Abstract: Based on the simulations of a two-dimensional multicell storm under the condition of inverse shear (Uz<0) in the lower levels by a mesoscale numerical model, a new mechanism for the cell regeneration and development within the multicell storm at the “sub-optimal shear” state is proposed. In the presence of a low-level inverse shear, the circulation caused by the surface cold pool is not counteracted by that by the ambient wind shear, and the density current extends out faster, making the multicell storm stay at the “sub-optimal shear” state. As a result, a new cell is triggered by the strong vertical perturbation ahead of the mature convection, rather than the split up from the updraft before the gust front at the leading edge of the surface cold pool . The latter is the mechanism at the “optimal” state proposed by Lin et al. (1998). In the new mechanism suggested in this paper, the regenerated cell grows fast owing to the incident warm moist air from the upstream of the multicell storm, and tends to prevent the moist airflow from entering into the mature convection at its western side. Consequently, the mature convection would weaken, be replaced, and eventually decay. Actually, these two different mechanisms come into play in a way depending on the relationship between the circulation of the low-level shear and that of the cold pool. When the circulation of the cold pool is stronger than that of the wind shear, the multicell storm is at the “sub-optimal shear” state, and the new convective cell is produced by the disturbance ahead of the mature cell. When the circulation of the cold pool is weaker, the cell regeneration is dominated by the mechanism at the “optimal” state with the new cell split from the gust front updraft. Therefore, these two mechanisms are not contradictive. With a moderate low-level inverse shear, they can alternately operate within a multicell storm.