THE NERC MST RADAR FACILITY AT ABERYSTWYTH
NASA-Ames files are designed to be self-descriptive. Once the 'File Format Index' is known - 1001 in the case of the surface wind data files - it is possible to read and understand the data without the need to refer to external sources. Any amount of metadata can be included in the header section of the file. The information included in these files has been chosen to match the Global Attributes used by the Climate and Forecasts metadata convention for NetCDF files; this is the preferred format for British Atmospheric Data Centre datasets.
For the time being, only those aspects of the file format which are essential for reading the data will be described. For a full description of the NASA-Ames formats, consult the Gaines and Hipskind [1998] document.
The following description only refers to data from 19th December 2001 onwards. Click here to find out about the file format used for data between 9th November 1995 and 28th November 2001.
The surface wind file for 1st June 2003 (wind-sensors_frongoch_20030601.na) will be used as an example. Text in green represents actual file contents. Text in red is for explanatory purposes only.
- Header Lines
- Line 1: 55 1001
Integer 1 corresponds to the total number of header lines, nr_header_lines
Integer 2 corresponds to the File Format Index
- Line 7: 2003 06 01 2004 05 13
Integers 1 - 3 correspond to the year, month and day on which the observations were made.
Integers 4 - 6 correspond to the year, month and day on which the file was created.
- Line 12: 999.99 999.99 99.99 99.99
These are the numbers which represent missing data values for the 'primary variables' (see below)
- Line 21: 1440
Corresponds to the number of data lines in the file, nr_data_lines
- Line 7: 2003 06 01 2004 05 13
After reading the above mentioned lines, wind forward to line (nr_header_lines + 1) where the data begin. The data can be read with a simple loop structure of the form (shown here in Fortran syntax):
do data_line_nr = 1,nr_data_lines read_data_line end do
Reading data line
Each data line contains 5 floating point numbers, shown here for the first 4 lines:
0.0 -0.18 3.27 0.76 1.40
60.0 -0.14 3.14 0.79 1.19
120.0 -0.38 3.21 0.77 1.16
180.0 -0.60 3.51 0.58 1.29
- Value 1: Time (s) since 00:00:00 UT on the day in question
Technically speaking this is the first independent variable rather than a primary variable.
- Value 2: Mean eastward wind (m s-1)
or mean zonal velocity over 60 s sample time
- Value 3: Mean northward wind (m s-1)
or mean meridional velocity over 60 s sample time
- Values 4 and 5: Miniumum/maximum gust wind speed relative to mean speed
For data recorded before 15th April 2003, the basic wind information was sampled at 10 s intervals, and at 1 s intervals for dates thereafter. The eastward and northward components of the wind represent 60 s means of these samples. The gust winds represent unaveraged 10 s values for data recorded before 15th April 2003, and 3 s means (of 1 s samples) therefafter. Values 4 and 5 represent, respectively, the ratios of the minimum and maximum gust speeds (occuring within each 60 s interval) to the mean speed.
- Value 2: Mean eastward wind (m s-1)
These files have no header lines and just consist of a sequence of wind speed and direction values. The data are recorded at 1 minute intervals, starting at 00:00:00 UTC on the day in question, with time increasing in steps of 1 minute from left to right. This gives 240 lines of 6 speed/direction pairs each as shown below, in green, for the first 3 lines of the file for 1st June 2000 (sw000601). Text in red is for explanatory purposes only and does not appear in the files.
4.94 171.8 4.73 167.0 4.25 163.4 3.92 171.9 5.02 169.8 5.08 169.6 S(1) D(1) S(2) D(2) S(3) D(3) S(4) D(4) S(5) D(5) S(6) D(6) 5.04 165.0 3.88 165.3 4.44 173.0 4.52 172.4 3.83 172.3 5.31 174.0 S(7) D(7) S(8) D(8) S(9) D(9) S(10) D(10) S(11) D(11) S(12) D(12) 4.81 179.3 4.60 171.2 5.44 169.6 4.83 176.6 5.15 165.0 5.27 174.4 S(13) D(13) S(14) D(14) S(15) D(15) S(16) D(16) S(17) D(17) S(18) D(18)
| S(i) |  |
wind speed (m/s) |
| D(i) | This nominally gives the meteorological
convention wind direction (° clockwise from North), i.e. FROM
which the wind is blowing. However, NOTE that there is an
error in the way in which the data have been written and the
actual meteorological convention direction is given by: |
|
| i | The index is shown here in the range [1 - 1440],
which means that the corresponding time (seconds after 00:00:00
UTC) is given by: |