  Scattering of a Gaussian wavepacket from rectangular potential barriers with
increasing widths or heights is studied numerically. It is seen that during a
certain time interval the time-evolving transmission probability increases
compared to the corresponding unperturbed cases. In the literature this effect
is known as superarrival in transmission probability. We present a
trajectory-based explanation for this effect by using the concept of quantum
potential energy and computing a selection of Bohmian trajectories. Relevant
parameters in superarrivals are determined for the case that the barrier width
increases linearly during the dispersion of the wavepacket. Nonlinear in time
perturbation is also considered.
