Dear Jonathan and Robb,
three comments on your exchange ...
1. As Jonathan noted, "Is visibility sometimes measured or reported at
levels above the surface? If so we ought to put "surface" in both
horizontal and vertical standard names." This would, of course, also
require a different standard name for visibility at other altitudes.
Alternatively, a scalar dimension could be defined giving the height
above the surface from which visibility was measured.
2. In the suggestions concerning direction, should we be explicit about
whether the direction is "toward" or "from" (as in winds from the NW or
winds toward the SE)? We could have 2 standard names (direction_toward
and direction_from) Also presumably direction would have to have units
something like "degrees east of north".
3. Jonathan, I note that we have precipitation and snowfall amount, but
not rainfall amount. I guess it's the difference between precip. and
snowfall. I think (in the standard name table) our "help" comments
should indicate that precipitation amount, rate, etc. includes all forms
of precipitation, just to make sure that no one stores rainfall under
precipitation. Do you think this is a good idea?
Karl
Jonathan Gregory wrote:
> Dear Robb
>
> Thanks for your posting about this and sorry for the delay in response. I
> presume it's the quantities in your long names that you would like standard
> names for. Your list raises a number of questions (below).
>
> I'm glad you're working on this; it would be good to define equivalences
> between CF and standards such as METAR. When we have settled such questions as
> below, we should probably set up a table describing the mapping as we have for
> GRIB and AMIP (they're linked from the standard name table HTML document).
>
> Thanks for your help. Best wishes
>
> Jonathan
>
>
> observation time: Can we use just "time" for this? We use the standard name of
> time for forecast validity/analysis time, so that would be consistent. Since
> some of the quantities in the report apply to an interval of time, such as peak
> wind speed and accumulated precipitation, the time coordinate variable should
> have boundary coordinates to record the start and end times of the interval.
>
> time nominal: What is this used for? Do METARs record two different times?
>
> Peak wind speed: The peak wind speed within a time interval does not need a
> special standard name, as it can be given a standard name of wind_speed, with a
> cell_methods attribute of the variable indicating that it is a maximum in time.
>
> Wind gust: What is the difference between this and the peak wind speed?
>
> Peak wind direction: I presume this is the direction of the wind which has the
> maximum speed within the time interval. Is that right? If so, that is a new
> kind of idea for CF, because it defines a quantity in terms of the within-cell
> variation of another quantity. I am not sure whether we should simply define a
> standard name for this, or whether there's something more general we should
> introduce.
>
> Peak wind time: This is a related question. Whatever its standard name, I
> suggest this should be encoded in the same way as other times i.e. time-units
> since reference-time, not as a character string.
>
> Wind from direction minimum/maximum: I am not sure what these directions mean.
>
> Visibility in air direction: I presume this is the direction in which the
> horizontal visibility was measured. Is that right? If so, we probably don't
> need a specific standard name for it. We could introduce a standard name of
> plain "direction", which will be a coordinate for the variable of visibility,
> like this:
>
> dimensions:
> visibility_direction=1;
> variables:
> float visibility_direction(visibility_direction);
> visibility_direction:standard_name="direction";
> float visibility(visibility_direction);
> visibility:standard_name="visibility_in_air";
>
> It could alternatively be put in a scalar coordinate variable, to avoid
> defining a dimension of size one.
>
> variables:
> float visibility_direction;
> visibility_direction:standard_name="direction";
> float visibility;
> visibility:standard_name="visibility_in_air";
> visibility:coordinates="visibility_direction";
>
> Why would you use a character string for the direction, rather than a floating-
> point number?
>
> Visibility in air: I agree we need vertical_visibility_in_air, so probably we
> should introduce a new standard name of horizontal_visibility_in_air and make
> the existing standard name of visibility_in_air an alias for it. But I note
> that for horizontal visibility you introduce "surface". Is visibility sometimes
> measured or reported at levels above the surface? If so we ought to put
> "surface" in both horizontal and vertical standard names.
>
> low/medium/high cloud: Are these three defined in terms of cloud type or
> altitude range, or are they simply a set of (up to) three layers that can be
> reported, regardless of their height? In the former case, we need definitions
> for low/medium/high. In the latter case, the definition is the cloud base
> altitude which you are also supplying. In that case I don't think we need new
> standard names, because the information can be structured like this:
>
> dimensions:
> cloud_altitude=3;
> variables:
> cloud_altitude(cloud_altitude); // coordinate variable
> cloud_altitude:standard_name="cloud_base_altitude";
> fraction(cloud_altitude); // data variable
> fraction:standard_name="cloud_area_fraction";
>
> weather: If we define a standard_name of weather_phenomenon, we should define
> also the values it can have, by either pointing to or copying the table. Do you
> have a URL for "WMO #306, code table 4658"? I guess it is similar to the
> abbreviations in http://www.asy.faa.gov/safety_products/metar-taf99.html.
>
> amounts of precipitation: New standard names aren't needed for these, as you
> can use precipitation_amount and snowfall_amount. The default interpretation
> for these quantities is an accumulation within the time interval, because they
> are extensive in time (the values depend on the length of the time interval).
> The time interval is recorded in the boundary variable of the time coordinate
> variable. If the quantities are in units of distance, such as inches, the
> standard_names should be lwe_thickness_of_precipitation_amount and
> lwe_thickness_of_snowfall_amount (lwe=liquid water equivalent). For some reason
> the former isn't already in the table and obviously should be added.
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Received on Mon Mar 08 2004 - 08:59:22 GMT
