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Um Analysis

This page describes how to analyse the UM with different packages.

General comments

Atmosphere data on pressure levels

Note that this remark is not package-specific. The output data on pressure levels cannot be used straight away. The surface pressure changes every timestep and if it is lower than a certain pressure level, the data on this level is undefined. To account for the fraction of the time that a certain pressure level is defined, it is needed to divide the variables on pressure levels (e.g. temperature, humidity, ..) by the variable HEAVYSIDE FN ON P LEV/UV GRID (STASH code 30301).

Ferret

Ferret is an analysis package specifically designed for atmospheric and oceanic data. It supports 4-dimensional (xyzt) datasets in the Net CDF format.

Ocean data on depth levels

Ferret incorrectly interprets the depth levels of oceanic data that has been converted from the model output data with Xconv or convsh. The depth levels of the z-axis therefore need to be replaced manually. The following command replaces the depth levels for standard Had GOM with 38 levels:

 define axis/edges/z/depth/units=m `TEMP,ret=zaxis`={0,10,20,30,40,50,60,70,80,90,100,\
   110,120,130,141.2,155.8,176.1,204.4,242.8,293.4,358.2,439,\
   537.5,655.3,793.7,954,1137.2,1344.2,1575.6,1831.8,2113,2418.6,\
   2744.9,3085,3430,3775,4120,4465,4810,5155,5500}

where the variable TEMP can be any oceanic variable on depth levels except the vertical velocity.

The HiGOM ocean model is the ocean component of the HiGEM model, uses the same number of levels, but located at different depths:

 define axis/edges/z/depth/units=m `TEMP,ret=zaxis`={0,10.1,20.24,\
   30.45,40.75,51.18,61.8,72.69,83.97,95.81,108.44,122.22,137.62,155.32,176.27,\
   201.77,233.55,273.87,325.56,391.96,476.68,583.2,714.29,871.46,1054.56,1261.85,\
   1490.4,1736.67,1997.07,2268.33,2547.73,2833.09,3122.75,3415.48,3710.39,4006.83,\
   4304.35,4602.62,4901.41,5200.57,5499.99}

Combining output

Model output files usually contain 1 or a small number of time steps, while for analysis many timesteps are needed. Ferret can access data from multiple files transparently through the use of descriptor files. The script make_des_time automatically generates a descriptor file for a list of UM output files in Net CDF format with the correct time axis. The script will produce an incorrect descriptor file if the variables in the output files use different time axes. Also, the first line needs to point to the ksh on your local system.