  In this paper, we derive a pinching estimate on the traceless Ricci curvature
in term of scalar curvature and the C^{1} norm of the Weyl tensor under the
Laplacian G_{2} flow for closed G_{2} structures. Then we apply this estimate
to study the long time existence of the Laplacian G_{2} flow and prove that the
C^{1} norm of the Weyl tensor has to blow up at least at a certain rate under
bounded scalar curvature.
