The middle and upper atmosphere (MU) radar incoherent scatter observations of electron density Ne profiles for 1986--1995 at Shigaraki (34.85 ¿N, 136.10 ¿E) show a forenoon density depression (bite-out) in summer both on individual days and in statistical averages. The depression takes place generally between 0700 and 1200 LT, above the F2 peak for low solar activity and around the F2 peak for high solar activity. The maximum electron density NmF2 shows, in both percentage and time duration, a greater depression for low solar activity than for high solar activity. With increasing altitude, the forenoon Ne depression develops earlier for low solar activity. In combination with the MU radar observations of ion drifts and plasma temperatures, our modeling indicates the following: (1) Transport processes such as the enhancement of electron temperature Te, the poleward neutral wind, and the reduced upper boundary ionization density help to reduce Ne and result in a negative ∂Ne/∂t in the morning hours in summer. Among those processes the morning enhancement of Te is found to be most important for the presence of the forenoon bite-out, since it greatly increases the O+ diffusion such that ionization outflow is significantly intensified. (2) The forenoon bite-out occurs only in summer, not in other seasons, because the smaller atmospheric composition ratio [O>/[N2> in summer leads to a smaller daytime background Ne and thus to an enhanced Te, and the after-sunrise photochemical contributions (mainly the O+ production) are so small that ∂Ne/∂t also tends to be smaller. These two conditions give prominence to the transport contributions. (3) The solar activity dependences of the forenoon bite-out can be explained basically in terms of the low-electron-density background. (4) The height of formation of the forenoon bite-out is also related to the sunrise effect on photoionization.