docs: def set_U1U2(self, U1, U2):
          """Set background zonal flow.
  
          Parameters
          ----------
  
          U1 : number
              Upper layer flow. Units: m/s
          U2 : number
              Lower layer flow. Units: m/s
          """
          if len(np.shape(U1)) == 0:
            U1 = U1 * np.ones((self.ny)) 
          if len(np.shape(U2)) == 0:
            U2 = U2 * np.ones((self.ny)) 
          #self.Ubg = np.array([U1,U2])[:,np.newaxis,np.newaxis]
          self.U1 = U1
          self.U2 = U2
          self.Ubg = np.array([U1,U2])