Results from the three-dimensional nonlinear simulations of the gradient drift instability to study structuring in high-latitude plasma patches are presented. Simulations demonstrate that the existence of the mesoscale structures (10 km to 100 m) on both leading and trailing edges and inside a patch is due to (1) the nonlinear development of the gradient drift instability (GDI) or due to (2) occasional reversal of the direction of convection. The high-resolution simulation data have a dynamic range that allows us to make detailed comparisons with the observed density and velocity fluctuation spectra. We use the measured ion density and horizontal velocity data from the Dynamic Explorer 2 (DE 2) spacecraft examined by Kivan¿ and Heelis <1997> to compare them with our simulations. Density structures in the leading and trailing edges of patches have been examined, and comparison of statistical characteristics of the average density gradient on these edges with those in the work by Coley and Heelis <1998> has been performed. The dynamics and evolution of the density fluctuations over the realistic time scale are shown, thereby providing a significant evidence of the existence of the GDI at high latitudes as a structure-generating mechanism and a first principle understanding of the hierarchy of the instabilities involved in the observed structuring in high-latitude plasma patches.