Dynamics and selection of giant spirals in Rayleigh-Benard convection

For Rayleigh-Benard convection of a fluid with Prandtl number sigma = 1.4, we report experimental and theoretical results on a pattern selection mecha nism for cell-filling, giant, rotating spirals. We show that the pattern se lection in a certain limit can be explained quantitatively by a phase-diffu sion mechanism. This mechanism for pattern selection is very different from that for spirals in excitable media.