  Flux-rope-based magnetohydrodynamic modeling of coronal mass ejections (CMEs)
is a promising tool for the prediction of the CME arrival time and magnetic
field at Earth. In this work, we introduce a constant-turn flux rope model and
use it to simulate the 12-July-2012 16:48 CME in the inner heliosphere. We
constrain the initial parameters of this CME using the graduated cylindrical
shell (GCS) model and the reconnected flux in post-eruption arcades. We
correctly reproduce all the magnetic field components of the CME at Earth, with
an arrival time error of approximately 1 hour. We further estimate the average
subjective uncertainties in the GCS fittings, by comparing the GCS parameters
of 56 CMEs reported in multiple studies and catalogs. We determined that the
GCS estimates of the CME latitude, longitude, tilt, and speed have average
uncertainties of 5.74 degrees, 11.23 degrees, 24.71 degrees, and 11.4%
respectively. Using these, we have created 77 ensemble members for the
12-July-2012 CME. We found that 55% of our ensemble members correctly reproduce
the sign of the magnetic field components at Earth. We also determined that the
uncertainties in GCS fitting can widen the CME arrival time prediction window
to about 12 hours for the 12-July-2012 CME. On investigating the forecast
accuracy introduced by the uncertainties in individual GCS parameters, we
conclude that the half-angle and aspect ratio have little impact on the
predicted magnetic field of the 12-July-2012 CME, whereas the uncertainties in
longitude and tilt can introduce a relatively large spread in the magnetic
field predicted at Earth.
