Abstract
AbstractGlobal positioning system (GPS) radio occultation (RO) observations, first made of Earth’s atmosphere in 1995, have contributed in new ways to the understanding of the thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS), an important component of the climate system. The UTLS plays an essential role in the global radiative balance, the exchange of water vapor, ozone, and other chemical constituents between the troposphere and stratosphere, and the transfer of energy from the troposphere to the stratosphere. With their high accuracy, precision, vertical resolution, and global coverage, RO observations are uniquely suited for studying the UTLS and a broad range of equatorial waves, including gravity waves, Kelvin waves, Rossby and mixed Rossby–gravity waves, and thermal tides. Because RO measurements are nearly unaffected by clouds, they also resolve the upper-level thermal structure of deep convection and tropical cyclones as well as volcanic clouds. Their low biases and stability from mission to mission make RO observations powerful tools for studying climate variability and trends, including the annual cycle and intraseasonal-to-interannual atmospheric modes of variability such as the quasi-biennial oscillation (QBO), Madden–Julian oscillation (MJO), and El Niño–Southern Oscillation (ENSO). These properties also make them useful for evaluating climate models and detection of small trends in the UTLS temperature, key indicators of climate change. This paper reviews the contributions of RO observations to the understanding of the three-dimensional structure of tropical UTLS phenomena and their variability over time scales ranging from hours to decades and longer.
Highlights
The thermal structure and variability of the tropical upper troposphere–lower stratosphere (UTLS) are fundamental to many aspects of the climate system
A detailed understanding of the processes that determine the temperature structure of the tropical tropopause is important for our understanding of stratospheric water vapor content and vertical wave propagation [with potential effects on the associated wave forcing of stratospheric winds, e.g., related to the quasi-biennial oscillation (QBO)]
Unlike throughout the free tropical troposphere, temperature near the tropical tropopause shows strong spatial and temporal variability (e.g., Randel et al 2003; Scherllin-Pirscher et al 2017a). This may be heuristically understood based on the long radiative time scales (;60 days) that are observed near the tropical tropopause: a seemingly unimportant heating rate of 0.1 K day21 could result in a temperature perturbation of ;6 K over 60 days
Summary
Tropical Temperature Variability in the UTLS: New Insights from GPS Radio Occultation Observations. ALEXANDER,f RICCARDO BIONDI,g THOMAS BIRNER,h JOOWAN KIM,i WILLIAM J. RANDEL,j SEOK-WOO SON,k TOSHITAKA TSUDA,l AND ZHEN ZENGd a Zentralanstalt fu€r Meteorologie und Geodynamik, Graz, Austria b Wegener Center for Climate and Global Change, University of Graz, Graz, Austria c Institute for Geophysics, Astrophysics, and Meteorology, Institute of Physics, University of Graz, Graz, Austria d COSMIC Program Office, University Corporation for Atmospheric Research, Boulder, Colorado e NorthWest Research Associates/Colorado Research Associates, Boulder, Colorado f Australian Antarctic Division, Hobart, Tasmania, Australia g Dipartimento di Geoscienze, Università degli Studi di Padova, Padua, Italy h Ludwig Maximilian University, Munich, Germany i Department of Atmospheric Sciences, Kongju National University, Gongju, South Korea j Atmospheric Chemistry, Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, Colorado k School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea l Research Institute for Sustainable Humanosphere, Kyoto University, Uji, Japan (Manuscript received May 2020, in final form September 2020)
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