The Naval Research Laboratory high-latitude ionosphere-magnetosphere mesoscale coupling model has been generalized to include scale-size-dependent magnetospheric coupling. With this model, the nonlinear evolution of plasma interchangelike instabilities and processes in the high-latitude ionosphere has been studied using computer simulation techniques. We find that the principal effect in the high-latitude ionosphere of magnetospheric coupling, i.e., increased inertial effets, is most pronounced for long-wavelength interchange-driven ionospheric fluctuations of several tens of kilometers and greater. For scale sizes of a few kilometers to tens of kilometers the phenomenology and time scales of the nonlinear evolution of ionospheric interchange modes can be characterized as neither purely inertial (magnetospheric control) nor purely collisional (ionospheric control). For scale sizes less than a few kilometers the nonlinear interchange evolution can be described as a purely collisional process. Density and electric field fluctuation power spectra have been computed from the simulations and found to be in agreement with experimental observations. With scale-size-dependent magnetospheric coupling, spectral breaks at scale sizes of the order of a few kilometers are observed in the density and eletric field spectra. Applications to mesoscale dynamics, structure, and the stability of convecting inhomogeneities and large-scale plasma density structures in the high-latitude ionosphere have been made.