In this Brief Communication, we use results from two-dimensional simulation
s to calculate for what product of perturbation wave number (k) and initial
amplitude (a(0)) the Richtmyer impulsive model will fail in its prediction
of shock wave driven instability growth as a function of initial shock Mac
h number. For "large" ka(0), we suggest a formula which bounds the maximum
value of the ratio of the amplitude growth rate over the interface speed by
one minus the ratio of the interface speed and the transmitted shock speed
. (C) 2000 American Institute of Physics. [S1070-6631(00)00208-7].