  We apply the 3-dimensional radiative transport code Wind3D to 3D hydrodynamic
models of Corotating Interaction Regions to fit the detailed variability of
Discrete Absorption Components observed in Si IV UV resonance lines of HD 64760
(B0.5 Ib). We discuss important effects of the hydrodynamic input parameters on
these large-scale equatorial wind structures that determine the detailed
morphology of the DACs computed with 3D transfer. The best fit model reveals
that the CIR in HD 64760 is produced by a source at the base of the wind that
lags behind the stellar surface rotation. The non-corotating coherent wind
structure is an extended density wave produced by a local increase of only 0.6
% in the smooth symmetric wind mass-loss rate.
