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For this page:
INTERNAL LINKS

EXTERNAL LINKS
THE NERC MST RADAR FACILITY AT ABERYSTWYTH
FILE FORMAT FOR LD40 LASER CEILOMETER CLOUD BASE DATA
File contents - Click here to find out about the contents of other files.
The files contain time series of cloud base altitudes (and of laser penetration depths) for up to three cloud layers. The data were recorded at 1 minute intervals from a Vaisala LD40 laser ceilometer operating at the NERC MST Radar site. Refer to the instrument page for full details of the the measurement procedure and a description of the data products.

Data availability:
Data are available from 8th August 2005 to the present. There are a few gaps in the dataset owing to problems with the instrument and with the data acquisition computer. Refer to Instrument performance weblog for further details.

File naming convention: Click here to find out about the file naming convention used for other files.

lidar-ld40_capel-dewi_YYYYMMDD_cloud-base.na

YYYY is a 4-digit year [2005 - ]
MM is a 2-digit month [01 - 12]
DD is a 2-digit day [01 - 31]

File location - Click here to find out about the location of other files.

/badc/mst/data/laser-ceilometer/

Archiving convention -Click here to find out about the archiving conventions used for other files

YYYY/MM

File format -Click here to find out about the formats used for other files.
Data are written in NASA-Ames (ASCII) files with a File Format Index of 1001. For a complete description of the NASA-Ames file formats, consult the Gaines and Hipskind [1998] document. Only those aspects of the file which are essential for reading the data are described below.

The file for 1st January 2006 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: 204 1001
Integer 1 corresponds to the total number of header lines, nr_header_lines
Integer 2 corresponds to the File Format Index

Line 7: 2006 01 01 2006 09 05
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 10: 10
The number of primary variables (nr_primary_vars).

Line 13: 99999.9 99999.9 99999.9 99999.9 99999.9 99999.9 99999.9 99999.9 9 99999999
Missing datum values, in order, for the primary variables -
see below. Missing datum values tend to imply that no information could be derived for a particular cloud layer, e.g. because the laser pulses could not penetrate a lower layer or because no clouds existed. Refer to the instrument status flag description below for details on how to identify a lack of observations.

Lines 13 - 22
The names of the primary variables, in order, as described below

Line 27: 1440
The number of data lines (nr_data_lines ) within the file.

Lines 28 - nr_header_lines:
These lines contain optional header information describing the instrument and the data product definitions. Some of this information is repeated below in association with the relevant data products but refer also to the instrument page.

Data reading loop
After reading the above mentioned lines, wind forward to line (nr_header_lines + 1) where the data begin. Since the number of header lines may change from file to file, it is important to read nr_header_lines from line 1 rather than assuming that it is always equal to 204. 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 a data line
The data lines contain 11 values: the first independent variable (time) followed by the 10 primary variables. The first 10 data lines from the file for 1st January 2006 are shown by way of example:
    0.0   800.0 99999.9 99999.9   170.0 99999.9 99999.9  1060.0   875.0 0 00000000
   60.0   620.0   685.0 99999.9   235.0   170.0 99999.9   945.0   790.0 0 00000000
  120.0   745.0 99999.9 99999.9   135.0 99999.9 99999.9   975.0   830.0 0 00000000
  180.0   650.0 99999.9 99999.9   150.0 99999.9 99999.9   895.0   740.0 0 00000000
  240.0   605.0 99999.9 99999.9   135.0 99999.9 99999.9   835.0   670.0 0 00000000
  300.0   610.0 99999.9 99999.9   145.0 99999.9 99999.9   850.0   685.0 0 00000000
  360.0   635.0 99999.9 99999.9   280.0 99999.9 99999.9  1005.0   785.0 0 00000000
  420.0   535.0 99999.9 99999.9   200.0 99999.9 99999.9   825.0   625.0 0 00000000
  480.0   745.0   865.0 99999.9   215.0    90.0 99999.9  1050.0   925.0 1 00000000
  540.0   500.0   625.0  1415.0   230.0   100.0    90.0  1600.0  1495.0 0 00000000
Value 1: Time (s) since 00:00:00 UTC for the day in question. Note that the time stamps for the data are derived from the instrument's internal clock - not from that of the data acquisition computer. They contain fields for hours and minutes but not for seconds. Only one of the 4 messages for each minute is saved. The instrument's internal clock can only be synchronised manually. Since this requires a special connection to be made via the serial service interface, it is only done on an infrequent basis. The accuracy of the internal clock's time decreases with increasing time.

Value 2: Cloud base altitude of first cloud layer (m). Altitude implies above mean sea level. Accuracy: +/- 5 m.

Value 3: Cloud base altitude of second cloud layer (m). Accuracy: +/- 5 m.

Value 4: Cloud base altitude of third cloud layer (m). Accuracy: +/- 5 m.

Value 5: Laser penetration depth into first cloud layer (m). Accuracy: +/- 5 m.

Value 6: Laser penetration depth into second cloud layer (m). Accuracy: +/- 5 m.

Value 7: Laser penetration depth into third cloud layer (m). Accuracy: +/- 5 m.

Value 8: Maximum possible altitude of cloud detection (m). No information may be derived for any clouds which may exist above this altitude.

Value 9: Estimated visibility in air looking vertically (m)

Value 10: Estimated precipitation rate index (0 - 3), where 0 implies no precipitation and 3 implies heavy precipitation.

Value 11: Instrument status flag (00000000 - 99999999). A value of 99999999, for this 8 digit number, implies that no observations were made. See the instrument status flag web page for an explanation of other values.


Internal Links:
Return to the top of the page
Access to the data
Description of the instrument, of the measurement procedure and of the meaning of the data products.
External Links:
Location of surface meteorological data files on the BADC
Page maintained by David Hooper
Last updated 5th September 2006