Vaisala 905 nm lidar ceilometer data

Data

Throughout the CLARE period a lidar ceilometer owned by ESTEC was operating on the roof of the control room at Chilbolton. This instrument records lidar backscatter coefficient as a 30-second average, and also calculates the heights of the first three cloud bases from the backscatter profile. The height resolution is 30 metres. The data are stored in 6-hour ascii/hexidecimal files, and the backscatter coefficient of each pixel is contained in a signed two-byte integer in units of 10-7 (srad m)-1.

The raw data for the entire campaign is stored in the data directory, and the filenames have the form

cYMMDDHH.dat 
where Y is the last digit of the year (i.e. 8 corresponds to 1998), MM is the month, DD is the day and HH is the start hour in UTC.

No attempt has been made to verify the calibration of the instrument. The manufacturers claim the calibration is accurate to 25%. NOTE: The ceilometer data acquisition system was using the PC clock which is now known to drift somewhat, although during the CLARE period it was reset every few days so should be accurate to within around 30 seconds.

Quicklooks

Gif images of each day of data can be found in the quicklooks directory. Additionally, higher resolution images are available for some of the flight days in the quicklooks_hr directory.

Reading the data

In IDL, you can read the data using a utility written at RAL called ceil.pro, which has a pretty graphical front end. Just type

.run ceil
and work out the rest for yourself. This program was used to generate the high-resolution quicklooks above.

In MATLAB there is a simple program written at the University of Reading called LCload.m for reading data from a single file into the matlab workspace. Usage information is given at the top of the file, but basically to load a file called c9010100.dat you'd type

[ceil_utc,ceil_date,ceil_range,ceil_b,cloud1,cloud2,cloud3]=LCload('c9010100.dat');

For those who want to work at a more fundamental level, I've written a library of C functions which you may find useful in libceil.tar.gz. It's been compiled on several types of UNIX and includes two example utilities. loadceil is a MEX file for loading data into matlab; because it's compiled it's much faster than LCload.m. ceil2xpm writes XPM-format images of the ceilometer data and was used to create the daily quicklooks above. Have a look at the README file in the tarball for more information.

For help on the matlab or C programs contact Robin Hogan. For help on the IDL program or requests for more data, contact Charles Wrench.