Upper Troposphere - Lower Stratosphere (UTLS)

The Upper Troposphere and Lower Stratosphere (UTLS) program aims to study the coupled dynamical, chemical and microphysical processes in the tropopause region. The primary motivation for understanding these coupled processes is their essential role in climate change, and the desire to critically diagnose and improve model simulations of this region. Water vapor, ozone, aerosols and cirrus cloud in the UTLS region, controlled by these coupled processes, have important impacts on the Earth’s radiation budget. To understand and further quantify the controlling mechanisms of these radiatively significant species, integrated studies of dynamical, physical and chemical processes are required. The program currently focuses on four aspects of these coupled processes:

  1. Dynamical and microphysical processes that control water vapor and clouds in the UTLS, especially that in the tropical tropopause layer (TTL).
  2. The role of dynamics, on large and small scales, in redistributing UTLS chemical constituents, including transport and mixing between stratosphere and troposphere.
  3. Characterization and maintenance of the tropopause, its long term change and variability.
  4. Multi-phase chemistry that controls the budgets of ozone and radical species in the UTLS.

Figure. This schematic portrays important processes that couple dynamics, chemistry and cloud microphysics in the UTLS region. The dashed green line represents the time average tropopause. In the tropics, maximum outflow from deep convection occurs near ~12-14 km, while the cold point tropopause occurs near 17 km. The intervening region has characteristics intermediate between the troposphere and stratosphere, and is termed the tropical transition layer (TTL). Extratropical stratosphere-troposphere exchange occurs in tropopause folds and intrusions linked with synoptic weather systems; these events transport stratospheric air high in ozone into the troposphere. Transport above the subtropical jet (STJ) couples the TTL and the extratropical lower stratosphere. In addition, synoptic scale uplift (‘warm conveyor belts’) and deep convection brings near-surface emissions of gases and particles into the upper troposphere, where they can strongly influence global-scale chemistry. Gravity wave generation and breaking also contribute the mixing of chemical constituents in the UTLS.

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