Parameterization of effective ice particle size for high-latitude clouds

摘要

A parameterization has been developed for mean effective size D_(ge) in terms of ice water content (IWC) and temperature using in situ measurements of ice crystal spectra, cloud particle shapes and particle cross-sectional area A from four research projects conducted in latitudes north of 45°N. The cloud microphysical measurements were made using PMS 2D optical probes, a PMS forward scattering spectrometer probe (FSSP), and Nevzorov total water and liquid water content probes. The IWCs derived from particle spectra using three different methods were compared with IWC measured with the Nevzorov probe (IWC_(Nev)). The contribution of small particles to the total mass was estimated by integrating a gamma distribution function that was fitted to match the measured FSSP concentrations. The D_(ge) was calculated from the derived IWC and total cross-sectional area per unit volume A_c. This analysis indicates that there are significant differences among the schemes used to derive the IWC. It was found that the IWC derived based on the Cunningham scheme and IWC_(Nev) have the highest correlation: r~2 = 0.78. After considering small particles the derived IWC almost matched the IWC_(Nev). The average estimated contribution of small particles to the A_c was 43%. The average estimated contribution of small particles to the total IWC, however, was 20%. Since D_(ge) is directly proportional to the ratio IWC/A_c, the addition of small particles reduced the derived D_(ge) considerably. The largest changes in D_(ge) associated with small particles, however, occur at the coldest temperature and at low IWC, reaching up to 45% for temperatures less than -25 ℃. Generally, D_(ge) and IWC increase with increasing temperature. Good agreement between the parameterized D_(ge) and derived D_(ge) from measurements were found when small particles were included.