Self-service unpacking

Contents

Correction
An error in the worked example (-37.2 should have read +16.3) was corrected on 2.2.04. Thanks to Peter Droogers.

Introduction
The scenario data-sets (TYN SC 1.0 and TYN SC 2.0) are very large. To reduce data transfer volumes, the data-set is supplied in the form of a set of raw ASCII files. The user must download the raw files and then unpack them into the scenario data-sets. There are three ways in which the unpacking can be achieved:

  1. using fortran software (requires Unix and possibly some programming) - see the unpacking web-page, the data-set readme file, and the software page
  2. by making manual calculations from the raw files, using the procedure specified below - this is only suitable if you are using a very small number of grid-boxes
  3. by writing your own software to automate the procedure specified below

Download
The raw files are supplied only via ftp. For access to the raw files, you must go through the data-set request procedure.

When you have obtained access to the raw files, you will find all the raw data files in the data subdirectory. At this point, if you have access to a Unix system, you may find it easier to download the archived version of the raw files, rather than each individual raw file. If you follow the archive route, all you need to download is:

If you do not follow the archive route, you will need to download everything in the data subdirectory except the archive* files. For either route, you only need to be download the files for the climate variables of interest to you.

If you have downloaded any archives, the raw files must be unarchived at the Unix prompt:

Whether you have followed the archive or non-archive route, many of the raw files will be in a compressed form. They will require decompression with 'uncompress'.

Ingredients
The raw files used as the ingredients in creating the scenario data-sets are given in the table below:

file type contents format TYN SC 1.0 TYN SC 2.0
scenario selector key values for individual scenarios ASCII ordered list scenarios.txt scenarios.txt
response pattern GCM patterns of change from 2070-2099 standard grid *.2080s.ateam.* hd*
global warming time-series of global temperature change (2001-2100) ASCII columns with headers *-*.ann *-*.ann
residual detrended inter-annual variability from 1901-2000 standard grid ateam.iavar* iavar*
climatology 1961-1990 average climate standard grid obs* obs*
minimum minimum permissible value standard grid min.???.ann min.???.ann
maximum maximum permissible value standard grid max.???.ann max.???.ann

Definition
Each datum (x) in any scenario is defined by the following equation:

xvgsiym = ovim + o'viym + ( pvgsim * tgsy )

The calculated value must be checked to ensure that it lies within the permissible range. If the value is out-of-range, it must be corrected to the minimum or maximum permissible value, as appropriate.

The symbols used in the above equation are given in the two tables below: main characters in the first table, subscript characters in the second table. The 'units' given in the first table presume that the climate variable is precipitation; for other climate variables substitute the relevant unit wherever 'mm' appear. The 'file type' given in the first table is a backwards reference to the 'Ingredients' table.

symbol variable units file
x scenario datum mm -
o (observed) climatology mm 'climatology'
o' (observed) residual mm 'residual'
p response pattern mm °C-1 'response pattern', modified by 'scenario selector'
t global warming °C 'global warming'

symbol variable
v climate variable
g climate model (GCM)
s SRES emissions scenario
i grid-box
y year
m month

Procedure
The unpacking of the raw files into the scenario data-set comprises:

The sequence of operations that might be performed in carrying out the unpacking is summarised below. This is given as an example, and is not prescriptive: the user is at liberty to use any sequence that satisfies the above equation. As an example we show how to obtain, for the 0.5° grid-box (363, 286) containing Norwich (52° 38' N, 1° 19' E), the projection of precipitation in July 2081 under the scenario given by using the climate model HadCM3 to represent the A2 emissions scenario.

  1. Convert the response pattern into a normalised response pattern (p). Example: the change in precipitation in 2070-99, relative to 1961-90, is -23.9 mm ('response pattern' file: hd.had3.a2m.2080s.pre); the amount of global warming ('scenario selector' file: scenarios.txt), i.e. 3.931 °C, is used to convert this into the change in precipitation per degree of global warming, i.e. -6.1 mm °C-1. (If a missing value has crept into a grid box in the response pattern, simply take the average value from the surrounding grid boxes.)
  2. Obtain the time-series of local climate change (p * t); this varies only at century time scales. Example: the normalised pattern, i.e. -6.1 mm °C-1, is multiplied by the amount of global warming in 2081 under the HadCM3 A2 scenario ('global warming' file: hadcm3-a2.ann), i.e. 3.734 °C-1, to give an anomaly of -22.7 mm.
  3. Add the inter-annual and multi-decadal variability (o') derived from the observed period (1901-2000). Example: the long-term change of -22.7 mm is modified by the residual (from the 'residual' file: iavar.1901-2000.pre) for July 1981, i.e. +16.3 mm, to give a combined anomaly of -6.4 mm.
  4. Add the mean climate (o) from the observed period (1961-90). Example, the combined anomaly of -6.4 mm is added to the climatological mean of 56.9 mm ('climatology' file: obs.clim6190.globe.lan.pre) to give a combined precipitation of 50.5 mm.
  5. Check that the value lies within the permissible range. Negative precipitation is clearly nonsensical, so the minimum permissible precipitation ('minimum' file: min.pre.ann) is 0 mm.

To follow this example through, use the relevant files. Most of the files are standard grids. The 'global warming' files are in a self-explanatory columnar format. The 'scenario selector' file was designed to inform my own fortran software, and contains:

  • the names of the 'response pattern' and 'global warming' files to use for each scenario.
  • the value by which to divide the pattern files to obtain responses expressed in units per degree of global warming; these values are duplicated in the table below for clarity; it may be noted that the pattern files for ECHam4 are already stored in units per degree of global warming:

°C PCM CGCM2 CSIRO2 HadCM3 ECHam4
A1FI 3.045 4.382 4.855 4.863 1.000 (use A2)
A2 2.462 3.548 3.938 3.931 1.000
B2 1.894 2.462 3.139 3.070 1.000
B1 1.541 2.023 2.592 2.521 1.000 (use B2)

The 'minimum' and 'maximum' files were included in order to simplify the additions that had to be made to an existing code base. They are not likely to be convenient if you are writing your own software. You can achieve the same effect by applying the values in the table below to every grid-box:

climate variable units minimum maximum
temperature °C none (in theory -273.15) none
precipitation mm 0.0 none
diurnal temperature range °C 0.1 none
vapour pressure hPa 0.0 none
cloud cover % 0.0 100.0

Please bear in mind that I do not offer any support for software design, coding, debugging, or execution. Your software is your responsibility! If you have any further questions, please read the data-set documentation and the Frequently Asked Questions (FAQ).