Standard names: Adding reference epoch sentence to anomaly terms

[feggleton]

Proposer's name Francesca Eggleton (not original proposer)
Date 20-10-2020

In #64 it was mentioned that we should add a sentence about using the reference_epoch into the definition of terms which are anomalies or related. The following terms were suggested:

air_pressure_anomaly | Pa | | 26
air_temperature_anomaly | K | | 25
brightness_temperature_anomaly | K | |
geopotential_height_anomaly | m | | 27
ratio_of_sea_water_potential_temperature_anomaly_to_relaxation_timescale | K s-1 | |
ratio_of_sea_water_practical_salinity_anomaly_to_relaxation_timescale | s-1 | |
sea_water_temperature_anomaly | K | |
surface_temperature_anomaly | K | |

change_over_time_in_sea_water_absolute_salinity
isccp_cloud_area_fraction
sea_water_absolute_salinity

The comment can be seen here: #64 (comment)

Sentence suggested: "To specify the reference (baseline) epoch to which the quantity applies, provide a scalar coordinate variable with standard name reference_epoch."

Please add any other names or comments about this below.

[taylor13]

Although "climatology" is used in isccp_cloud_area_fraction, it simply expands the acronym: ISCCP = International Satellite Cloud Climatology Project. There is no climatological period needed or used in defining isccp_cloud_area_fraction.

Similarly, the use of climatology in the description of change_over_time_in_sea_water_absolute_salinity and sea_water_absolute_salinity may not imply a climatological period is needed in defining these quantities. Perhaps an ocean expert can comment.

[larsbarring]

@taylor13 I added those three standard names to the list based on a simple search for the term climatology in standard name descriptions, just so that any such standard name should not be accidentally overlooked. The intention was by no means to imply that they ought to have a reference period specified but rather to give the opportunity for others to step in and say, as you just did, that this is not necessary/relevant. As a matter of understanding (and not arguing with you!): as the specified "climatology" represents the "ISCCP epoch" could there be a value for data users to have this specified in the metadata even though the project as such is widely known?

@feggleton I think the suggested sentence to add is well crafted, but from @davidhassell's comment I got the impression that adding such a sentence to the definition of a standard name may have implications for the existing usage of the term. Is that right?

[taylor13]

Understood. Thanks for identifying all the variables that warranted some review.

Regarding the "ISCCP epoch", my understanding is that although the algorithms developed to determine the isccp_cloud_area_fraction were developed during a particular time, these algorithms do not depend to any significant degree on the climate of that period. They can be applied to any cloud field.

[larsbarring]

Thanks for clarifying, much appreciated! /Lars

[martinjuckes]

Does the reference_epoch have to be a scalar? E.g. I could have 3 time series in an array for 3 different reference times, float tanom(time,series);

float series(3):
    series:standard_name = "reference_epoch" ;
    series:flag_values = 1,2,3;
    series:flag_meanings = "tar_reference", "ar4_reference", "ar5_reference" ;

where TAR, AR4 and AR5 refer to different IPCC assessment reports, which used different reference periods.

Could the rule here be the same as for any other coordinate variable: the dimensions of the reference time for variable X should be a subset of the dimensions of X.

The definition of air_pressure_anomaly says that it is the anomaly relative to a climatology ... which implies that the reference could have 12 monthly values. This would require a coordinate variable with a climatology attribute. A note saying that bounds or climatology attributes should be used to specify the extent to the reference period(s) could be added.

[github-actions]

This issue has had no activity in the last 30 days. This is a reminder to please comment on standard name requests to assist with agreement and acceptance. Standard name moderators are also reminded to review @feggleton @japamment

[larsbarring]

@martinjuckes: I think that your examples/use cases make a lot of sense. As does the rule you refer to

... the same as for any other coordinate variable: the dimensions of the reference time for variable X should be a subset of the dimensions of X.

The description of teh reference_epoch standard name should then be expanded to reflect this wider usage.

[sethmcg]

I have a few additional standard names that take a reference_epoch:

keetch_byram_drought_index uses in its calculations the annual average precipitation, which is defined over some reference period.

modified_fosberg_fire_weather_index uses KBDI as an input, so it should inherit the reference_epoch from it.

nfdrs_severe_fire_danger_index is based on percentile values of ERC and BI, which need to be defined with regard to some (common) reference period.

What do we need to do to add some reference_epoch boilerplate added to the descriptions of those standard names?