  We use spatio-temporal cross-correlations of slopes from five Shack-Hartmann
wavefront sensors to analyse the temporal evolution of the atmospheric
turbulence layers at different altitudes. The focus is on the verification of
the frozen flow assumption. The data is coming from the Gemini South
Multi-Conjugate Adaptive Optics System (GeMS). First, the Cn2 and wind
profiling technique is presented. This method provides useful information for
the AO system operation such as the number of existing turbulence layers, their
associated velocities, altitudes and strengths and also a mechanism to estimate
the dome seeing contribution to the total turbulence. Next, by identifying the
turbulence layers we show that it is possible to estimate the rate of decay in
time of the correlation among turbulence measurements. We reduce on-sky data
obtained during 2011, 2012 and 2013 campaigns and the first results suggest
that the rate of temporal de-correlation can be expressed in terms of a single
parameter that is independent of the layer altitude and turbulence strength.
Finally, we show that the decay rate of the frozen-flow contribution increases
linearly with the layer speed. The observed evolution of the decay rate
confirms the potential interest of the predictive control for wide-field AO
systems.
