PARAMETERIZATION OF TROPICAL CIRRUS ICE CRYSTAL SIZE DISTRIBUTIONS AND IMPLICATIONS FOR RADIATIVE-TRANSFER - RESULTS FROM CEPEX

Average ice crystal size distributions are parameterized as functions of temperature and ice water content (IWC), based on observations in c irrus produced as outflows of deep convection made during the Central Equatorial Pacific Experiment (CEPEX), as the sum of a first-order gam ma function, describing ice crystals with melted equivalent diameters (D-m) less than 100 mu m, and a lognormal function, describing larger ice crystals. The fit parameters are chosen to minimize the chi-square d function describing the difference between observed and parameterize d distribution functions. The parameterization is mass conserving, acc urately represents small ice crystals, and is easily integrable. The p arameterization gives accurate estimates of mass, area, and number con tained in different size ranges. The radiative properties estimated fr om midlatitude parameterizations are compared with those estimated fro m this parameterization using anomalous diffraction theory. As opposed to some previous studies, small crystals do not dominate the mass and radiative properties bf cirrus. Comparison with midlatitude observati ons shows that size distribution shape can vary substantially dependin g on where, when, and how the cirrus is measured and on how it forms.