code: def tn_days_below(tasmin, thresh='-10.0 degC', freq='YS'):
      r
      thresh = utils.convert_units_to(thresh, tasmin)
      f1 = utils.threshold_count(tasmin, '<', thresh, freq)
      return f1
docs: r"""Number of days with tmin below a threshold in
  
      Number of days where daily minimum temperature is below a threshold.
  
      Parameters
      ----------
      tasmin : xarray.DataArray
        Minimum daily temperature [℃] or [K]
      thresh : str
        Threshold temperature on which to base evaluation [℃] or [K] . Default: '-10 degC'.
      freq : str, optional
        Resampling frequency
  
      Returns
      -------
      xarray.DataArray
        Number of days Tmin < threshold.
  
      Notes
      -----
      Let :math:`TN_{ij}` be the daily minimum temperature at day :math:`i` of period :math:`j`. Then
      counted is the number of days where:
  
      .. math::
  
          TX_{ij} < Threshold [℃]