Determination of the Size and Concentration of Cloud Drops with an FSSP

Abstract

A detailed size calibration of the Forward Scattering Spectrometer Probe (FSSP) over the size ranges of interest to cloud physics (2–47 μm), is presented. The calibration includes laboratory tests with glass spheres, Mie scattering calculations and airborne cloud measurements. Each size bin of the FSSP was found to have errors of 0–5 μm in diameter. The magnitude and sign of the errors were dependent on airspeed for airspeeds greater than 55 m s⁻¹. This sizing error can lead to errors of 70% or more in derived cloud liquid water content. In addition, a concentration algorithm was derived from laboratory tests with pseudo-random pulse generators which properly corrects for probe dead time. After application of both calibration algorithms the instrument is capable of yielding mean diameters that agree with predicted mean diameters to within a few tenths of a micron and liquid water contents which are in good agreement with an independent liquid water measurement. Even after corrections, the instrument still produces significant artifical broadening of a cloud drop spectrum.