High Albedos of Cirrus in the Tropical Pacific Warm Pool: Microphysical Interpretations from CEPEX and from Kwajalein, Marshall Islands

Abstract

Recent studies suggest that extensive shields of cirrus clouds over the equatorial Pacific ?warn pool? may have a significant influence on the global climate, yet details of the links between cloud microphysical properties, upper-tropospheric latent and radiative heating rates, and climate are poorly understood. This study addresses whether relatively reflective ice crystals with dimensions smaller than about 100 µm near the tops of tropical cirrus clouds, produced by deep convection when the sea surface temperature exceeds 300 K, are principally responsible for the high albedos observed in this region. In situ measurements of ice crystal size distributions and shapes, acquired during the Central Equatorial Pacific Experiment (CEPEX), are used to derive cloud ice water content (IWC), particle cross-sectional area (A), and other microphysical and optical properties from particles with sizes down to 5 µm. These measurements are needed to ascertain the microphysical properties primarily responsible for determining cloud optical depth and albedo in visible wavelengths and were acquired by a Learjet flying in tropical cirrus and occasionally in convection between altitudes of 8 and 14 km (?20°C to ?70°C). Previously unanalyzed microphysical measurements in the vicinity of Kwajalein, Marshall Islands, acquired in the mid-1970s from a WB57F aircraft between altitudes of 5 and 17 km, are also used to study the variation in microphysical properties from cirrus base to top, using a combination of constant-altitude penetrations and steep ascents and descents through cloud. Analysis shows that IWC, A, and various measures of particle size all tend to decrease with decreasing temperature and increasing altitude, although considerable scatter is observed. Small ice crystals make up more than half the mass and cause more than half the extinction on average in the upper, colder parts of the cirrus; however, the predominantly large particles in the lower, warmer parts of the cirrus contain at least an order of magnitude greater mass and are dominant in producing the high observed albedos. An examination of the lidar and radiometer data acquired onboard the NASA ER-2, which overflew the Learjet during CEPEX, supports the conclusion that the higher, colder regions of the cirrus typically have volume extinction coefficients that are only about 10% of those in the lower, warmer regions.