An asymptotic technique to solve Markov's parabolic equation for the second-order spaced position and frequency coherence function is discussed. Rather than employing separation of variables, the technique is based on the quasi-classic representation in terms of complex trajectories and is also valid in the case of a nonhomogeneous background medium and does not demand the statistical homogeneity of fluctuations. It has no constraints relevant to the initial conditions in the form of an incident plane wave and produces in automatic fashion different known rigorous solutions, in particular, to the case of quadratic structure function.