Hi all,

Just to summarize some of what has been said before ....

I think it is pretty clear that the solid phase can take several forms, as Martin points out.  I think the more subtle issue is that frozen water is not quite the same as solid water, if we take the strict definition of freezing as being water that has passed from the liquid state to the solid state.  Some solid water forms by passing directly from the gaseous state to the solid state (which sometimes is called "vapor deposition", not "freezing").  This process is actually important in forming particles large enough to precipitate (which often melt before arriving at the surface).  See https://en.wikipedia.org/wiki/Wegener%E2%80%93Bergeron%E2%80%93Findeisen_process .

best,
Karl

On 4/13/18 3:22 AM, Martin Juckes - UKRI STFC wrote:
Dear John,


I think the primary defining characteristic is the 3-way partitioning of water 
into liquid, vapor and solid phases. The existing CF standard names referring 
to liquid and vapor phases (e.g. cloud_liquid_water_mixing_ratio, 
rainfall_flux, atmosphere_moles_of_water_vapor, humidity_mixing_ratio) do not 
provide any explanation of the precise definition of these terms -- it is 
assumed that people know what is meant. This is OK if we are not looking at the 
edge cases (e.g. the aggregations of water molecules which occur in vapor at 
high humidity or the details of stratospheric aerosols) -- I'm not an expert 
here either, so I hope we can omit that level of detail here. Existing names 
make reference to snow, hail, graupel, cloud ice and water content of aerosols 
-- I'm not sure it this is 100% complete, but perhaps it is enough to list 
these. Will the following meet your concern:

"Solid precipitation refers to the precipitation of water in the solid phase. Water 
in the atmosphere exists in one of three phases: solid, liquid or vapor. The solid phase 
can exist as snow, hail, graupel, cloud ice, or as a component of aerosol."


regards,

Martin


________________________________
From: John Graybeal <jbgrayb...@mindspring.com>
Sent: 13 April 2018 06:32
To: CF Metadata List
Cc: Jonathan Gregory; Juckes, Martin (STFC,RAL,RALSP)
Subject: Re: [CF-metadata] CMIP6 data request: Precipitation of solid phase 
water

I agree with Martin, use solid for atmosphere and frozen for soil. It doesn’t 
jump out at me that solid water is (umm, almost?) always frozen, even though 
it’s obvious that frozen water is always solid.

I think it would be useful if the definition listed the forms (and pathways) 
that solid water can take, so that those of us less atmospherically advanced 
will be able to appreciate what it’s meant to include.

john

---------------------------------------
John Graybeal
jbgrayb...@mindspring.com


On Apr 6, 2018, at 23:49, Martin Juckes - UKRI STFC <martin.juc...@stfc.ac.uk> 
wrote:

Dear Jonathan,


my vote would be to leave soil water terms unchanged and allow the use of 
"solid" in the atmosphere.  There is sufficient difference between the 
behaviour of water in soil and in the atmosphere to justify, in my mind, this slight 
divergence in usage. After all, solid water from the atmosphere can only become frozen 
soil water, I believe, by melting, sinking into the soil and then re-freezing,


regards,

Martin


________________________________
From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of Jonathan Gregory 
<j.m.greg...@reading.ac.uk>
Sent: 06 April 2018 16:38
To: cf-metadata@cgd.ucar.edu
Subject: Re: [CF-metadata] CMIP6 data request: Precipitation of solid phase 
water

Dear Martin

I agree with you that frozen_precipitation_flux seems a bit more surprising in
some way than solid_precipitation_flux. If we put "solid" instead of "frozen",
should we change (by alias) the existing names that have "frozen", which are

frozen_water_content_of_soil_layer
lwe_thickness_of_frozen_water_content_of_soil_layer
mass_fraction_of_frozen_water_in_soil_moisture
mass_fraction_of_unfrozen_water_in_soil_moisture
soil_frozen_water_content
surface_frozen_carbon_dioxide_amount
volume_fraction_of_frozen_water_in_soil

I'm sure there must be others with useful comments to make about this.

Best wishes

Jonathan


----- Forwarded message from Martin Juckes - UKRI STFC 
<martin.juc...@stfc.ac.uk> -----

Date: Fri, 6 Apr 2018 15:06:15 +0000
From: Martin Juckes - UKRI STFC <martin.juc...@stfc.ac.uk>
To: Jonathan Gregory <j.m.greg...@reading.ac.uk>
Subject: Re: [CF-metadata] CMIP6 data request: Precipitation of solid phase
       water

Dear Jonathan,


It may be that the situation of interest in soil is a change brought about by decreasing temperature, and 
that the nature of the resulting substance is somewhat complex because of the matrix of soil that is carrying 
it. In the atmosphere I feel that applying the term "frozen" is a bit of a stretch, though I agree 
that it would be possible to state that as our intended meaning within the CF convention. On the other hand, 
we already use "liquid" (as in cloud_liquid_water) and "vapor" for the other two phases, 
so there is an argument for sticking to the standard partition solid/liquid/vapor.


Taking your other comments into account, the term would be either "frozen_precipitation_flux" or 
"solid_precipitation_flux". "frozen_precipitation" looks misleading to me, but perhaps 
that is a matter of taste.


As you say, it would be good to hear other opinions (I'll be on leave for the 
next few days, and will pick up the discussion at the end of next week),


regards,

Martin



________________________________
From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of Jonathan Gregory 
<j.m.greg...@reading.ac.uk>
Sent: 06 April 2018 14:38
To: cf-metadata@cgd.ucar.edu
Subject: Re: [CF-metadata] CMIP6 data request: Precipitation of solid phase 
water

Dear Martin

That's a good point about snow and ice. I think we should use modified help-
text to make clear that "frozen water" means any form of solid water. I can't
recall the reason for "frozen" rather than "solid" - I guess because it felt
more obvious, if less systematic. I think I'm happy to understand "frozen
water" as "water in the solid phase", however it got into that state. I wonder
if others feel differently.

Best wishes

Jonathan

----- Forwarded message from Martin Juckes - UKRI STFC 
<martin.juc...@stfc.ac.uk> -----

Date: Thu, 5 Apr 2018 17:29:08 +0000
From: Martin Juckes - UKRI STFC <martin.juc...@stfc.ac.uk>
To: "cf-metadata@cgd.ucar.edu" <cf-metadata@cgd.ucar.edu>,
       "j.m.greg...@reading.ac.uk" <j.m.greg...@reading.ac.uk>
Subject: Re: [CF-metadata] CMIP6 data request: Precipitation of solid phase
       water

Dear Jonathan,


I hadn't spotted the existing usage of frozen_water. My only reservation is that for such names the help text 
says '"frozen_water" means ice', which would exclude snow. The existing usages of 
"frozen_water" are all soil quantities for which the subtlety of the distinction between ice and 
snow is irrelevant. We could modify the help text for atmospheric variables, but is the term 
"frozen" appropriate for all solid phase water in the atmosphere?  Freezing is a process of 
transforming to solid phase through a reduction in temperature, which might be considered inappropriate for 
some atmospheric ice and snow formation pathways ... I'm not sure about this, what do you think?


regards,

Martin


________________________________
From: CF-metadata <cf-metadata-boun...@cgd.ucar.edu> on behalf of Jonathan Gregory 
<j.m.greg...@reading.ac.uk>
Sent: 05 April 2018 18:18
To: cf-metadata@cgd.ucar.edu
Subject: [CF-metadata] CMIP6 data request: Precipitation of solid phase water

Dear Martin

I agree with the need, but I note that the guidelines propose the phrase
frozen_water for solid water, and this is already used in several standard
names.

Although "precipitation" is used in the world at large for species other than
water, so far in CF standard names it's used only for water. Hence we can omit
"water" for consistency.

In view of these two points, would frozen_precipitation_flux be OK?

Best wishes

Jonathan

----- Forwarded message from Martin Juckes - UKRI STFC 
<martin.juc...@stfc.ac.uk> -----

Date: Tue, 3 Apr 2018 11:46:50 +0000
From: Martin Juckes - UKRI STFC <martin.juc...@stfc.ac.uk>
To: "cf-metadata@cgd.ucar.edu" <cf-metadata@cgd.ucar.edu>
Subject: [CF-metadata] CMIP6 data request: Precipitation of solid phase water

Dear All,


The CMIP6 data request includes a request for a variable representing the precipitation 
flux of water in a solid phase (including snow and ice). This variable was also in CMIP5. 
The current CMIP6 data request follows CMIP5 usage in adopting the standard name 
"snowfall_flux" for this variable, which is not really correct. There may have 
been a time when all solid precipitation in CMIP models was snow, but I think we need a 
more precise name now.


We have "rainfall_flux" for precipitation of liquid phase water, but in other terms the construction 
"liquid_water" is used to refer to the liquid phase, e.g. 
"mass_concentration_of_liquid_water_in_air", so "solid_water" is a natural extension.


I propose a new standard name for solid phase precipitation:


precipitation_flux_of_solid_water [kg m-2 s-1]

In accordance with common usage in geophysical disciplines, "flux" implies per unit area, called 
"flux density" in physics. "solid_water" refers to all forms of the solid phase of water.


regards,

Martin
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