Comparisons of Airborne CVI and FSSP Measurements of Cloud Droplet Number Concentrations in Marine Stratocumulus Clouds

Comparisons of counterflow virtual impactor (CVI) and forward scattering spectrometer probe (FSSP) measurements of cloud droplet number concentrations obtained by two different aircraft in marine boundary layer stratocumulus clouds are presented. The cut sizes of the CVIs have been calculated using a mathematical model (D LH50) and using Stokes' theory (D S50). For most of the cases the agreement between D LH50 and D S50 is within 3%. Based on the results obtained with the mathematical model the agreement between CVI and FSSP concentrations is within 25% for most of the cases analyzed in this study and also within 10% for several of the cases. The reasonable agreement between CVI and FSSP concentrations supports the assumption that a droplet releases a single residual particle upon evaporation. Furthermore, in this study it is shown that droplet shattering can qualitatively be identified and it occurs often when there are elevated concentrations of drizzle droplets in the clouds. Poor agreement between the CVI and FSSP was found in a few instances. For one of these cases the discrepancy obtained between the CVI and FSSP seems to have been caused by losses in the CVI mainly by cloud droplets with larger sizes.