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We study microstructure selection during directional solidification of a thin metallic sample. We combine in situ X-ray radiography of a dilute Al-Cu alloy solidification experiments with three-dimensional phase-field simulations. We explore a range of temperature gradient G and growth velocity V and build a microstructure selection map for this alloy. We investigate the selection of the primary dendritic spacing Λ and tip radius ρ. While ρ shows a good agreement between experimental measurements and dendrite growth theory, with ρ∼ V− 1/2, Λ is observed to increase with V (∂ Λ/∂ V> 0), in apparent disagreement with classical scaling laws for primary dendritic spacing, which predict that∂ Λ/∂ V< 0. We show through simulations that this trend inversion for Λ (V) is due to liquid convection in our experiments, despite the thin sample configuration. We use a classical diffusion boundary-layer approximation to semi …