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Non-universal definition of specific humidity #36
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Pinging @svahl991, @ligiabernardet. |
I am in favor of making this change. CCPP after all is a model-independent effort and shouldn't give priority to one or another. |
I am also in favor of the change. Different standard names for different physical quantities is supposed to be one of the defining features of the CCPP. |
Thank you for bringing this issue up! Agreed that specific humidity can be ambiguous as some models define moist air as dry air + water vapor (hence moist air only includes the gaseous components of air) whereas other models include condensates in moist air in addition to dry air+water vapor. If condensates are included then there is the question of which ones (which is model specific). A way to avoid these ambiguities (except which condensates are in moist air) could be: water_vapor_mixing_ratio_wrt_dry_gas ps. the same argument can be made for hydrostatic pressure (does the weight of the column incl. condensates or not?) |
Thanks @PeterHjortLauritzen for your comment. I think rule 5, quoted above, proposes a clear way to avoid this ambiguities:
which gives the following options: water_vapor_mixing_ratio_wrt_dry_air About which condensates are included in the total mass or not, I think there is no ambiguity within a given model. Each model includes all available information to approximate total mass as best as it can. If someone faces a special case where the ambiguity does need to be removed, further names could be added (for instance, water_vapor_mixing_ratio_wrt_moist_air_and_cloud_water). I think this may be beyond the scope of this issue though. |
Your proposal probably avoids ambiguity except for water_vapor_mixing_ratio_wrt_moist_air. Different modeling groups define moist air differently. In the UK Met Office's Joy of U.M. documentation section 1.5 defines moist air as dry air + all phases of water. Whereas your definition only includes dry air and water vapor. Link to documentation here: ftp://wxmaps.org/pub/kjin/UM/p015.pdf I also find it a little inconsistent to use "_mass" in one of the definitions and no mention of mass in the other definitions. That said, if there is consensus to use water_vapor_mixing_ratio_wrt_dry_air then I won't drag out this discussion further ... |
@MayeulDestouches, can you elaborate on this? Where is the model boundary? Can parameterizations coming via the CCPP make different assumptions? |
FYI The UK MetOffice uses the CCPP standard names as part of the JEDI-UM model interfaces. They don't use it for the physics. (JEDI adopted the CCPP standard names to describe variables in the model interfaces.) |
About
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Maybe my understanding was a bit naive: if a global scale models has a cloud liquid water variable and a cloud ice variable but no rain, snow or graupel, it can only compute |
@MayeulDestouches: I agree that _wrt_dry_air would be a good compromise. ps. I think "_wrt_moist_air_and_cloud_condensed_water" would be a bit confusing if a model has prognostic rain, graupel etc. since they are not necessarily part of a cloud ... |
@PeterHjortLauritzen, I proposed So a model with precipitating hydrometeors could compute both (1) and (2), while a model with only cloud liquid water and cloud ice water can only use (1). This is supposed to (partly) answer the question of which condensates were included or not in the (soon deprecated) |
Variable
specific_humidity
is defined in rule 5 of the CCPP Standard Name Rules.This definition as an alias of
mixing_ratio_of_vapor_wrt_moist_air
is not universal. At the UK Met Office for instance, what we call specific humidity ismixing_ratio_of_vapor_wrt_total_mass
. This definition is used consistently across all our code, from NWP to climate. The definition here,_wrt_moist_air
, is used in the UFS/GFS physics (according to @climbfuji).There seems to be no agreement on what specific_humidity really means. I would have liked the definition to be changed to the Met Office's one, but a better and more acceptable solution would be to simply remove this ambiguous name in the CCPP standard.
water_vapor_mixing_ratio_wrt_dry_air
is already a CCPP name, I propose to addwater_vapor_mixing_ratio_wrt_moist_air
andwater_vapor_mixing_ratio_wrt_total_mass
as a replacement forspecific_humidity
.Any comment welcome.
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