Currently users are prevented from setting an attribute named missing_value on their cubes. However, since Iris will never automatically create one, this means there is no way to save a cube to a netCDF file and include a missing_value attribute.

For indicating missing values iris will always save to netCDF with a _FillValue attribute. The CF conventions recommend that if both _FillValue and missing_value attributes are used then they should have the same value (sensible), but do not prohibit them from being different. Therefore I propose that missing_value becomes an attribute that is allowed to be set on cubes, since the effects of doing so do not conflict with CF conventions.