  We present both full quantum mechanical and semiclassical calculations of
above threshold ionization (ATI) of a hydrogen atom in the tunneling regime by
a few-cycle linearly polarized infrared laser pulse. As a quantum treatment, we
applied the direct integration of the time dependent Schr\"odinger equation
(TDSE). In the semiclassical approximation (SCA), it is assumed that wavepacket
propagation in the post-tunneling process can be well described within the
classical framework. With these two methods, we analyze the similarities and
deviations for ionization of the hydrogen atom. We found that the 3 dimensional
semiclassical method can describe reasonably well the momentum correlation
pattern of the ATI peaks. We also show good agreement between the results
obtained by TDSE method and the semi-classical method. Furthermore, with the
semiclassical approximation we clearly identify and separate the regions in
momentum distributions of the ejected electrons according to initial
conditions. We illustrate the corresponding regions with typical electron
trajectories.
