179
changes with time, each droplet is still representative of the same amount of oil
assigned at the release time. We then compute the scaling factor, sf, for each droplet
from its initial mass m t0 and the mass of oil it represents, m R , as follows: sf = m R /m t0 .
This scaling factor is unique for each droplet and remains invariant during the run.
Oil mass of a droplet at each time, m t , is estimated from the droplet diameter and
effective density at the corresponding times; it is then multiplied by the scaling factor to obtain the effective oil mass: m
sf m
t
Ef
t
= ⋅ . This effective oil mass is further
summed for all the droplets found in each post-processing domain 3D grid box and
at a given time.
The 3D post-processing domain has 0.02-degree × 20-m grid boxes in the horizontal and vertical directions, respectively, spanning from the surface down. For
concentration estimates in the surface layer, the vertical boundaries are taken
between 0 and 1 m. After the oil mass is computed for a given droplet and given
output time, the corresponding post-processing domain grid box is determined,
and the effective oil mass of all the droplets found in that grid box at a given time
are summed up. After looping over all the droplets and all the times, we yield the
cumulative effective oil mass in the time-space field. Concentrations are obtained
by normalizing the total oil mass to the mass of water in the corresponding grid
box, and the daily averages are further determined from the 2-hourly output products. Bathymetry is taken into the account to compute volumes of the grid boxes;
the volume of oil is assumed to be much smaller than the volume of water in a 3D
grid box.
11.4 Modeling Results and Analyses
11.4.1 Surface Oil Expression
The extent of oil slicks on the ocean surface was closely monitored during the spill
using satellite observations and can thus be used for qualitative model verification.
Surface oil concentrations from the control run in the top surface layer 0–1 m for
May 13, 2010, show similar features as the Roffer’s Ocean Fishing Forecasting
System (ROFFS) analysis (Fig. 11.1a). However, there is not much oil reported by
the observations along the coastal areas, which are present in model results
(Fig 11.1b). Reported oiled coastal areas (Nixon et al. 2016) agree with Romero
et al. (2017) and the CMS output. Surface oil concentrations for May 13, 2010, indicate wider spread of the oil slick around the blowout location and a well-defined
extent of the oil presence south of it. The oil slick is further split around 27
°
N and
88.5
°
W into eastward and westward branches; the eastward branch follows the strong
Loop Current, while westward propagation results from inclusion of wind drift
effects (see also Le Hénaff et al. 2012, Fig. 11.2 with similar surface oil behavior).
The DB_VDROPJ_treated simulation yields greater surface concentrations than
the DB_control scenario in most of the areas, except of the oil slick branch extending eastward along the 28
o
N. DB_VDROPJ_untreated (not shown) resulted in even
higher concentrations due to larger droplets surfacing quickly.
11 Far-Field Modeling of a Deep-Sea Blowout: Sensitivity Studies of Initial…
changes with time, each droplet is still representative of the same amount of oil
assigned at the release time. We then compute the scaling factor, sf, for each droplet
from its initial mass m t0 and the mass of oil it represents, m R , as follows: sf = m R /m t0 .
This scaling factor is unique for each droplet and remains invariant during the run.
Oil mass of a droplet at each time, m t , is estimated from the droplet diameter and
effective density at the corresponding times; it is then multiplied by the scaling factor to obtain the effective oil mass: m
sf m
t
Ef
t
= ⋅ . This effective oil mass is further
summed for all the droplets found in each post-processing domain 3D grid box and
at a given time.
The 3D post-processing domain has 0.02-degree × 20-m grid boxes in the horizontal and vertical directions, respectively, spanning from the surface down. For
concentration estimates in the surface layer, the vertical boundaries are taken
between 0 and 1 m. After the oil mass is computed for a given droplet and given
output time, the corresponding post-processing domain grid box is determined,
and the effective oil mass of all the droplets found in that grid box at a given time
are summed up. After looping over all the droplets and all the times, we yield the
cumulative effective oil mass in the time-space field. Concentrations are obtained
by normalizing the total oil mass to the mass of water in the corresponding grid
box, and the daily averages are further determined from the 2-hourly output products. Bathymetry is taken into the account to compute volumes of the grid boxes;
the volume of oil is assumed to be much smaller than the volume of water in a 3D
grid box.
11.4 Modeling Results and Analyses
11.4.1 Surface Oil Expression
The extent of oil slicks on the ocean surface was closely monitored during the spill
using satellite observations and can thus be used for qualitative model verification.
Surface oil concentrations from the control run in the top surface layer 0–1 m for
May 13, 2010, show similar features as the Roffer’s Ocean Fishing Forecasting
System (ROFFS) analysis (Fig. 11.1a). However, there is not much oil reported by
the observations along the coastal areas, which are present in model results
(Fig 11.1b). Reported oiled coastal areas (Nixon et al. 2016) agree with Romero
et al. (2017) and the CMS output. Surface oil concentrations for May 13, 2010, indicate wider spread of the oil slick around the blowout location and a well-defined
extent of the oil presence south of it. The oil slick is further split around 27
°
N and
88.5
°
W into eastward and westward branches; the eastward branch follows the strong
Loop Current, while westward propagation results from inclusion of wind drift
effects (see also Le Hénaff et al. 2012, Fig. 11.2 with similar surface oil behavior).
The DB_VDROPJ_treated simulation yields greater surface concentrations than
the DB_control scenario in most of the areas, except of the oil slick branch extending eastward along the 28
o
N. DB_VDROPJ_untreated (not shown) resulted in even
higher concentrations due to larger droplets surfacing quickly.
11 Far-Field Modeling of a Deep-Sea Blowout: Sensitivity Studies of Initial…