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 août 2011 à 10:36 -0400, Nan Galbraith a écrit :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 [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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