[CF-metadata] CF convention for vector quantities

From: Bert Jagers <Bert.Jagers>
Date: Fri, 24 Feb 2006 18:37:03 +0100

Hi All,

First of all thanks for your replies to my question about CF2 and standard
names two/three weeks ago. Main conclusions from that discussion seem to be:

* standardized Datum attribute needed for specifying a reference level
(don't include reference level in standard name)
* name spaces can be used to resolve perception issues related to standard
naming conventions, e.g. sea_surface_height for water level in other water
bodies than seas, but one must be carefull when mapping between such name
spaces.

Now I have some new questions related to vector quantities ...

Over past the two days I had a brief discussion with Rich Signell with
respect to the extension of CF convention to include some attributes for
vector quantities. A plotting tool that I developed should automatically
detect vector quantities and process and plot them accordingly. As far as I
am aware there is currently no part in the CF convention that makes such
automatic detection possible (except for implementing the standard names of
all known vector quantities). Vector quantities include quantities such as:

"eastward_sea_water_velocity" and "northward_sea_water_velocity"
"sea_water_x_velocity" and "sea_water_y_velocity"
"eastward_atmosphere_dry_static_energy_transport_across_unit_distance"
"eastward_atmosphere_water_transport_across_unit_distance"
"eastward_momentum_flux_correction"
"eastward_sea_ice_velocity"
"product_of_eastward_sea_water_velocity_and_temperature"
"product_of_eastward_wind_and_air_temperature"
"product_of_eastward_wind_and_geopotential_height"

... and on and on and on....

I would prefer not to have to implement all these names. I see two
alternatives:

1) Introduce dummy quantity "sea_water_velocity" and refer to the associated
components by means of an appropriate attribute, e.g. "Components".
Alternatively both component could refer to the both components, e.g.
Components : uflow vflow
where
variable standard_name
uflow eastward_sea_water_velocity
vflow northward_sea_water_velocity
This solution does not yet resolve associated issue A (see below).

2) Introduce standard prefixes for vector components to e.g.
"eastward_component_of_sea_water_velocity" and
"northward_component_of_sea_water_velocity" and stick to
these names, that is
"product_of_eastward_sea_water_velocity_and_temperature"
becomes
"eastward_component_of_product_of_sea_water_velocity_and_temperature"
... or some equivalent postfix.

However, two other issues are related to this question:

A) Distinction between components in east and north direction and components
in the two grid directions: in which direction are the two components
defined? It seems that "sea_water_x_velocity" and "sea_water_y_velocity" are
used for components in the grid direction although in local coordinate
systems x and y are generally associated with the two main coordinate
directions whereas the grid may be curvilinear. Most likely
"direction_of_sea_water_velocity" and "sea_water_speed" should form together
a vector quantity. Central question: which types of vector components are
needed?

B) Staggering of data. The ROMS and Delft3D applications that we are looking
at work both with staggered numerical schemes. So, in fact the two velocity
components are provided at two different location sets. Before we can deal
with these vector quantities correctly we need some standardization on
staggered data. I know there have been some preliminary discussions on this
matter in the context of CF2, but what is a realistic time schedule for
these discussions to continue and settle?

Best regards,

Bert
Received on Fri Feb 24 2006 - 10:37:03 GMT