FAAM CORE Cloud Droplet Probe


The CDP measures cloud droplet number and size in the nominal size range 3 to 50 microns. It is an optical particle counter (OPC) and uses a laser to illuminate droplet-laden air between the instruments two arms. Light scattered from each particle is counted with the intensity of the light scattered into a fixed solid angle giving a measure of particle size. The instrument is fitted to the aircraft in one of the PMS canisters under the aircraft wings following the design used on NCAR's G-V research aircraft.


Instrument specifications

Nominal size range 3µm to 50µm
Sample area 0.199 mm-2
Number concentration range 0 - 10000 cm-3
Number of size bins 30
Sampling frequency 1Hz + PBP or 10Hz (no PBP)
Refractive index 1.30 - 1.70 non-absorbing
Light collection angles 1.7° - 14°
Air speed range 10 - 200 ms-1
Laser wavelength 658 nm
Temperature -40 to +40 degrees C
Altitude 0 - 50,000 feet

Instrument description

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 (nominal) 1.7° to 14° range, are collected and directed to a 50/50 optical beam splitter.

These forward scattered photons are focused through an optical mask and directed to a pair of photodetectors, referred to as the sizer and the qualifier. 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 (into a number known as a 'pulse height') and categorized into one of the 30 sizing bins (the boundaries of which are known as the bin 'thresholds').

This data is transmitted to the on-board computer, via a RS422 serial packet, which is then logged and viewed by the instrument operator in real-time, using the Particle Analysis Display System (PADS).

The core FAAM CDP can be set to collect data at 1Hz or 10Hz sampling, over 30 size bins. For 1Hz sampling the CDP data now also includes Particle-by-Particle (PBP) data. For the first 256 particles detected during each sample:

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


Sizing response calibrations are conducted on a flight-by-flight basis wherever possible. Each individual processed calibration file is provided, along with a 'Master' calibration, which contains the mean of all individual calibrations for the campaign. Each calibration file (including the Master) contains metadata on the beads used in the calibration. The calibration time-line along with download links for all calibration data can be found on the CDP calibration page linked above, or alternatively from the contact at the bottom of the page.

The CDP calibration method is linked here: pdf CDP calibration method (36 KB)


The CDP calibration provides a .csv file containing the calibrated scattering cross section bin boundaries for the 30 bins (and the associated uncertainties). The user can then convert these scattering cross sections into diameters for the sample of interest.

FAAM provides two software tools to enable the data-user to convert the data:

  • MieConScat uses Mie theory to produce a table of scattering cross sections as a function of diameter, based on given instrument optical geometry, laser wavelength and particle refractive index.
  • CStoDConverter reads in the calibrated scattering cross section .csv file and uses these values to determine the bin mean and bin width (and associated errors) in terms of diameter. CStoDConverter uses scattering data created by MieConScat, so both programs are required.

Both programs are provided as Windows executables, and C++ and FORTRAN source code is included which compile on other operating systems. Each software contains user instructions, and are provided courtesy of Dr Phil Rosenberg, hosted at SourceForge.


CDP data is available to the scientific community via the Centre for Environmental Data Analysis (CEDA). The data should be considered in 3 stages:

(1) Raw data: (filename: core-cloud-phy_faam_yyyymmdd_rN_bNNN_rawpads.zip), which contains un-processed, un-calibrated data as recorded by the on-board data acquisition system.

(2) Processed data: (filename: core-cloud-phy_faam_yyyymmdd_vNNN_rN_bNNN.nc) which contains processed uncalibrated data. CDP processed data is where particle concentration has been corrected for aircraft airspeed and the sample area. Total concentration per second is calculated by summing individual channel (n) concentrations, which are calculated as follows:

concentration_n = counts_n / sample volume

Where counts_n = raw counts per second for channel n, and sample volume = sample area x TAS, where the sample area is in meters (1.99 x 10-7) and TAS (True Air Speed) is in meters per second.

(3) Processed, calibrated data. Unlike the majority of FAAM core instrumentation, the CDP calibration has been designed to allow data-user flexibility based on the sample of interest (e.g. water, aerosol, dust etc). Therefore, calibrations should be applied by the data-user. See calibration section above.


The validity of CDP data if affected by the applied sample area. The sample area is periodically measured (after any gross changes in alignment) by groplet gun beam mapping.

The default manufacturer sample area is 0.24 mm-2.

For CDP #1:

Before June 2014 the sample area was 0.6 mm-2

Between June 2014 and January 2015 the sample area was 0.517 mm-2

Between January 2015 and October 2018 the sample area was 0.252 mm-2

From October 2018 to present the sample area has been 0.199 mm-2

For CDP #2:

Before October 2018 the sample area was 0.24 mm-2 (manufacturer default)

From October 2018 to present the sample area has been 0.268 mm-2


DMT CDP-2 Manual. http://www.dropletmeasurement.com/resources/manuals-guides.

FAAM OPC calibration paper. P.D. Rosenberg et al, Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign, Atmos. Meas. Tech., 5, 1147-1163, doi:10.5194/amt-5-1147-2012, 2012.

Further details

Contact Dr Chris Reed at FAAM

Additional information