Environmental Science and Engineering Seminar

Wednesday, October 6, 2021
4:00pm to 5:00pm
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South Mudd 365
Talk 1. Interactions between ocean currents and surface waves. Talk 2. Destructive Interference: Potential of Frequency Combs for Future Greenhouse Gas Monitoring
Bia Villas Boas, Postdoctoral Scholar, Environmental Science and Engineering, Caltech,
Newton Nguyen, Graduate Student, Environmental Science and Engineering, Caltech,

Talk 1

The energetic balance between ocean, atmosphere, ice, and land regulates the Earth's climate. This balance is governed by processes that couple different components of the system in a number of complex interactions that happen at the boundaries. In particular, the marine atmospheric boundary layer provides a medium for the atmosphere and the ocean to constantly exchange energy, momentum, heat, freshwater, gases, and other tracers. These fluxes are largely modulated by interactions between surface winds, waves, and currents. Despite wave motions being strongly coupled to the upper-ocean circulation and the overlying atmosphere, efforts to improve climate and wave models have evolved somewhat independently. However, surface wave physics is key to improving climate models and better representing the coupling between the ocean and the atmosphere. In this talk, I will address questions aimed at bridging the gap between ocean and wave models and advancing our understanding of how currents and waves Interact. To do that, I will apply theory and numerical modeling to assess the relative impact of current divergence and vorticity in modifying several properties of the waves, including direction, period, directional spreading, and wave height. I will show that wave parameters contain information about the currents and have the potential to help to detect and characterize strong gradients in the velocity field, which is particularly relevant for upcoming in situ and spaceborne programs.

Talk 2

Long-term monitoring of greenhouse gases (ghgs) requires sub-percent (0.1%) accuracy, and observations should also be dense enough to constrain top-down calculations of ghg emissions. However, logistical difficulties make it costly to expand the current measurement network. Dual Comb Spectroscopy (DCS) has emerged as a cost-effective technique to measure ghgs, like CH4, CO2, and H2O, with high accuracy. By employing laser frequency combs, DCS provides broad-band measurements with extremely high spectral resolution (0.0067 cm-1) at open paths with a range of up to 10 km. However, in order to obtain sub-percent accuracy in ghg retrievals with DCS, tabulations of spectroscopic parameters used to calculate the absorption cross-sections of these molecules need to be more accurate and complete. Here, we show that retrieved concentrations of ghgs from a multi-month DCS field deployment disagree by up to 76 ppb for CH4 and 3.5 ppm for CO2 when using different spectroscopic databases, which do not meet the 0.1% accuracy threshold. We show that this is due to errors in parameters determining pressure and temperature broadening effects aliasing to retrieved ghg concentrations. Finally, we demonstrate the ability of the DCS to obtain vertical ghg gradients, and we determine the information content that variations in pressure and temperature have on these profile retrievals – a future DCS application. Given these findings, more accurate spectroscopy is therefore necessary before the full potential of the DCS technology can be exploited for highly accurate, long-term ghg monitoring

For more information, please contact Bronagh Glaser by email at bglaser@caltech.edu or visit Environmental Science and Engineering.