  We establish the variational principle of Kolmogorov-Petrovsky-Piskunov (KPP)
front speeds in temporally random shear flows inside an infinite cylinder,
under suitable assumptions of the shear field. A key quantity in the
variational principle is the almost sure Lyapunov exponent of a heat operator
with random potential. The variational principle then allows us to bound and
compute the front speeds. We show the linear and quadratic laws of speed
enhancement as well as a resonance-like dependence of front speed on the
temporal shear correlation length. To prove the variational principle, we use
the comparison principle of solutions, the path integral representation of
solutions, and large deviation estimates of the associated stochastic flows.
