Project Overview

Summary:

Dr. Yangxing Zheng at Florida State University’sCenter for Ocean-Atmospheric Prediction Studies (COAPS) was awarded an RFP-VI grant at $422,399 to conduct the RFP-VI project titled, “Modeling Modification of Surface Oil Transport by Air/Sea Interactions and Tropical Storms”. The project consisted of 1 institution (Florida State University), 1 principal investigator (Zheng), 1 co-PI (Dr. Mark Bourassa), and 1 research scientist (Stephen Van Gorder).

     Oil spills pose a serious threat to marine resources and can be highly destructive to nearby wetland and estuarine animal habitats. In order to limit the damage resulting from an oil spill and to facilitate efficient containment and cleanup efforts, response managers rely heavily on reports about a spill’s location, size and extent, as well as forecasts of surface oil locations. In the hours, days, and even months following any spill, information about surface winds and ocean waves is critically important to estimating the spill location and forecasting how the released oil will be transported. When dealing with large spills, floating oil can also substantially modify the wind and waves, which in turn modifies the movement of the spill. The prior work of Zheng et al. (1) used an idealized model to show that the above interactions between oil, wind, and waves substantially affected the oil’s motion; albeit with less impact than the strong currents associated with Gulf eddies. Numerical ocean and atmospheric models do not yet take into consideration these effects on oil transport. In this study, modifications of the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) model (2) will account for these interactions under much more realistic environmental conditions and in a system that can ultimately be used to generate improved oil spill forecasts.

     Oil-related modification of the ocean surface will cause changes in surface winds and stress that will also modify waves and currents. This wave-induced transport is typically ignored in oil trajectory forecasts including the Deepwater Horizon forecasts (3). Such motion is already parameterized in some ocean models. However, a common practice in modeling oil spills is to make an adjustment in surface transport due to ‘wind drift,’ which includes an approximation for wave induced motion based on one specific set of conditions (4). Results from the Deep-C Consortium have shown that wave transport is highly variable. For example, distant strong wind events such as winter fronts or hurricanes can greatly increase wave-related transport. But while wave-related transport is smaller than transport from strong deep water currents; it is very important in shelf water and responsible for substantial shoreward transport. The modified COAWST model includes coupling with waves, making it well suited for including wave-related transport in a relatively realistic manner. The majority of the oil-related modifications has already been completed and tested for another project. An improved form of this model will be used to examine transport associated with tropical cyclones and winter fronts, as well as study how modified air/sea coupling changes this transport.

     The idealized model for how oil modifies surface stress (1) assumed a surface entirely covered by oil; however, observations following the Deepwater Horizon accident indicate that the oil coverage after a spill is patchy. This patchiness can easily be added to the existing model in the parameterizations of wind stress and evaporation. As a reasonably accurate approximation, evaporation of water is negligible where there is a layer of oil, which appears to be the dominant process in changing the surface temperature in a slick (radiative properties have small changes). Observations during the Deepwater Horizon spill found temperatures could be 5 °C greater than surrounding water for patches of oil (10). The energy budget can be used to constrain estimates of fractional coverage: too much coverage will result in temperatures that are too great. The fully coupled feedback between the slick, ocean and atmosphere will require estimates of oil coverage and temperature changes and tuning through comparison to observed sea surface temperatures.

     The coupled model will be used to model oil transport and coverage during the Deepwater Horizon spill, which will provide comparisons to observations. The modifications to the atmospheric and ocean model can be shared with the broader community and the modified coupled model will be a legacy of this project.

Research Highlights

     Dr. Zheng’s research, which included 2 conference presentations to date and 6 datasets submitted to the GoMRI Information and Data Cooperative (GRIIDC), which are available to the public.  The modifications of the coupled COAWST modeling system (with very high resolutions in space and time) are the primary accomplishments in this research showing the effects of floating oil on the coupling among atmosphere, ocean, and wave models. These effects are due to the presence of the floating oil include the changes to wind stress, ocean current, ocean waves, momentum and heat fluxes, which in turn affect the movement of floating oil. Research scientist Mr. Van Gorder and the graduate student Daneisha Blair greatly benefitted from the 2019 COAWST workshop on February 25–28 in Raleigh, North Carolina. Dr. Zheng is grateful for the support to present the research on the 2019 Gulf of Mexico Oil Spill & Ecosystem (GOMOSES) conference in New Orleans, LA on February 4–7. The gradient student Daneisha Blair greatly thanks the support to present research in the American Geophysical Union Ocean Science Meeting in San Diego, CA on February 17–21, 2020. Significant outcomes of their research (all related to GoMRI Research Theme 1) are highlighted below.

• They have successfully completed work concerning the impact of surface oil on the movement of surface oil using the innovatively parameterized roughness length z0.
• They have successfully completed work concerning the impact of surface oil on the movement of surface oil by modifying the momentum and heat fluxes at the air-sea boundary.
• ROMS modeling with and without the effects of tides indicates tides do not have a significant impact on surface oil movement.
• They have successfully run the modified COAWST model with the modified Grenier and Bretherton Planetary Boundary Layer (GB PBL) and Mellon-Yamada-Nakanishi-Nino (MYNN2) PBL schemes treating surface oil as a passive tracer to examine the impacts of different schemes on surface oil transport.
• After comparison with observations, they find that GB PBL scheme is better than MYNN PBL scheme in capturing realistic features of ocean state in the Gulf of Mexico.
• They have successfully run the modified COAWST model with the modified Bourassa-Vincent-Wood (BVW) flux model and GB PBL schemes to fully determine momentum and heat fluxes, atmospheric stabilities at the air-sea boundary with and without the presence of oil. Three 14-day runs in the fully coupled COAWST model system are considered: (1) GB PBL scheme and BVW z0 without oil’s effect and surface flux without oil’s effect as a baseline run; (2) GB PBL and BVW z0 without oil’s effect, surface flux with oil’s effects included; and (3) GB PBL and BVW z0 with oil’s effect included and surface flux with oil’s effects included. The effects of surface oil at the boundary are examined and highlighted in surface transport of oil from the model outputs.
• Totally six datasets from these model outputs have been submitted to GRIIDC (i.e., four datasets consisting of 2-day runs with hourly outputs and two datasets consisting of three 14-day runs with 6-hourly outputs).
• The effects of surface oil on the surface transport of oil will be carefully examined and compared to the available observations during DWH period. Some parts of model outputs will be used for the graduate student to complete the thesis work.
• Summary of conclusions
  • The combined impacts of changes to surface stress and heat fluxes modify transport to increase towards the spill, resulting in stronger gradients of oil concentration near the edges of the spill.
  • The magnitude of the temperature change due to reduction in evaporation is sensitive to the concentration of oil, allowing this impact to be well tuned through comparisons to satellite observations of brightness temperature.
  • In our coupled model, the gradient in surface fluxes appears to cause atmospheric waves to propagate away from the oil, largely in the down wind direction. These waves slightly modify winds and temperatures downwind of the oil spill. These changes in sea surface temperature would be similar in magnitude to the changes in temperature at the spill location, if the amount of oil on the surface was about 60% of what was found to be a good tuning value for the DWH spill.
    • This suggests that for many spills, the changes in surface fluxes due to oil on the surface will be small (exceptions being spills where the rate of oil reaching the surface is sufficiently large).
    • It also suggests that satellite observed changes in sea surface temperature could provide some insights into oil concentration, but that this approach would only be useful for large rates of oil reaching the surface.

 Proposal Abstract - RFP-VI PI Yangxing Zheng