[CF-metadata] Request for new standard names for climatological statistics based on thresholds

Bärring Lars Lars.Barring at smhi.se
Tue Mar 28 09:02:02 MDT 2017


Hi Dan, all

Thanks for these pointers. Obviously I somehow managed to miss this thread when searching the mail archive (which I actually did try despite all evidence to the contrary). I am sorry for that.

Anyway, in an attempt to restart this discussion let me my summarising the previous thread:

It started off from the need to distinguish between dry and wet days according to a threshold of some small precipitation amount (say 0.2 mm[/day]), that typically had been observed at limited precision (thus discretized). For floating point data from models this not a problem and the arguments were that it would be more appropriate to take actual limits induced by the discretization into account. 

There was a suggestion to use the existing standard name but adding a (non CF) logical flag attribute to handle non-strict inequality. But this met opposition as it would split the description of the data into two places (the standard name and the logical flag attribute). 

It was noted that a strict and a non-strict inequality are indeed two different things and should not be mixed up in the context of standard names.
The last comment was to suggest that the way forward would be to rename (redefine) all the relevant standard names to include non-strict inequalities. But, as was noted, this seems cumbersome.

I hope this is a reasonable summary of that thread. With this background, let me add some real use-cases that may shed some new light on the need to make the distinction.

I have in previous posts referred to the two WMO/WCRP/etc. expert teams ETCCDI and ET-SCI defining and producing core sets of climate indices (aka climate indicators). I am member of neither and have no vested interests, other than I do see the need to make this kind of data more easily available in a unified way.

ETCCDI defines Summer Days ("SU") as the number of days when the daily maximum temperature is strictly above +25 degC. This perfectly handled by the standard name number_of_days_with_air_temperature_above_threshold.

ET-SCI defines Hot Days ("TXge30" previously known as "SU30") as the number of days when the daily maximum temperature is equal to or above +30 degC. In structure it similar to SU with the crucial exception of a non-strict inequality. And, this small but crucial difference in definition cannot be changed for any number reasons.

Now, in various user-oriented tools and web services the requirement is to allow the users to change the threshold value to enable them to explore the data and carry out simple sensitivity studies on their own. Thus, there is a concrete need to handle the situation of having the ETCCDI index SU modified to use the threshold value +30 degC (in which case it might perhaps be be called "SU30"). This was in fact the very reason for renaming the ET-SCI index "SU30" to "TXge30" to avoid misunderstandings. Likewise, ET-SCI index TXge30 can be modified to use the threshold value +25 degC (which then would be called "TXge25"). 

To facilitate the dissemination of these kinds of datasets to a wider community it would indeed be helpful to have standard names that reflect the perhaps small but still crucial difference between strict and non-strict inequalities.

I therefore like to again raise the suggestion to introduce the two constructs "._at_or_above_threshold" and "._at_or_below_threshold" for the relevant standad names. 

As I have outlined, we have real use-cases that go beyond what was the driver for the discussion in the previous thread. Moreover, with nine new standard names requested in my initial post the additional burden on the standard name space is modest. However, if this is a concern the use cases I have outlined currently cover the following standard names
number_of_days_with_air_temperature_above_threshold
number_of_days_with_air_temperature_below_threshold
number_of_days_with_lwe_thickness_of_precipitation_amount_above_threshold
The remaining six I included without having an exhaustive overview of what climate indices are used out there, and as they are identical in their structure they could be changed at the same in one go without having to revisit this issue again every time the need pops up. 


Regards,
Lars 


--
Lars Bärring

FDr, Forskare
PhD, Research Scientist

SMHI  /  Swedish Meteorological and Hydrological Institute 
Rossby Centre 
SE - 601 76 NORRKÖPING 
http://www.smhi.se

E-post / Email: lars.barring at smhi.se
Tel / Phone: +46 (0)11 495 8604  
Fax: +46 (0)11 495 8001 
Besöksadress / Visiting address: Folkborgsvägen 17





-----Original Message-----
From: Hollis, Dan [mailto:dan.hollis at metoffice.gov.uk] 
Sent: den 27 mars 2017 13:06
To: Bärring Lars; cf-metadata at cgd.ucar.edu
Subject: RE: Request for new standard names for climatological statistics based on thresholds

Hi Lars,

I raised a very similar question a couple of years ago:
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2014/057605.html

The outcome was inconclusive. One suggestion was to add a Boolean attribute that indicated whether the threshold value was included or not. Others thought that adding more standard names was the way to go, while others thought that a single name was sufficient.

I encourage you to read through the rest of the thread and see if it helps with your current request:
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2014/thread.html#57605

For info, we continue to use the existing standard names even though they do not strictly match the definitions of our 'days of rain' statistics.

Regards,

Dan


Dan Hollis   Climatologist
Met Office   Hadley Centre   FitzRoy Road   Exeter   Devon   EX1 3PB   United Kingdom
Tel: +44 (0)1392 884535   Mob: +44 (0)7342058682   Fax: +44 (0)1392 885681
E-mail: dan.hollis at metoffice.gov.uk   Website: http://www.metoffice.gov.uk For UK climate and past weather information, visit http://www.metoffice.gov.uk/climate


-----Original Message-----
From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Bärring Lars
Sent: 24 March 2017 08:10
To: cf-metadata at cgd.ucar.edu
Subject: [CF-metadata] Request for new standard names for climatological statistics based on thresholds

Dear all,
 
Several standard names oriented towards climate indices for various impacts are based on thresholds, and the standard name includes the construct "..._above_threshold" or "..._below_threshold".  However, several well-established climate indices use non-strict inequalities in their definition. 
 
For model output using floating point precision the difference between using a strict and a non-strict inequality is small or even negligible, but for observational data discretized to some limited precision (typically one or no decimal digit) this makes a difference. 
 
At a workshop last week people involved in WMO/CCl Expert Team on Sector-specific Climate Indices (ET-SCI) and the joint CCl/WCRP/JCOMM Expert Team on Climate Change Detection and Indices (ETCCDI), as well as the European ECA&D programme and several research projects discussed this. 
 
The outcome of these discussions is to suggest new standard names similar to the existing ones but using the contructs "..._at_or_above_threshold" and "..._at_or_below_threshold". In all other respects these new standard names should be patterned after the following existing ones:
 
number_of_days_with_air_temperature_above_threshold
number_of_days_with_air_temperature_below_threshold
number_of_days_with_lwe_thickness_of_precipitation_amount_above_threshold
number_of_days_with_surface_temperature_below_threshold
number_of_days_with_wind_speed_above_threshold
spell_length_of_days_with_air_temperature_above_threshold
spell_length_of_days_with_air_temperature_below_threshold
spell_length_of_days_with_lwe_thickness_of_precipitation_amount_above_threshold
spell_length_of_days_with_lwe_thickness_of_precipitation_amount_below_threshold
 
The specific use cases for these extension are several ET-SCI defined indices that involves non-strict inequalities. 
 
The alternative of changing the ET-SCI definitions to use a strict inequality is not an option because they have been painstakingly defined in collaboration with user communities and/or are directly related to well-established operational usage. 
 
Likewise, to just adjust the threshold in order to turn the non-strict inequality to a strict equality (say from 30 C to 29.9 C or 29.99 C or ...) is not attractive and prone to cause confusion.
 
Kind regards,
Lars

--
Lars Bärring

FDr, Forskare
PhD, Research Scientist

SMHI  /  Swedish Meteorological and Hydrological Institute Rossby Centre SE - 601 76 NORRKÖPING http://www.smhi.se

E-post / Email: lars.barring at smhi.se
Tel / Phone: +46 (0)11 495 8604
Fax: +46 (0)11 495 8001
Besöksadress / Visiting address: Folkborgsvägen 17


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