In order to obtain striation power spectra and to understand the nonlinear saturated state of the gradient drift instability, which is believed to be responsible for the striations in plasma clouds coupled to the background ionosphere, a two-level (one for the plasma cloud and one for the background ionosphere), two-dimensional numerical simulation has been performed. The plasma cloud is taken to be in the F region, the background ionospheric level is in the lower F or the E region, and the temperature is taken to be zero at both levels. The plasma cloud is initially assumed to be Gaussian in one direction (y) and uniform in the direction (x) of the ambient electric field, and the ambient magnetic field is uniform in the z direction. The cloud is initialized with random perturbations, and a high-resolution numerical simulation is carried out over the entire two-dimensional (x, y) mesh. In the linear phase of the simulation, striations grow on the back side of the cloud and image striations appear in the background ionosphere. The nonlinear phase is characterized by pinching of the original perturbations within the cloud, production of secondary perturbations, a bubbling through of the back side striations to the front side of the cloud, and the appearance of large image striations in the background ionosphere. The modal development shows taht at late times in the nonlinear phase the power spectrum for the density fluctuations obeys a power law dependence in k space. Specifically, the x one-dimensional power spectrum ∝kx-nx where nx?2--3 (for wavelengths between 2 and 100 km), and the y one-dimensional power spectrum ∝kx-ny where ny?2 (for wavelengths between 2 and 100 km). This nonlinear modal evolution favors long wavelengths, whereas the linear theory for our model system show that the growth rate maximizes at short wavelengths. Such nonlinear inversion may be responsible for explaining the long-wavelength preferred striation scale size seen in barium plasma clouds.