Phase field modeling is carried out to investigate the convective and morphological instability during directional solidification of a succinonitrile/acetone alloy. Considering the presence of gravity, we have found that the planar interface could become wrinkled even beyond the Mullins-Sekerka instability; this is originated from the lateral solute segregation induced by the flow. For the cases slightly above the onset of instability, morphologies of shallow cells are affected by the convection as well. The cells with different wavelengths and depths can coexist due to the flow-induced segregation. The coupling of long-(convective mode) and short wavelengths (morphological mode) is illustrated for the first time, which cannot be predicted by the linear stability theory. As the growth rate is further increased, the effect of the buoyancy decreases. (c) 2006 Elsevier B.V. All rights reserved.