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Orbital Forcing
Control of the orbital forcing parameters is via a new panel in the UMUI, under Atmos→Science→General Physics→Solar
With both calculation methods set OFF, the model will default to a fixed set of parameters corresponding to the year 2000, as hardcoded into UM6.1. This is the default behaviour.
If one of the calculation methods is set ON, the orbital parameters can be set for a specified year and held fixed through the run, or allowed to vary according to the model year as the run progresses. If allowed to vary, the rate at which the orbital parameters change can be artificially accelerated by specifying an acceleration factor greater than 1. For instance, an acceleration factor of 10 would mean that changes in orbital forcing that would normally take 100 years will be applied over 10 model years instead.
The Berger78 method implemented for the online calculation is valid for ± 1Myr. More recent calculations have provided values with some usable accuracy ± 250 Myr (i.e. Laskar04, although the values we want are probably only meaningful back to −65 Myr), which can be used by providing a correctly formatted table and using the “Calculate using data table” option.
Back to −1Myr there is very little difference between the various different tables/online method, except in the eccentricity, where Laskar04 values can differ by a few percent. Under the data table method, the table is searched, line by line, from the top everytime a new value is required. Given the size of the full Laskar04 table, this can be quite a time consuming process, should one want to use a “realistic” time varying forcing for very old periods where the Berger78 method is inaccurate. If you really did want to do this, you’d be better off constructing a new table containing just the subset of dates required for the run.
(note: dates must go back in time down the table, as in the web-published Laskar04 data)
Different published calculations use different reference years as Year 0 - i.e. all dates in Berger78 are relative to 1950, whilst Laskar04 is relative to 2000. This is the year that should be put as the <offset year>. The published values can then be cut and pasted into the relevant column.
A. L. Berger, J. Atm. Sci, 35, 2362–2367 (1978)
J. Laskar et al., Astronomy and Astrophysics 428, 261–285 (2004)
http://users.skynet.be/erik.desonville/en/astro.paleo.htm (links to papers/data sets)
The new Absolute_verylong format changes the UM date-coding conventions for the file names, and simply specifies the year as an 9 digit integer in the file name, with a “+” or “-“ at the end to signify whether it is an AD or BC date. For example: xbyvra@pyo71c1 becomes xbyvra#py000002471c1+. This new format is now selectable from the list in Sub-Model Independent → File and Directory Naming
The long_output_names.mod also lets the UM use negative dates, so that actual paleo years can be specified. Currently, output file utilities based on conv.sh (including xconv) that convert these output files into other formats (i.e. netCDF) cannot cope with negative dates, so cannot be used on such files. This should be fixed shortly.