Abstract:
The latest version of the oil application of the Connectivity Modeling System (CMS), or oil-CMS, is used to simulate the far-field oil modeling of the Deepwater Horizon (DWH) blowout in the Gulf of Mexico (2010-04-20). The presented simulations feature multi-phase droplets that have a log-normal droplet size distribution (DSD) at the initial release time. The simulation is 100 days long, in which the oil spill stops after 87 days. Oil particles were released at 28.736°N, 88.365°W at a depth of 1222 m (300 m above the oil well), which is taken as a trap height. The model time step is 1200 seconds; 3,000 particles are released every two hours, equivalent to a total of 3,132,000 oil particles released over 87 days of the oil spill. Each liquid oil droplet contains three (3) pseudo-components corresponding to differential oil densities: 10% light oil with a density of 800 kg/m^3, 75% oil with 840 kg/m^3, and 15% heavy oil with 950 kg/m^3 density. The half-life decay rates of oil fractions were 30 days, 40 days, and 180 days, respectively, accounting for the biodegradation. The surface evaporation half-life was set to 250 hours; horizontal diffusivity is set to 10 m^2/s. At the initial release time, the droplets have a log-normal DSD with standard parameters µ=117 mm and σ = 0.62, based on best-known estimates from the observational and experimental data. These parameters assume the oil is not treated with chemical dispersants. CMS has a horizontal resolution of 0.02º. The vertical grid is 0-2500m with 20-m increments, except the top two layers bounded as 0-1m and 1-20m and the last layer is 2500-4500m. The transport and evolution of the oil particles are tracked by the oil-CMS model during the 100 days of the simulation, recording each droplet’s horizontal position, depth, diameter, and density every two hours. This dataset contains mass, diameter, and depth statistics of the evolving particles at five times over their history; their initial mass and diameter distributions; horizontally-cumulative oil concentration as a function of time; daily-averaged oil concentrations at the surface and near the release depth; and the three-dimensional oil concentration fields.
Data Parameters and Units:
The numerical modeling output of the oil-CMS representing the trajectory, history, and attribute of individual oil droplets is post-processed to compute the spatio-temporal evolution of mass-conserved oil concentrations or to extract selected data for droplet statistics. The dataset consists of fifteen files.
DWHbase_iDSD_3fr_0_1m_untr.nc and DWHbase_iDSD_3fr_800_1200m.nc specify the daily vertically-averaged oil concentration data in the top 1 m and in the layer between 800 and 1200 m, respectively. Parameters are time (days after the initial blowout, [days]), lat (latitude, [decimal degrees N]), lon (longitude, [decimal degrees E]), zbound (vertical layer boundaries, [m]), depth (bathymetry, [m]), oil_conc (oil concentration, [ppb]).
DWHbase_iDSD_3fr_3D.nc specifies the three-dimensional spatial distribution of oil concentration. Parameters are time (days after the initial blowout, [days]), lat (latitude, [degrees N]), lon (longitude, [degrees E]), zlevs_bnd (vertical layer boundaries, [m]), zlevs (depth of center of layer, [m]), depth (bathymetry, [m]), oil_conc (oil concentration, [ppb]).
ilmass2D_DWHbase_iDSD_3fr.nc specifies the horizontally-cumulative oil concentrations throughout the water column at two-hourly intervals. Parameters are time (days after the initial blowout, [days]), lat (latitude, [decimal degrees N]), lon (longitude, [decimal degrees E]), zlevs_bnd (vertical layer boundaries, [m]), zlevs (depth of center of layer, [m]), oil_conc (oil concentration, [ppm]).
DWHbase_iDSD_3fr_oilmass_landed.txt and DWHbase_iDSD_3fr_oilmass_exited.txt list droplets that landed and no longer actively transported by the CMS (“landed”) or exited by any reason, including leaving the domain or becoming too small (“exited”). Parameters are time since the beginning of the simulation [seconds], water depth [m], latitude [decimal degrees], longitude [decimal degrees], oil droplet mass (scaled to the representative oil mass for that particular droplet, [kg]).
DWHbase_iDSD_3fr_diam_mass_t0.mat contains the initial distribution of particle size. Units are part_mass (droplets' mass, [kg]) and part_diam (droplets' diameter, [m]).
Droplets5d_stats_DWHbase_iDSD_3fr_95days.mat contains the statistics of droplets released during the first 5 days of simulation, with evolution of each droplet tracked for 95 days. A variable “times” lists several selected representative times, in seconds, counting from the droplet release time; it is used to form a cell variable “Dtimes”. Cell variables contain droplet diameters “Ddiam”, droplet oil mass (scaled) “Dmass”, and droplet depth “Ddepth”, extracted from model raw trajectory files at output times corresponding to those in “Dtimes” variable. Dimensions of these cell variables reflect the number of trajectory raw output files used (ten output trajectory files that store model output for the droplets released during the first five days; total of “nums” trajectory files], and the number of selected time records relative to each droplet’s release time).
Droplets_stats_DWHbase_iDSD_3fr_t0_6h_24h_5d_10d.mat contains the statistics of all available droplets released during the simulation, as they evolve in size, diameter, and depth at initial time, after 6 hours, 24h, 5 days, and 10 days past the release. The cell variable “Dtimes” lists these selected times in string format. Cell variables contain droplet diameters “Ddiam”, droplet oil mass (scaled) “Dmass”, and droplet depth “Ddepth”, extracted from model trajectory files at selected output times corresponding to those in “Dtimes” variable. Dimensions of these cell variables reflect the number of trajectory raw output files [total of “nums” files], and the number of selected time records relative to each droplet’s release time).
DSD_iDSD_3fr_{6h,24h,5d,10d,52d,57d}.txt list the size and depth of each droplet present at six hours, 24 hours, five days, ten days, 52 days, and 57days after the modeled blowout. Parameters are droplet diameter [m] and depth [m].
The time scale for the 3D oil concentrations and oil mass: daily averages.
Methods:
Numerical simulations and post-processing were performed using a Pegasus supercomputer at the Center of Computational Science, University of Miami, in 2019. The website for the main CMS code may be found at github.com/beatrixparis/connectivity-modeling-system. The oil-CMS module is still under development. Ocean hydrodynamic forcing for the oil-CMS model uses HYbrid Coordinate Ocean Model (HYCOM) for the Gulf of Mexico region on a 0.04º. horizontal grid with 40 vertical levels from the surface to 5500 m. It provides daily average 3-D momentum, temperature, and salinity forcing fields for the CMS model. The surface wind drift parameterization employs surface winds and wind stresses from the 0.5º-degree Navy Operational Global Atmospheric Prediction System (NOGAPS) and adds a wind factor of 0.03 of surface winds to the upper-layer ocean currents. To obtain representative oil concentrations or oil mass on a regular horizontal and/or vertical grid, modeled oil droplets need to be scaled to account for the total amount of oil spilled into the ocean during the actual blowout, as estimated from the reports (730,000 tons). The data for the oil concentrations are daily average values in ppb units; the oil mass units are kg of crude oil. Horizontal 0.02-degree grid of the oil concentrations covers the entire Gulf of Mexico domain and beyond (18.1ºN-30.8º N, 98.0ºW-77.0ºW), and the vertical grid as indicated in the file metadata Post-processed NetCDF files were created using Matlab software package, v. R2017a, and many are compressed using Matlab capability to reduce NetCDF files size; maximum compression or ‘DeflateLevel’ = 9 is used.
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