  We use a quantum trajectory-based semi-classical method to account for
Coulomb interaction between the photoelectron and the parent ion in the
classically forbidden, sub-barrier region during strong-field tunneling
ionization processes. We show that---besides the well-known modification of the
tunneling ionization probability---there is also an influence on the
interference pattern in the photoelectron spectra. In the long-wavelength
limit, the shift of the intra-cycle interference fringes caused by sub-barrier
Coulomb effects in the laser polarization direction can be derived
analytically. We compare our results with \emph{ab initio} solutions of the
time-dependent Schr\"odinger equation and find good agreement in the
long-wavelength regime, whereas the standard strong field approximation fails.
We show that the nodal structure along low-order above-threshold ionization
rings is also affected by sub-barrier Coulomb effects.
