In this paper, we tackle the problem of scheduling an Ocean-Atmosphere application used for climate prediction on thegrid. An experiment is composed of several 1D-meshes of identical DAGs composed of parallel tasks. To obtain a good??completion time, we divide groups of processors into sets each working on parallel tasks. The group sizes are chosen by computing the best makespan for several grouping possibilities. We improved this heuristic method by different means. The improvement yielding to the best makespan is the representation of the problem as an instance of the Knapsack problem. As this heuristic is firstly designed for homogeneous platforms, we present its adaptation to heterogeneous platforms. Simulations show improvements of the makespan up to 12\%.