Investigation of tropical cirrus, their variability, evolution, and relation to the upper tropospheric water vapor.

摘要

This thesis investigates three topics concerning tropical cirrus: (1) comparison of satellite-based cirrus retrieval schemes; (2) variability of tropical cirrus in response to the Mt. Pinatubo eruption and the 1997–1998 El Niño/1998 La Niña; (3) formation and evolution processes of tropical cirrus and their relation to the upper tropospheric water vapor (UTWV).; Two satellite-based cirrus retrieval schemes are compared: the split-window method and the ISCCP cloud scheme. The comparison shows that both cloud schemes identify about 0.2–0.3 cirrus cloud amount in the tropics, although there are detailed differences of classification for about half of these clouds. Further assessments in comparison with an analysis of collocated radiance measurements in the IR (6.7 μm) and microwave (183 GHz) water vapor channels are conducted to evaluate the two cirrus retrieval schemes.; Comparison of three satellite-based cirrus datasets (produced by the ISCCP, the split-window, and the 31 analysis) indicates that the Mt. Pinatubo volcanic aerosol did not have a significant, systematic, and widespread effect on both cirrus amount and optical thickness. The 1997–1998 El Niño is accompanied by a cirrus decrease in the west Pacific and a cirrus increase in the central/east Pacific and west Indian ocean; however, no large-scale, mirror image changes in cirrus are observed during the 1998 La Niña.; The formation and evolution processes of tropical cirrus are studied in a Lagrangian trajectory approach by following air expelled from convective systems and examining the changes of cirrus properties along forward trajectories. It is found that cirrostratus (cirrus anvils) decay quickly and evolve into thin cirrus and thin cirrus frequency drops off very slowly with time. Examination of tropical cirrus and UTWV shows that cirrus are associated with a moister upper troposphere and a slower decrease rate of UTWV The amount of water in the cloud is too small for evaporation of cirrus ice crystals to moisten the upper troposphere. Rather, it is the turbulence/wave transport that brings water vapor upward to maintain the high UTWV level when cirrus occur. Finally, the maintenance of the UTWV is discussed by using a simple, conceptual model.