Intellectual Merit: Simulation of the subsurface and surface dispersal of oil in the Gulf of Mexico will be conducted with the objective of producing probabilistic envelopes of the spread of different size classes of oil as they age over time. The proposed model system is ready to respond. The SABGOM hydrodynamic model of the Gulf of Mexico and South Atlantic Bight has been successfully coupled with LTRANS, a fully three-dimensional Lagrangian particle tracking model capable of simulating sub-grid scale turbulent motion as well as time-varying particle attributes like diameter, density, and rise/sinking velocities. At distances greater than a few hundred meters above the deepwater source (depending on ambient current speed and stratification), the dispersal of oil depends mainly on the behavior of oil droplets which are fractionated into different sizes. These oil droplets can have orders of magnitude differences in ascent rates (e.g., 6 mm/s and 0.06 mm/s for 300 micron and 30 micron diameter particles, respectively) and change in diameter as they age. Emulsification, interaction with suspended particulate matter, dissolution and other processes can also affect droplet behavior. Our Lagrangian approach is ideally suited for simulating oil dispersal because differences in initial droplet characteristics and time-varying droplet behavior are readily incorporated. In this project, the coupled SABGOM/LTRANS model system will be run for the time period of the Deepwater Horizon oil spill, maps and animations of model output will be produce. The model results will be compared with available observations and will be made available to the oil spill response community.
In the near-term, a series of LTRANS simulations will be run using the existing flow field from recent SABGOM model simulations. The Lagrangian dispersion runs will be initialized with a continuous source of particles representing the near-field plume above the well. Each run will simulate the far-field dispersion of those particles based on a specific set of assumptions about particle behavior. As more complete information on the size and composition of gas bubbles and oil droplets emerge, the most realistic particle distributions from the LTRANS ensemble of runs will be selected. As part of this effort, an improved hindcast from the SABGOM model for use with LTRANS will be prodiced and the model skill will be quantified against physical oceanographic observations. In addition, Eulerian and Lagrangian predictions of oil dispersal will be quantitatively compared with observations in order to use the strengths of both approaches to provide the most realistic predictions for the oil response community.
Broader Impacts: Mid-term results will be open-source models and model results using existing and likely new, particle-tracking technology for the geosciences and oil-spill response communities. Incorporation of the model into the framework of the Community Surface Dynamics Modeling System (CSDMS) will ensure that the coding structures are suitable for coupling with other models and future distribution for research and educational purposes. In addition, the team members from the USGS will ensure that LTRANS can run with CF-compliant model output, making it functional with over seventeen coastal models, allowing simulations and forecasts to be made throughout the US coastal waters. In addition to providing timely information for oil spill responders, this project will lay the ground work for future efforts that investigate the interaction between oil and larval transport of commercially and ecologically important organisms in the Gulf of Mexico.
Intellectual Merit: Both ambient stratification and ambient currents can cause dissolved and finely dispersed oil to separate from a rising gas droplet plume. I have re-examined laboratory data distinguishing the two effects and have derived a simpler relationship to determine which effect (stratification or current) is more dominant. Unlike the earlier DeepSpill field study conducted off the coast of Norway in 2000, the phase separation at the Deepwater Horizon site was found to be caused principally by stratification, hence suggesting a stratification-based plume model to predict the characteristics of oil and gas intrusion layers following the Deepwater Horizon oil spill. In the process we modified a previously developed analysis for linear density stratification to apply to quadratically varying stratification. Using such a model we found that the dominant trap height of dissolved and dispersed hydrocarbons was between 350 and 400 m above the seafloor. This is in excellent agreement with the average elevation from over 100 CTD casts. However, we did not predict the degree of variability in trap height shown by these observations. Our modeling also predicts smaller secondary intrusions spaced roughly 300 meters apart and these have been observed in CTD casts. Major findings: We have provided a rational basis for explaining the existence and location of subsurface intrusions of oil and gas resulting from the Deepwater Horizon spill. Our work provides the "inner boundary conditions" of several Lagrangian particle tracking models that simulate far field oil transport and fate, and helps provide a constraint on the oil budget calculations (i.e., how much oil was found below the surface versus on the surface). Chemical dispersants applied to sub-surface spills break the oil into small droplets. In combination with separately funded ongoing research, our work on phase separation is helping to determine the efficacy of this remediation strategy by indentifying just how small droplets need to be in order to enter intrusions as opposed to being transported to the surface with the parent plume. Broader Impacts: Our research has promoted a better understanding and prediction of the formation and dispersal of sub-surface oil plumes. This information has been communicated to oil spill managers, members of the science community and students via presentations and publications.
