Acquiring the Met Office Assimilated data held at the BADC

This file contains basic background information to help you use the Met Office Assimilated data held at the BADC.

  1. Checking the data directories
  2. Transferring Data from the BADC
  3. Reading the data files

The Met Office assimilated data was supplied to the BADC by the Met Office as IEEE Unix binary files in Met Office "PP" format. For the vast majority of users, the binary files can be read directly, without going through a ascii-binary conversion step. The files were written on HP machines and have been read directly on SUN, SGI and AIX. We have also read them under DEC Unix using the f77 -convert big_endian compiler option.

We supply an example read program to read the binary data, and 3rd party software written by other users of the data, which we do not support.

1. Browsing the Data Directories

The Met Office Assimilated Data directories can be browsed using the web at /badc/ukmo-assim/data/. We suggest that you look here to check whether the data you require is available. Please note that since these data are restricted, you need to apply for access to the Met Office assimilated data, before downloading the data over the WWW. The online application for access to the Met Office Assimilated data includes the Met Office Agreement to be electronically accepted.

The data are updated every night from the Met Office - check the updates file for the latest file additions.


2. Transferring Data from the BADC

All data and software is located beneath the directory /badc/ukmo-assim from the Browse Archive facility. Here you will find a README file and the following subdirectories


3. Reading the data files

3.1 The file structure

Each file contains an analysis for a single date, or in cases where the analyses have been produced every six hours, for a single time.

The file contains fixed length 28288 byte records. The first record contains a text header incorporating the standard UARS headers, and a list of meta data including geographical and vertical ranges and parameters available in the file.

Each subsequent record contains a parameter field for a single pressure level. The records are ordered by decreasing pressure, and all records for one parameter are grouped together in the file. Thus zonal wind fields for all 22 pressure levels are followed by the fields for the meridional winds and so on.

Schematically the file has the following structure (the letters pl denote pressure level):

 

    ________________________________________________________
   |                                                        |
   | Record 1 : SFDU label and UARS Header Record           |
   |________________________________________________________|
   |                                                        |
   | Record 2 : Zonal Wind Field at zeroth pl (1000hPa)     |
   | Record 3 : Zonal Wind Field at 1st pl                  |
   | Record 4 : Zonal Wind Field at 2rd pl                  |
   |                                                        |
    ....
   |                                                        |
   | Record 23 : Zonal Wind Field at 21st pl (0.316hPa)     |
   |________________________________________________________|
   |
   | Record 24 : Merid. Wind Field at zeroth pl (1000hPa)   |
   |                                                        |
    ....
   |                                                        |
   | Record 46 : Geopot. Height Field at zeroth pl (1000hPa)|
   |                                                        |
    ....
   |                                                        |
   | Record 68 : Temperature Field at zeroth pl (1000hPa)   |
   |                                                        |
    ....
   |                                                        |
   | Record 90 : Vertical  Wind Field at zeroth pl (1000hPa)|
   |________________________________________________________|

This file structure is also used for the monthly averaged data which are stored in the "means" sub-directory.

Using the Met Office -supplied access routines is recommended, (see below) since you are then shielded from the underlying file structure. For a very detailed description of the file structure and the contents of each field refer to the documentation produced by the Met Office.

3.2 The Met Office Access Routines

Software routines to read these data have been supplied by the Met Office. There is also a routine to calculate Ertel Potential Vorticity from the analyses.

Two basic approaches may be used to read the Met Office analyses. Firstly, a complete field may be read at a single level (eg. for map plotting); this method minimises interpolation errors since the values are on the original horizontal grid points. Alternatively, a set of profiles may be derived at specific locations; this method is intended for comparing the analysis data with observed profiles.

To access this data use the following routines:

    To open a file:                         UKAOPN
    To read a field at one level:           UKAFLD
    To define profiles at several levels:   UKAPRF
    To calculate Ertel potential velocity:  PV_UKA
    To close a file:                        UKACLS
    To print a diagnostic message:          UKAPRT

Users should note the the routines which extract a field of data (UKAFLD and PV_UKA), return the field as a 1-dimensional array of 7008 points. Within this array, gridpoints are grouped in rows at a constant latitude, with high northerly latitudes appearing first.

Detailed documentation on these routines was produced by the Met Office and is available online from the BADC.

3.3 Third Party Software

In the software directories for the Met Office Assimilations, there is a subdirectory for 3rd Party software. If you develop software to use with the Met Office assimilation data, then we encourage you to let us have a copy so that we can make it available to other researchers.