HIGH ALBEDOS OF CIRRUS IN THE TROPICAL PACIFIC WARM POOL - MICROPHYSICAL INTERPRETATIONS FROM CEPEX AND FROM KWAJALEIN, MARSHALL ISLANDS

Recent studies suggest that extensive shields of cirrus clouds over th e equatorial Pacific ''warm pool'' may have a significant influence on the global climate, yet details of the links between cloud microphysi cal properties, upper-tropospheric latent and radiative heating rates, and climate are poorly understood. This study addresses whether relat ively reflective ice crystals with dimensions smaller than about 100 m u m near the tops of tropical cirrus clouds, produced by deep convecti on when the sea surface temperature exceeds 300 K, are principally res ponsible for the high albedos observed in this region. In situ measure ments of ice crystal size distributions and shapes, acquired during th e Central Equatorial Pacific Experiment (CEPEX), are used to derive cl oud ice water content(IWC), particle cross-sectional area (A), and oth er microphysical and optical properties from particles with sizes down to 5 mu m, These measurements are needed to ascertain the microphysic al properties primarily responsible for determining cloud optical dept h and albedo in visible wavelengths and were acquired by a Learjet fly ing in tropical cirrus and occasionally in convection between altitude s of 8 and 14 km (-20 degrees C to -70 degrees C). Previously unanalyz ed microphysical measurements in the vicinity of Kwajalein, Marshall I slands, acquired in the mid-1970s from a WB57F aircraft between altitu des of 5 and 17 km, are also used to study the variation in microphysi cal properties from cirrus base to top, using a combination of constan t-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 ave rage in the upper, colder parts of the cirrus; however, the predominan tly large particles in the lower, warmer parts of the cirrus contain a t lease an order of magnitude greater mass and are dominant in produci ng the high observed albedos, An examination of the lidar and radiomet er data acquired onboard the NASA ER-2, which overflew the Learjet dur ing 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.