Numerical simulations of the F region ionosphere over Arecibo, Puerto Rico, are presented. We deduce from these simulations that the dynamic behavior of the Arecibo ionosphere, particularly the so-called ''midnight collapse'' phenomenon, is strongly influenced by the upward propagating semidiurnal tidal component of the neutral meridional wind excited below thermospheric levels. Presence of a terdiurnal component in the wind field improves the agreement and could result from nonlinear interactive coupling between the neutral wind and ion drag. The steep underside density gradients occurring in conjunction with the collapse are due to vertical shear in the meridional wind field associated with the propagating semidiurnal tide. These gradients are capable of triggering the gradient drift instability, thus accounting for plasma irregularity formation and observations of VHF scintillations associated with the collapse phenomenon.