Parameterization of Tropical Cirrus Ice Crystal Size Distributions and Implications for Radiative Transfer: Results from CEPEX

Abstract

Average ice crystal size distributions are parameterized as functions of temperature and ice water content (IWC), based on observations in cirrus produced as outflows of deep convection made during the Central Equatorial Pacific Experiment (CEPEX), as the sum of a first-order gamma function, describing ice crystals with melted equivalent diameters (Dm) less than 100 mm, and a lognormal function, describing larger ice crystals. The fit parameters are chosen to minimize the chi-squared function describing the difference between observed and parameterized distribution functions. The parameterization is mass conserving, accurately represents small ice crystals, and is easily integrable. The parameterization gives accurate estimates of mass, area, and number contained in different size ranges. The radiative properties estimated from midlatitude parameterizations are compared with those estimated from this parameterization using anomalous diffraction theory. As opposed to some previous studies, small crystals do not dominate the mass and radiative properties of cirrus. Comparison with midlatitude observations shows that size distribution shape can vary substantially depending on where, when, and how the cirrus is measured and on how it forms.