advection_part

    # Advection: upwind approach: 
    [nz,nx]=np.shape(fin)
    for i in range(1,nx-1):
        for j in range(0,nz):
            if vx[j,i]>=0:
                dfdtx=vx[j,i]*(fin[j,i-1]-fin[j,i])/dx
            else:
                dfdtx=vx[j,i]*(fin[j,i]-fin[j,i+1])/dx
            dfdt[j,i]=dfdt[j,i]+dfdtx
    for i in range(0,nx):
       for j in range(1,nz-1):
            if vz[j,i]>=0:
                dfdtz=vz[j,i]*(fin[j-1,i]-fin[j,i])/dz
            else:
                dfdtz=vz[j,i]*(fin[j,i]-fin[j+1,i])/dz
            dfdt[j,i]=dfdt[j,i]+dfdtz