Microphysical and radiative properties of tropical clouds investigated in TC4 and NAMMA



[1] The size, shape and concentration of ice particles in tropical anvil cirrus and in situ cirrus clouds have a significant impact on cloud radiative forcing, and hence on global climate change. Data collected in tropical anvil and cirrus clouds with a 2D-S probe, an optical imaging probe with improved response characteristics and the ability to remove shattered artifacts, are analyzed and discussed. The data were collected with NASA DC-8 and WB-57F research aircraft near Costa Rica during the 2007 Tropical Composition, Cloud and Climate Coupling (TC4) field project, and with the DC-8 near Cape Verde during the 2006 NASA African Monsoon Multidisciplinary Analyses (NAMMA) campaign. Data were collected in convective turrets, anvils still attached to convection, aged anvils detached from convection and cirrus formed in situ. Unusually strong maritime convection was encountered, with peak updrafts of 20 m s−1, ice water contents exceeding 2 g m−3 and total particle concentrations exceeding 10 cm−3 at 12.2 km. Ice water contents in the anvils declined outward from the center of convection, decreasing to <0.1 g m−3 in aged anvil cirrus. The data show that microphysical and radiative properties of both tropical anvils and cirrus are most strongly influenced by ice particles in the size range from about 100 to 400 μm. This is contrary to several previous investigations that have suggested that ice particles less than about 50 μm control radiative properties in anvils and cirrus. The 2D-S particle area and mass size distributions, plus information on particle shape, are input into an optical properties routine that computes cloud extinction, asymmetry parameter and single scattering albedo. These optical properties are then input into two-stream radiative code to compute radiative heating profiles within the various cloud types. The results produce short- and long-wave heating/cooling vertical profiles in these tropical clouds. A simple parameterization based on 2D-S measurements is derived from the particle mass size distribution that yields an area size distribution. The parameterized area size distribution can then be used in large-scale numerical simulations that include radiative transfer packages.