def u8p(d,t):
      """
      Compute horizontal wind u at mesh cell centers a.k.a. p-points
      :param d: open NetCDF4 dataset
      :param t: number of timestep
      """
      u = d.variables['U'][t,:,:,:]
      return 0.5*(u[:,:,0:u.shape[2]-1]+u[:,:,1:])