HR: 11:20h
AN: A11H-05    [PDF]
TI: Tropical cirrus lifecycle and relation to the upper tropospheric water vapor
AU: * Luo, Z
EM: luo@cira.colostate.edu
AF: Cooperative Institute for Research in the Atmosphere, Colorado State University, W. Laporte Ave, Fort Collins, CO 80523-1375 United States
AU: Rossow, W B
EM: wrossow@giss.nasa.gov
AF: NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025 United States
AB: Tropical cirrus lifecycle, evolution and relation to the upper tropospheric water vapor (UTWV) are examined by analyzing satellite derived cloud data, UTWV data, and NCEP/NCAR reanalysis wind field. Building upon the existing ISCCP data and the TOVS product, a global (except polar region), six-hourly cirrus dataset are developed from two infrared radiance measurements roughly at 11 and 12 micron. The analysis in this study is conducted in a Lagrangian framework, which assumes that cirrus, like other tracers, drift with the upper tropospheric wind, while at the same time, going through their lifecycle from formation to maturation to decay. The Lagrangian trajectory analysis shows that the decay of deep convection is immediately followed by the growth of cirrostratus and then the decay of cirrostratus is followed by the growth of cirrus. Cirrus properties continuously evolve along the trajectories as they gradually thin out and move to the lower levels. Typical tropical cirrus systems last for 19 - 30 hours, with a standard deviation of about 16 hours. This is much longer than cirrus particle lifetimes (0.8 - 8 hours), suggesting that there are other processes (e.g. large-scale lifting) that oppose particle sedimentation to maintain tropical cirrus. Consequently, tropical cirrus can advect over large distances, about 600 - 1000 km, during their lifetimes. For almost all current GCMs, this distance spans more than one grid box requiring that the water vapor and cloud water budgets include an advection term. Based on their relationship to convective systems, detrainment cirrus are distinguished from in situ cirrus. It is found in this study that more than half of the tropical cirrus are formed in situ. The interaction between cirrus and UTWV is explored by comparing the evolution of the UTWV along composite clear trajectories and trajectories with cirrus. Cirrus are found to be associated with a moister upper troposphere and a slower decrease rate of UTWV. Moreover, elevated UTWV level has a longer duration than cirrus. The amount of water in cirrus is too small for evaporation of cirrus ice particles to moisten the upper troposphere. Rather, it is large-scale and small-scale transport that brings water vapor upward to maintain the high UTWV level. Finally, the maintenance of UTWV is discussed by using a simple, conceptual model.
DE: 0320 Cloud physics and chemistry
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting