Changes between Version 18 and Version 21 of Ticket #152


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Timestamp:
01/12/17 10:20:00 (2 years ago)
Author:
taylor13
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  • Ticket #152 – Description

    v18 v21  
    1010The first method can be used for the common case that the cell_method applies to a single area-type. In this case, the cell_methods attribute may include a string of the form `name: method where type`. Here name could, for example, be area and type may be any of the strings permitted for a variable with a standard_name of area_type. As an example, if the method is `area: mean where sea_ice`, then the data would represent a mean over only the sea ice portion of the grid cell. When this first option is adopted, none of the variables in the netCDF file should be given a name identical to the string that names the `area_type`.  This restriction is imposed so that it will be clear that the metadata should not be interpreted following the second option (described in the next paragraph), which takes precedence.
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    12 The second method for indicating that a statistic applies to only a portion of a cell is more general because it can reference multiple area-types.  This may be needed when a variable has a dimension that ranges across various area types.  In this case, the cell_methods entry is of the form `name: method where typevar`. Here `typevar` is a string-valued auxiliary coordinate variable or string-valued scalar coordinate variable (see Section 6.1, "Labels") with a `standard_name` of `area_type`. The variable `typevar` contains the name(s) of the selected portion(s) of the grid cell to which the method is applied. This method provides a convenient way to store output from land surface models, for example, since they deal with many area types within each surface gridbox (e.g., vegetation, bare_ground, snow, etc.).
     12The second method for indicating that a statistic applies to only a portion of a cell is more general because it can reference multiple area-types.  This may be needed when a variable has a dimension that ranges across various area types.  In this case, the cell_methods entry is of the form `name: method where typevar`. Here `typevar` is a string-valued auxiliary coordinate variable or string-valued scalar coordinate variable (see Section 6.1, "Labels") with a `standard_name` of `area_type`. The variable `typevar` contains the name(s) of the selected portion(s) of the grid cell to which the method is applied. This method provides a convenient way to store output from land surface models, for example, since they deal with many area types within each surface grid cell (e.g., vegetation, bare_ground, snow, etc.).
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    1414== 2. Caption of example 7.6
     
    6161}}}
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    63 When the area fraction is varying with time, there are several different ways in which a time mean can be formulated. Three of these are illustrated in this example. Suppose, for instance, we are averaging over three time steps and the data at one grid point is -10, -6, -2 with area fractions .75, .50, .25. The values of the simple_mean, weighted_mean and partial mean are, respectively, (-10 -6 -2)/3 = -6, (-10*.75 - 6*.5 -2*.25)/(.75+.5+.25) = -7.33 , and (-10*.75 - 6*.5 -2*.25)/3 = -3.667. The partial mean provides the contribution to the mean over the entire grid from a specified area type. The simple mean is weighting each time period equally, while the weighted mean provides equal weighting to each unit area of `sea_ice`.
     63When the area fraction is varying with time, there are several different ways in which a time mean can be formulated. Three of these are illustrated in this example. Suppose, for instance, we are averaging over three time steps and the data at one grid point is -10, -6, -2 with area fractions .75, .50, .25. The values of the simple_mean, weighted_mean and partial mean are, respectively, (-10 -6 -2)/3 = -6, (-10*.75 - 6*.5 -2*.25)/(.75+.5+.25) = -7.33 , and (-10*.75 - 6*.5 -2*.25)/3 = -3.667. The partial mean provides the contribution to the mean over the entire grid cell from a specified area type. The simple mean is weighting each time period equally, while the weighted mean provides equal weighting to each unit area of `sea_ice`.
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    6565In example 7.8, `time` could be replaced by any other coordinate over which an average is taken, such as an ensemble index.