In situ measurements of effective diameter and effective droplet number concentration

The effective diameter of cloud droplets is usually derived from measurements of droplet size distribution measured by Particle Measuring Systems (PMS) probes. The disadvantage of this method is that PMS probes have a truncated size range. During the RACE project, an alternative method to measure the effective diameter used a cloud extinction meter and King and Nevzorov hot wire liquid water content and total water content (LWC/ TWC) probes installed on the National Research Council (NRC) Twin Otter. The effective diameter was derived from direct in situ measurements of the extinction coefficient (epsilon) and liquid water content (W) as D-eff=k(1)W/epsilon. This method of calculation of D-eff is free of problems related to deriving D-eff from the truncated particle size distribution, Since measurements of epsilon and W cover the whole size range of cloud particles, this method gives an accurate value of D-eff. This method can also be successfully applied for mixed and ice phase clouds, since the Nevzorov TWC probe provides measurements of total (ice plus liquid) water content. Effective number concentration was calculated as N-eff=k(2)epsilon(3)/W-2. Comparisons of D-eff and N-eff, calculated by this method, and directly from PMS Forward Scattering Spectrometer Probe (FSSP) spectra, are favorable in the subset of conditions when the FSSP is considered to measure the spectra fully and accurately.