Comparison between Standard and Modified Forward Scattering Spectrometer Probes during the Small Cumulus Microphysics Study

Abstract

Microphysical measurements performed during the Small Cumulus Microphysics Study (SCMS) experiment are analyzed in order to examine the instrumental limitations of forward scattering spectrometer probes (FSSPs). Complementary information collected with a modified version of the instrument, the Fast-FSSP, are used to address crucial issues such as the size calibration of the spectrometers and the effects on the measured spectra (distortion and broadening), of beam inhomogeneities, of variations of the sampling section, and of the coincidence of particles. Their impact on the calculation of liquid water content is evaluated by the comparison with measurements performed with a hot-wire probe and a particle volume monitor. In addition to the statistical approach that aims at evaluating the typical uncertainty of the measurements, special attention is given to the identification of circumstances under which some of the instrumental limitations combined are likely to affect significantly the accuracy of the measurements. The overall data quality is illustrated in the data summary of the 10 missions flown with the Météo-France Merlin-IV during SCMS. Droplet concentration measurements performed with the standard and the Fast-FSSP, and statistically processed for each flight separately, agree to within a bias lower than 10% and a standard deviation of ±20%. The derived liquid water content measurements, compared to the hot-wire probe, exhibit a larger standard deviation of ±30%, with a substantial degradation at high droplet concentration due to droplet spectra distortion by droplet coincidences in the detection beam.