New theory for radiatively driven shallow circulations

Credit: MPI-M

Credit: MPI-M

In a new study recently accepted in the Journal of Atmospheric Science, Ann Kristin Naumann, Bjorn Stevens, Cathy Hohenegger and Juan Pedro Mellado from the Max Planck Institute for Meteorology (MPI-M) show that shallow circulations driven by radiative differences can be as strong as those driven by surface temperature differences.

The scientists developed a simple conceptual model to understand how radiative cooling induces circulations in the lower part (1-3 km) of the tropical troposphere. These shallow circulations are believed to be important in determining the structure and organization of deep convection, including the Inter-Tropical Convergence Zone.

According to the theory developed by Naumann et al., radiatively driven shallow circulations can be as effective as those imprinted by surface temperature gradients, the latter having been the dominant paradigm for explaining how patterns of sea-surface temperatures influence the position and strength of deep convection. Their theory is evaluated using high-resolution (large-eddy) simulations and found to be able to explain the experimental numerical results.

Their work introduces a new way of thinking about tropical dynamics, as flows may be driven as much by differential long-wave cooling in the lower troposphere, as by surface temperature gradients arising from differences in near-surface wind speeds or insolation changes. In other words, the radiative effects of water vapor in the lower troposphere may play a very active role in the coupled dynamics of this important, and still mysterious, part of Earth's atmosphere.

Naumann, A. K., Stevens, B., Hohenegger, C., & Mellado, J.-P. (2017): A conceptual model of a shallow circulation induced by prescribed low-level radiative cooling. doi: 10.1175/JAS-D-17-0030.1

Dr Ann Kristin Naumann
Max Planck Institute for Meteorology
Phone: +49 40 41173 239
Email: ann-kristin.naumann@we dont want