[CF-metadata] Standard names for mean sea level change

Jonathan Gregory j.m.gregory at reading.ac.uk
Wed Jun 14 09:58:20 MDT 2017


Dear Roy and Alison

> To clarify, what I was getting at with the rationalisation of syntax was mixed use of 'average' and 'mean' for the same statistic and the fact that the names are opposite ways around. For example, instead of thermosteric_change_in_mean_sea_level and global_average_thermosteric_sea_level_change why not have average_thermosteric_sea_level_change and global_average_thermosteric_sea_level_change (possibly with all averages changed to means)?

The use of "average" rather than "mean" (my choice, I admit) was made to avoid
confusion with "mean sea level". However if we're introducing "mean" now anyway
I wouldn't mind changing "average" to "mean", since "mean" is more common.
However, there is some subtlety and potential for confusion still!

global_mean_[thermosteric_]sea_level_change is not the [thermosteric] change in
global mean sea level, because there is no such quantity as "global mean sea
level" without "change".  The "mean" here means a spatial average. Moreover,
you don't necessarily calculate these global quantities as a global mean of
local quantities, because they really refer to change in the volume of the
world ocean, divided by world ocean surface area. If they were really spatial
means, we could use cell_methods to describe them instead of distinct standard
names.

thermosteric_change_in_mean_sea_level is the thermosteric change in (local)
mean sea level. The "mean" here means a temporal average.

> Finally, thinking about it my concerns about these new names being abused could be alleviated by the following definition:
> 
> "Mean sea level" means the time mean of sea surface elevation at a given location over an arbitrary period sufficient to eliminate the principal tidal signal. Zero mean sea level change is an arbitrary level.

That is fine with me, especially if it alleviates Roy's concerns. I would
slightly prefer "tidal signals" instead of "principal tidal signal". Roy is
quite right that climate models don't usually have tides anyway. For precision
in the real world it is essential to specify a particular geopotential datum,
since MSL is vague. Nonetheless "above MSL" is a commonly used phrase.

Best wishes

Jonathan



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