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J. F. Read and R. T. Pollard

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A. Gettelman, W. J. Randel, S. Massie, F. Wu, W. G. Read, and J. M. Russell III

Abstract

The interannual variability of the tropical tropopause region between 14 and 18 km is examined using observations of convection, winds, and tropopause temperatures from reanalyses and water vapor from satellites. This variability is compared to a simulation using the Community Climate Model version 3 (CCM3) general circulation model forced by observed sea surface temperatures. A coherent picture of the effect of the El Niño–Southern Oscillation (ENSO) on the tropopause region is presented in the NCEP–NCAR reanalyses and CCM3. ENSO modifies convection in the Tropics, and the temperature and circulation of the tropical tropopause region, in agreement with idealized models of tropical heating. CCM3 reproduces most details of these changes, but not the zonal mean temperature variations present in the analysis fields, which are not related to ENSO. ENSO also forces significant changes in observed and simulated water vapor fields. In the upper troposphere water vapor is at maximum near convection, while in the tropopause region water vapor is at minimum in the regions of convection and surrounding it. Convection, cirrus clouds, temperatures, and transport are all linked to describe the water vapor distribution and highlight the role of transport in the tropopause region.

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P. G. Challenor, J. F. Read, R. T. Pollard, and R. T. Tokmakian

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This paper describes a new method for combining altimetry data with hydrography in order to produce absolute surface geostrophic currents from altimetry. This method is then applied to data from the Drake Passage allowing surface currents to be monitored every 35 days during the second half of 1992. The resulting currents show several regions of strong currents with water flowing to the east and other places where the currents are either zero or flowing to the west After comparison with a model it is suggested that this structure is a result of the bathymetry.

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D. L. Wu, E. F. Fishbein, W. G. Read, and J. W. Waters

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The quasi-2-day wave is known as a strong and transient perturbation in the middle and upper atmosphere that often occurs shortly after solstice. The excitation mechanisms of this transient wave have been discussed for years, but no clear answer has yet been attained. In this paper, propagating characteristics of the 2-day wave are studied based on 8-mon temperature measurements from the Microwave Limb Sounder onboard the Upper Atmosphere Research Satellite. The studies are focused on the wave events that happened in January 1993 and in July–August 1993. The observations suggests that winter planetary waves could be responsible for triggering the summer 2-day wave through long penetration into the summer stratosphere. A connection is evident in the evolution of the wave amplitude between the summer 2-day wave generation and winter wave penetration. The data also suggest that the enhancement of the wave amplitude is a manifestation of both a local unstable wave and a global normal-mode Rossby wave.

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W. G. Read, J. W. Waters, D. A. Flower, L. Froidevaux, R. F. Jarnot, D. L. Hartmann, R. S. Harwood, and R. B. Rood

Initial results of upper-tropospheric water vapor obtained from the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) are presented. MLS is less affected by clouds than infrared or visible techniques, and the UARS orbit provides daily humidity monitoring for approximately two-thirds of the earth. Best results are currently obtained when water vapor abundances are approximately 100–300 ppmv, corresponding to approximately 12-km height in the Tropics and 7 km at high latitudes. The observed latitude variation of water vapor at 215 hPa is in good agreement with the U.K. Universities's Global Atmospheric Modelling Project model. The ability to observe synoptic-scale features associated with tropopause height variations is clearly illustrated by comparison with the National Aeronautics and Space Administration Goddard Space Flight Center assimilation model. Humidity detrainment streams extending from tropical convective regions are also observed.

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J. W. Waters, W. G. Read, L. Froidevaux, R. F. Jarnot, R. E. Cofield, D. A. Flower, G. K. Lau, H. M. Pickett, M. L. Santee, D. L. Wu, M. A. Boyles, J. R. Burke, R. R. Lay, M. S. Loo, N. J. Livesey, T. A. Lungu, G. L. Manney, L. L. Nakamura, V. S. Perun, B. P. Ridenoure, Z. Shippony, P. H. Siegel, R. P. Thurstans, R. S. Harwood, H. C. Pumphrey, and M. J. Filipiak

Abstract

The Microwave Limb Sounder (MLS) experiments obtain measurements of atmospheric composition, temperature, and pressure by observations of millimeter- and submillimeter-wavelength thermal emission as the instrument field of view is scanned through the atmospheric limb. Features of the measurement technique include the ability to measure many atmospheric gases as well as temperature and pressure, to obtain measurements even in the presence of dense aerosol and cirrus, and to provide near-global coverage on a daily basis at all times of day and night from an orbiting platform. The composition measurements are relatively insensitive to uncertainties in atmospheric temperature. An accurate spectroscopic database is available, and the instrument calibration is also very accurate and stable. The first MLS experiment in space, launched on the (NASA) Upper Atmosphere Research Satellite (UARS) in September 1991, was designed primarily to measure stratospheric profiles of ClO, O3, H2O, and atmospheric pressure as a vertical reference. Global measurement of ClO, the predominant radical in chlorine destruction of ozone, was an especially important objective of UARS MLS. All objectives of UARS MLS have been accomplished and additional geophysical products beyond those for which the experiment was designed have been obtained, including measurement of upper-tropospheric water vapor, which is important for climate change studies. A follow-on MLS experiment is being developed for NASA’s Earth Observing System (EOS) and is scheduled to be launched on the EOS CHEMISTRY platform in late 2002. EOS MLS is designed for many stratospheric measurements, including HOx radicals, which could not be measured by UARS because adequate technology was not available, and better and more extensive upper-tropospheric and lower-stratospheric measurements.

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