12th Conference on Middle Atmosphere

Thursday, 7 November 2002: 2:50 PM
The origins of thin cirrus observed in the tropical tropopause layer over the Pacific Ocean: Convective influence case studies from the GTE Pacific Exploratory Missions
Henry B. Selkirk, Bay Area Environmental Research Institute, Moffett Field, CA; and L. Pfister and E. V. Browell
The presence of thin, and at times, sub-visible cirrus in the upper tropical troposphere presents challenges to our understanding of the dynamics and microphysics of deep convection over the tropics and the role that such convection plays in the radiative balance in this sensitive region of the global climate system. We recently reported airborne DIAL observations of thin, isolated high altitude cirrus in the central equatorial Pacific were obtained during December 1995 and February 1996 during the NASA Tropical Ozone Transport and Vortex Ozone Transport Experiments (TOTE/VOTE). The TOTE/VOTE thin cirrus took two forms: (a) thin, laminar layers that could be attributed to cooling of nearly saturated upper tropospheric air and (b) thicker, more textured structures that appeared to originate more directly from identifiable convective systems a day or two earlier. Does this typology usefully apply to other areas and seasons in the tropical Pacific? In particular, what are the characteristics of thin cirrus in regions where local convection is more frequent and more highly organized as in the Pacific warm pool? The primary analysis tool we use is the convective influence technique, which combines isentropic back trajectories with infrared brightness temperatures obtained from the GMS and GOES geostationary satellites.

We present here measurements acquired over the entire tropical Pacific basin and adjacent subtropics during the equinoctial seasons. As with the TOTE/VOTE observations, we examine aerosol scattering ratio measurements made by the NASA-Langley airborne DIAL system on board in the NASA DC-8 aircraft. These were taken as part of the four Pacific Exploratory Missions (PEM) in NASA’s Global Tropospheric Experiment: PEM-West A (September-October 1991), PEM-West B (February-March 1994), PEM-Tropics A (September-October 1996) and PEM-Tropics B (March-April 1999). Differences in details of the DIAL aerosol measurements among the four PEM missions preclude the use here of a precise operational definition of thin cirrus as was used in the TOTE/VOTE work. Nonetheless, it is clear that geometrically thin (1 km or less) and vertically isolated cloud layers near the tropopause occurred on each of the missions in a wide range of meteorological conditions. We apply the convective influence technique to characterize the thermal history of these layers and to identify any deep convective systems from which these layers may have originated. In the case of layers formed from cooling processes, we identify the region upstream whence the moistened layer may have emerged.

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