The treatment of air velocity - wind - in CF is a little bit
problematic, IMHO; wind is assumed to be a 2D vector.
Water velocities are defined more consistently, with eastward_
northward_ and upward_sea_water_velocity having parallel names/
definitions, and allowing the e/n terms to be used with or without
a vertical component.
I think the latter is a better way to handle platform motion,
because it's more flexible and consistent. It does put the onus
on the user to look for a vertical component if they're going to
calculate speeds, but that's true no matter how the definitions
and terms are written - unless we want different terms for e/n
when there is (or is not) a vertical component.
Sea water velocities (e/n/u, matching)
> eastward_sea_water_velocity: A velocity is a vector quantity.
> "Eastward" indicates a vector component which is positive when
> directed eastward (negative westward).
Air velocities (e/n handled separately from upwards)
> eastward_wind: "Eastward" indicates a vector component which is
> positive when directed eastward (negative westward). Wind is defined
> as a two-dimensional (horizontal) air velocity vector, with no
> vertical component. (Vertical motion in the atmosphere has the
> standard name upward_air_velocity.)
> upward_air_velocity: A velocity is a vector quantity. "Upward"
> indicates a vector component which is positive when directed upward
> (negative downward). Upward air velocity is the vertical component of
> the 3D air velocity vector.
If you have a 3-component air velocity record, I'm not sure how you're
supposed to record the e/n components in CF.
I agree with what you say about the vertical component of ship's motion.
In practice, it's also a matter of whether this data is available or not
that determines whether it's used.
Nan
On 8/30/11 11:08 AM, Bruno PIGUET wrote:
> Well, yes, the last sentence can be simplified.
>
> It is a copy/paste/adaptation of the sentence describing
> upward_air_velocity, where the 3D aspect is important. In our present
> case, I let readers/reviewers judge on that point.
>
> For your information, my current need is for airborne measurements,
> where vertical velocity cannot be assumed null.
> On ships, I would say that the assumption "vertical velocity is 0"
> depends on the time scale. If you are doing eddy correlation
> computation, and store your high-freq data in a netcdf/CF file, you'll
> deal with non-null instantaneous vertical motion. Probably not a
> mainstream use of netcdf/CF, though.
>
> Bruno.
>
> Le mardi 30 ao?t 2011 ? 10:36 -0400, Nan Galbraith a ?crit :
>> These look good, and are certainly needed.
>>
>> Just one question about the last line of each of the definitions,
>> referring to the 3D platform velocity. On ships, there's often only
>> a 2D vector, I suppose the vertical velocity is presumed to be 0.
>>
>> Would it be preferable to not specifically mention 3D, and just say
>> "the eastward (or northward/vertical) component of the platform
>> velocity vector" to make this "fit" 2D ship tracks as well?
>>
>> Thanks - Nan
>>
>>
>> On 8/30/11 10:12 AM, Bruno PIGUET wrote:
>>> Dear all,
>>>
>>> I would like to request the creation of new standard names for the
>>> local geographic components of the platform_speed_wrt_ground.
>>>
>>> Following the already existing definition of speed, velocity, and the
>>> naming scheme for the wind component (eastward_wind, northward_wind,
>>> upward_air_velocity), I propose :
>>>
>>> - platform_northward_velocity_wrt_ground
>>> "wrt" means with respect to. Standard names for platform describe the
>>> motion and orientation of the vehicle from which observations are made
>>> e.g. aeroplane or ship. A velocity is a vector quantity. The platform
>>> velocity with respect to ground is relative to the solid Earth beneath
>>> it i.e. the sea floor for a ship. It is often called the "ground speed"
>>> of the platform. "Northward" indicates a vector component which is
>>> positive when directed northward (negative southward).
>>> Platform_northward_velocity_wrt_ground is the northward component of the
>>> 3D platform velocity vector.
>>>
>>> - platform_eastward_velocity_wrt_ground
>>> "wrt" means with respect to. Standard names for platform describe the
>>> motion and orientation of the vehicle from which observations are made
>>> e.g. aeroplane or ship. A velocity is a vector quantity. The platform
>>> velocity with respect to ground is relative to the solid Earth beneath
>>> it i.e. the sea floor for a ship. It is often called the "ground speed"
>>> of the platform. "Eastward" indicates a vector component which is
>>> positive when directed eastward (negative westward).
>>> Platform_upward_velocity_wrt_ground is the eastward component of the 3D
>>> platform velocity vector.
>>>
>>> - platform_upward_velocity_wrt_ground
>>> "wrt" means with respect to. Standard names for platform describe the
>>> motion and orientation of the vehicle from which observations are made
>>> e.g. aeroplane or ship. A velocity is a vector quantity. The platform
>>> velocity with respect to ground is relative to the solid Earth beneath
>>> it i.e. the sea floor for a ship. It is often called the "ground speed"
>>> of the platform. "Upward" indicates a vector component which is positive
>>> when directed upward (negative downward).
>>> Platform_upward_velocity_wrt_ground is the vertical component of the 3D
>>> platform velocity vector.
>>>
>>>
>>> Bruno.
>>>
>>>
>>> _______________________________________________
>>> CF-metadata mailing list
>>> CF-metadata at cgd.ucar.edu
>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
--
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* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
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Received on Tue Aug 30 2011 - 10:01:43 BST
