def v8p(d,t):
      """
      Compute horizontal wind v at mesh cell centers a.k.a. p-points
      :param d: open NetCDF4 dataset
      :param t: number of timestep
      """
      v = d.variables['V'][t,:,:,:]
      return 0.5*(v[:,0:v.shape[1]-1,:]+v[:,1:,:])