def compute_relBIAS(da_fc, da_true, dims='time'):
    """ Compute the relative bias from two xr.DataArrays given some dimensions
    
    Parameters
    ----------
    da_fc : xr.DataArray
        Forecast. Time coordinate must be validation time.
    da_true : xr.DataArray
        Labels
    dims (str): 
        Dimensions over which to compute the metric
    
    Returns
    -------
    relBIAS : xr.DataArray
        Relative bias
    """
    error = da_fc - da_true
    rbias = error.mean(dims) / da_true.mean(dims)
    return rbias