I'd like to know a notion for a morphism between algebraic varieties in characteristic $p$ that plays the role of a $C^{\infty}$-fiber bundle. It should be, in particular, flat. I'm not assuming the morphism to be smooth; e.g. a family of nodal curves $X_t$ with $p_a=2$ and such that the $j$-invariant of the normalizations $\widetilde{X}_t$ is not constant in $t,$ would certainly qualify.

**Motivation:** If $f:X\to Y$ is a proper morphism of complex algebraic varieties, then by Morse theory, there exists a "stratification of $f";$ in particular, over each stratum of $Y,\ f$ is a $C^{\infty}$-fiber bundle. I wonder if this could be true in char. $p,$ but the first thing is to have an analogous notion.