Horizontal flow convergence
= Horizontal flow convergence
{title2=$C=-\nabla_h\mathbin\cdot\mathbf u$}
Horizontal flow convergence is the negative horizontal <divergence> of a <velocity> field. Positive convergence reduces the horizontal area of a material patch; for constant-depth linear <shallow water equations>, the surface equation is $\eta_t=HC$, so convergence raises the free surface and divergence lowers it. This is a fluid-flow notion, distinct from convergence of a numerical sequence.