converting the sigma level(model surface) to pressure level

[tjang]

Hello.

Recently, I'm studying IRIS for data analysis and visualization.

Reading UM field file is very simple but I don't know how to convert the sigma level(model surface) to pressure level.

IRIS supports some methods?

[Phil Elson]

I'm not 100% I've understood correctly (I don't have much knowledge of the UM vertical level systems in use) - are you at the point where you have model levels, with a reference surface pressure (2d) + sigma coordinates (1d), and want to convert the data to pressure levels (1d)? If so, I'm not sure Iris can do any vertical regridding at this stage (though I think the interpolation functions could handle it).

If you just want to get the pressure of a single gridpoint from some data with a hybrid pressure vertical coordinate scheme, and you have all of the data loaded appropriately into the cube, I think you can just access the "pressure" derived coordinate. The example at http://scitools.org.uk/iris/docs/latest/examples/graphics/cross_section.html conveniently has a hybrid height vertical coordinate system which is very similar. For ease of discussion the cube looks like:

air_potential_temperature / K       (atmosphere_hybrid_height_coordinate: 15; grid_latitude: 100; grid_longitude: 100)
     Dimension coordinates:
          atmosphere_hybrid_height_coordinate                           x                  -                    -
          grid_latitude                                                 -                  x                    -
          grid_longitude                                                -                  -                    x
     Auxiliary coordinates:
          model_level_number                                            x                  -                    -
          sigma                                                         x                  -                    -
          surface_altitude                                              -                  x                    x
     Derived coordinates:
          altitude                                                      x                  x                    x
     Scalar coordinates:
          forecast_period: 0.0 hours
          time: 2009-09-09 17:10:00
     Attributes:
          Conventions: CF-1.5
          STASH: m01s00i004
          source: Data from Met Office Unified Model 7.04

Apologies if I've got the wrong end of the stick.

Regards,