function [Qx_c,Qy_c] = diff_cen(Q,JM,KM,dx,dy)

Qx_c(JM,KM,:) = 0.5*(Q(JM+1,KM,:) - Q(JM-1,KM,:))/dx;
Qy_c(JM,KM,:) = 0.5*(Q(JM,KM+1,:) - Q(JM,KM-1,:))/dy;