Dear all,
Unfortunately I have had far too little time to contribute to the CF
mailing list over the past year, but I try to scan the titles of mails
coming in and now and then I read a post. I didn't intend to read this
thread until my eye accidentally caught the word "sedimentation" in body
text. As someone involved in sediment transport in rivers, estuaries,
coastal areas and to a much lesser oceans, I do have a strong
association with the word "sedimentation". As a result I have some
concerns with the appropriateness of the chosen word.
If I interpret the most recent discussion correctly, the proposal is to
use the word "sedimentation" for the slow downward motion of liquid
water in the atmosphere; not being an atmospheric scientist I'm trying
to grasp what that actually means. The discussion distinguishes it from
definitions including terms like "accretion" and "autoconversion" which
based on the description in the standard name table both seem to be
related the growth of droplets and ice crystals; flocculation is the
associated process in cohesive sediment modeling. Other processes that
thus would be included in your new definition of "sedimentation"
include: vertical advection due to general air velocity, turbulent
mixing, relative gravitational settling of the droplets/crystals, ... more?
In sediment transport, the words "sedimentation" and "deposition" are
generally used to mean the reverse of "erosion". The former two mean
that sediment falls from suspension onto the bed ("ocean floor"),
whereas "erosion" means the uptake of sediment from the bed by the flow.
Now, let us see how the word "sedimentation" was used before in CF
names. The term "sedimentation" has so far been used in three standard
names related to the ocean_mole_content of three chemical substances.
These names are associated with 2D quantities (quantities per unit area)
which matches my interpretation of the word (and the common use of this
word): the amount of the substances considered (carbon, elemental
nitrogen, and iron) in the ocean column is reduced due to the fact that
organic suspended sediment settles onto the ocean floor. Basically it
includes a transition from one medium (in the water column) to another
medium (in/on the ocean floor). The newly proposed use of the term
"sedimentation" does not match such a transition.
Thinking along these lines I looked at the word "deposition" which is
used more frequently in standard names. However, mostly it is used in
combination with "dry", "wet" or "turbulent" and seem to be associated
with the transition of substances from medium atmosphere to the medium
ocean (or land): a 2D flux field of 2D variable "atmosphere mass
content" in line with the use of the word "sedimentation" described
above. The word "deposition" without "dry" or "wet" is less uniquely
tied to a specific process. It has been used as the reverse of
"sublimation" in
tendency_of_mass_fraction_of_stratiform_cloud_ice_in_air_due_to_deposition_and_sublimation,
as the sum of "dry deposition" and "wet deposition" (= "turbulent
deposition" + "gravitational settling"), and in a non-specified manner
in
tendency_of_ocean_mole_content_of_iron_due_to_deposition_and_runoff_and_sediment_dissolution.
In ocean modeling I would tend to use the word "settling" for the
downward motion of sediment relative to the water without necessarily
hitting the ocean floor, but also the word "settling" has so far only
been used in combination with 2D variables of type "atmosphere mass
content" and deposition.
Best regards,
Bert
------
Tomoo Ogura wrote:
> Hi Philip,
>
> Thanks for drafting the definitions of precipitation and sedimentation.
>
> It appears to me that the definitions are appropriate for aerosols, too,
> (but not being an expert, maybe I'm not the right person to comment
> here ...)
>
> Best wishes,
>
> Tomoo
>
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> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
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Received on Tue Mar 08 2011 - 17:22:23 GMT
