuarine, and alluvial processes. Infrared (IR) imagery offers the unique tool to survey such systems, allowing not only to assess temperature variability of the thermal boundary layer, but also to derive surface flow fields through digital particle image velocimetry, optical flow techniques, or spectral methods. In this study, IR time-series imagery taken from a boat moored in the Hudson River estuary is used to determine surface flow, turbulent kinetic energy dissipation rate, and characteristic temperature and velocity length scales. These are linked to subsurface measurements provided by in situ instruments. Under the low wind conditions and weak stratification, surface currents and dissipation rate are found to reflect subsurface mean flow (r^2 = 0.89) and turbulence (r^2 = 0.75). For relatively low dissipation rates, better correlations are obtained by computing dissipation rates directly from wavenumber spectra rather than when having to assume the validity of the Taylor hypothesis. Furthermore, the subsurface dissipation rate scales with the surface length scales (L) and mean flow (U) using ε ∝ U^3/L (r^2 = 0.9). The surface length scale derived from the thermal fields is found to have a strong linear relationship (r^2 = 0.88) to water depth (D) with (D/L) ∼ 13. Such a relation may prove useful for remote bathymetric surveys when no waves are present.