  The microturbulent approximation of turbulent motions is widely used in
radiative transfer calculations. Mainly motivated by its simple computational
application it is probably in many cases an oversimplified treatment of the
dynamical processes involved. This aspect is in particular important in the
analysis of maser lines, since the strong amplification of radiation leads to a
sensitive dependence of the radiation field on the overall velocity structure.
To demonstrate the influence of large scale motions on the formation of maser
lines we present a simple stochastic model which takes velocity correlations
into account. For a quantitative analysis of correlation effects, we generate
in a Monte Carlo simulation individual realizations of a turbulent velocity
field along a line of sight. Depending on the size of the velocity correlation
length we find huge deviations between the resulting random profiles in respect
of line shape, intensity and position of single spectral components. Finally,
we simulate the emission of extended maser sources. A qualitative comparison
with observed masers associated with star forming regions shows that our model
can reproduce the observed general spectral characteristics. We also
investigate shortly, how the spectra are effected when a systematic velocity
field (simulating expansion) is superposed on the fluctuations. Our results
convincingly demonstrate that hydrodynamical motions are of great importance
for the understanding of cosmic masers.
