Picture: Clouds at sunset. Credit: Michael Weidner.

On horizontal scales of several tens to hundreds of kilometres, which we call “mesoscale,” mean vertical motion is very small compared to mean horizontal motion. Yet the vertical motion of the atmosphere exerts a critical influence on the formation of clouds.

  • Large‐scale descent is associated with fair weather, and
  • Large-scale ascent is associated with cloudiness.

Weather and climate modelling often assume that mesoscale vertical motion varies slowly in the three spatial dimensions and with time. However, recent observations over the tropical Atlantic showed strong variability in mesoscale vertical motion, implying that clouds do not only respond to vertical motion but may themselves trigger vertical motion in their vicinity. This study reports similar variability also near Darwin, Australia.

One way that clouds can trigger remote vertical motion is by emitting waves. We can think of these waves as similar to those created when stones are thrown into a pond.

This study examines vertical profiles of horizontal wind speed measured by instruments on ascending balloons near Darwin, Australia. These observations do indeed show waves that can provide a plausible explanation for the patterns of noteworthy variability in mesoscale motions.

These findings suggest a two‐way coupling of clouds to their environment with potentially important consequences for our understanding of weather and climate phenomena.

  • Paper: Stephan, C. C., Lane, T. P., & Jakob, C. ( 2020). Gravity wave influences on mesoscale divergence: An observational case study. Geophysical Research Letters, 47, e2019GL086539. https://doi.org/10.1029/2019GL086539