Clicking on the Trans button in the main xconv window brings up the Transformation/Interpolation interface.
The Transformation/Interpolation interface is used to transform spectral
coefficients to a grid, truncate spectral coefficients or change the horizontal
grid. Only sections which are relevant to the current field and configuration
will be usable the others will be inactive.
Spectral data can either be transformed to a gridpoint field or truncated. If Truncate spectral coefficients is selected then the new number of spectral coefficients must be entered. The truncation is done be chopping off the higher wave numbers, or if a number larger than current number of spectral coefficients is entered then the existing coefficients are padded out with zeros. If Spectral to gridpoint field is selected then a new grid must be defined. Two grid types can be selected either a Gaussian grid or a regular grid. If the data is to be transformed to the grid equivalent to a particular spectral truncation then enter the number of spectral coefficients, and press return. This will then automatically fill in the correct x and y grid parameters. Alternatively enter the number of columns (nlong) and press return, this will assume the first longitude is at 0 and fill in the correct column spacing for a global field. Similarly enter the number of rows (nlat) and press return, this will fill in the correct first latitude and row spacing for a global field which does not pass though the equator. If a different grid definition is needed then all the relevant boxes must be filled in. For a Gaussian grid only the number of rows is needed for the y dimension. Spectral coefficients must be transformed to a global grid, but see below for a method to transform to a limited area grid.
Gridpoint data can be interpolated onto a new grid, either using Bilinear interpolation or Area weighted interpolation, the new grid is defined in exactly the same way as above for spectral transformations. There are extra options available for gridpoint interpolation. If a field contains missing data values, then the relevant button should be selected, this will prevent xconv from taking missing data values as real data values. If this option is selected and the field does not contain missing data values the field is still interpolated correctly but it will take slightly longer to do. If Average over poles is selected then xconv will replace the polar values with an average value, if the interpolated data does not contain polar values then this option will have no effect. If the data is a Land/Sea mask, or any data which consists entirely of 0's and 1's (or 0's and -1's), then this option should be selected. The use of bilinear interpolation can result in fields being smeared over the output grid. It is recognised that this may not be appropriate for precipitation type fields, and xconv includes an option to remove all interpolated precipitation at gridboxes where the interpolated value is less than a user-defined cutoff value. If this option is required then Include precipitation cutoff should selected and a value for the precipitation cutoff should be entered. The precipitation cutoff is implemented in two steps, firstly any output field value less than the cutoff value will be set to zero. Secondly, for each output gridpoint, the nearest input field gridpoint is determined, if the value of precipitation at this latter point in the input data field is less than the cutoff value, then the precipitation at the output gridpoint is set to zero. The area weighted interpolation option does not allow fields with missing data to be handled differently or for a precipitation cutoff to be used. It is possible to interpolate onto a grid with a rotated pole, such as the limited area grids used in the UKMO Unified Model. To do this the longitude and latitude of the rotated north pole must be entered. Finally for gridpoint fields you can select Change x-y dimensions , this allows you to change the values of the x and y dimensions but not the number of rows or columns. This option only changes the dimension values, the data values are unaffected. This option can be useful when you have data from different sources on essentially the same grid, except for slight differences due to, for example, rounding error. If you wanted to difference this data some programmes e.g. vcs would complain that the data is on different grids, using this option you can force the x and y dimension values to be the same.
The Apply button must be clicked for any transformation or interpolation
changes to take effect.
The Reset button will reset the transformation or interpolation
information
to either its initial state if Apply has not yet been used or to the
state it was in when Apply was last used.
The Transformation/Interpolation interface can be applied successively
to the same field, for example you could convert T106 spectral data to a
UKMO Unified model limited area grid using the following method:
Transform the T106 spectral data to a 320x160 Gaussian grid, click on the
Apply button, you now have a gridpoint field which you can interpolate
onto a regular limited area grid with a rotated pole.
Index: Index Previous: Field and dimension names interface Next: Appendix 1. Convsh commands