We examine the gamma-gamma and lognormal distributions as they apply to the irradiance at a point detector produced by partially coherent beams propagating horizontally through atmospheric turbulence. Our investigation compares the probability density functions and probability of fade predicted by the distributions with results from a wave-optics simulation developed for partially coherent beam propagation. For a partially coherent beam that is not too far removed from a coherent beam, we find the wave-optics results tend to the gamma-gamma model for the weak fluctuation regime and the results are closer to the lognormal model for the strong fluctuation regime. We observe that increasing the initial beam size/Fried parameter ratio (w(0)/r(0)) or shortening the coherence length (l(c)) tends to narrow the probability density profile produced by the simulation.