IDAR elastic backscatter reflection from clouds, aerosols or precipitation is used to measure cloud base height and vertical visibility, and is particularly suited for low altitude measurements. This instrument can measure up to three layers of clouds simultaneously using a high powered pulsed laser diode in the infrared region at 910 nm. Pulse repetition can be varied from 2 to 120 seconds and fast repetition rates can be used to detect thin cloud patches below a solid cloud base.

Ceilometer backscatter profile data can be used to derive the distribution of the time averaged cloud fraction over the site and the distribution of boundary layer aerosols. This data can also be used to derive the atmospheric extinction ratio at low altitudes (near-ground to 25,000 ft [7.5 km].)

Continuous high resolution water vapour mass mixing ratio profile measurements are of interest and provide detail not possible with radiosondes. The ceilometer can show the variation in the height of the boundary layer, above which the water vapour mass mixing ratio can rapidly fall to a low level. This ability to make water vapour measurements by day is important in the study of the variability of water vapour profiles and convective activity in the boundary layer.

Originally, our measurements were taken in the standard measuring mode, where the CL31 digitally samples the return signal every 67ms from 0 to 50 microseconds, providing a spacial resolution of 10m with data-averaged messages received every 2 seconds. A B&B Electronics model ESR901 RS-232 Serial to Ethernet converter is installed in the Ceilometer base on the roof, connecting to the campus internet and mapped to a virtual COM port on the remote data collection computer. The data is recorded using a Pear script, sent to the server automatically via a ScriptFTP script, where it is later parsed and converted to image files using a Matlab and IDL routine. The image files are archived and presented on this site, password restricted link below. Each data message represents 770 value points for the vertical column.

Starting 13 October, 2006, the CL-31 data format was changed to 10m x 770 samples, 15 seconds data collection interval with data averaging, and with error H2 on, setting the range gate normalization so that all data, including noise, is normalized.

The CL-31 electronics were upgraded on 13 February, 2007 with a new engine supplied by Vaisala. There were no other changes in operation and no noticeable changes in data quality.