Bred vectors are constructed as finite amplitude, finite time
approximations of Lyapunov vectors. The fact that we use
them locally, rather than globally, greatly reduces the
number of independent bred vectors needed (Kalnay et al,
Since the bred vectors represent the instabilities of the
underlying flow, and these are finite time, finite space, it is
reasonable to assume that there may be instabilities that
the bred vectors “miss” if they are confined to a too small
Therefore, “sprinkling” bred vectors with small random
perturbations not only mimics observational errors but
keeps them “young” and gives them a better chance to
represent new instabilities that may appear in the analysis