Effects of Coincidence on Measurements with a Forward Scattering Spectrometer Probe

Abstract

A PMS Forward Scattering Spectrometer Probe (FSSP) may fail to detect a droplet that enters the illuminated volume in coincidence with another droplet, or such a coincident pair may be assigned an erroneous size. This effect is shown to distort the droplet size spectra measured by the FSSP, and the effects can be quite important in clouds having droplet concentrations exceeding 500 cm⁻³. The most common coincidence error is one that arises when a droplet within the sample volume of the FSSP is rejected or sized incorrectly because of the coincident passage of another droplet outside that sample volume. Droplet spectra are measured to be too broad, and to contain too many large but too few small droplets, as a result of this effect. Some implications for past and future studies using this probe are discussed.