The Cloud Droplet Probe (CDP) manufactured by Droplet Measurement Technologies, fulfills the core data-set requirement for droplet sizes over the range 2µm to 50µm. The probe is a wing mounted instrument, which houses a forward-scatter optical system, which includes a laser heating circuit, photodetectors and analog signal conditioning. It is operated as part of the Core Cloud Physics Console. CDP measures:

• Cloud droplet size distribution over the range 2µm to 50µm
• Average drop diameter
• Particle concentration (when sample area and velocity are known)
• Mass weighted diameter
• Mode distributed diameter
• Standard deviation
• Liquid water content (LWC).

For the first 256 particles detected during each sample (1Hz), the core CDP data set now also includes:

• Particle size in counts
• Individual particle time with µs resolution.
• Inter-arrival Particle Time (IPT) (the time in msec that has elapsed between quialified particles)

System Components

CDP is mounted on a wing canister following the design used on NCAR's G-V research aircraft. The basic components and theory of operation is as follows:

• The air sample is passed through the laser beam and the particle scatters light in all directions. Of this scattered light, the photons which are scattered in the 4° to 12° range, are collected and directed to a 33/66% optical beam splitter.
• These forward scattered photons are focused through an optical mask and are directed to a pair of photodetectors (one of which receives 33%, one receives 66% of the scattered light). These photodetectors determine that the particle falls within the 'depth of field', and if acceptable, the photon pulses are then converted to electrical pulses, and are amplified.
• The resulting analogue voltage value is digitised and categorized into one of the 10, 20, 30 or 40 sizing bins.
• This data is transmitted to the on-board computer, via a RS232 serial packet, which is then logged and viewed by the instrument operator in real-time, using the Particle Analysis Display System (PADS).

Data

CDP data are valid within the standard operating envelope of the aircraft, and is made available to the scientific community via the British Atmospheric Data Centre (BADC). The electronic and software features include user programmable sample rates, number of bins and bin thresholds. CDP is set to collect data at 1hz, over 30 size bins. Threshold values are available from FAAM (see Further Details, below). The software also enables real-time monitoring of household variables such as laser operating current, laser power and CDP internal temperature.

CDP was trialled in the laboratory as well as on the aircraft from September 2008. Comparisons with the Met Office FFSSP probe, Manchester Universities CDP and FSSP-100, and the University of Hertfordshire SID probes have allowed the performance of CDP to be assessed thoroughly and have confirmed the quality of data produced.

CDP calibration procedures include spinning pinhole tests and glass bead calibrations. These check both the alignment, cleanliness and accuracy of measurements. A rigorous cleaning, calibration and maintenance regime continues at FAAM laboratories which ensures CDP continues to produce high quality and dependable data for use in the scientific community.

Documentation

Further details