function[Qx_b,Qy_b]=so_diff_backward(Q,JM,KM,dx,dy)

Qx_b(JM,KM,:)=.5*(Q(JM-2,KM,:)-4*Q(JM-1,KM,:)+3*Q(JM,KM,:))/dx;
Qy_b(JM,KM,:)=(Q(JM,KM,:)-Q(JM,KM-1,:))/dy;