The observed impact of mesoscale vertical motion on tropical clouds

A recent study by Geet George, Bjorn Stevens (both from the Max Planck Institute for Meteorology) and Marcus Klingebiel (now at Leipzig University) along with Sandrine Bony and Raphaela Vogel (Laboratoire de Météorologie Dynamique in Paris) shows that aspects of the atmospheric circulation tend to control low-level cloudiness at the mesoscale (20-200 km) more than the conventionally studied thermodynamic aspects.

© MPI-M

There has been a long-standing lack of measurements of atmospheric circulation at the mesoscale because of observational challenges. With the NARVAL2 field campaign in 2016, however, measurements of mesoscale vertical motion were made for the first time using dropsondes. This provided an opportunity to study the influence of circulation on cloudiness in the northern Atlantic trade-wind regions, where the field campaign took place.

 

Geet George and co-authors analysed the retrieved data and found that the low-level vertical motion of the atmosphere can strongly control the cloud occurrence at cloud base. With rising motion at cloud base, measures of cloudiness such as cloud fraction, liquid water and precipitation all increase. In the study, they propose a simple mechanism of how the vertical motion influences cloudiness via the convective mass flux, which is essentially the rate of moist air ventilated into the atmospheric layer where clouds form. Moreover, factors which are known to control cloudiness at larger scales fail to explain the variability of cloudiness at the mesoscale.

 

Currently, climate models do not consider dynamical parameters of the atmosphere, such as vertical velocity or pressure velocity to determine cloudiness. With these findings, the authors argue for such parameters to be directly incorporated into cloud parameterization schemes, especially since the study shows that clouds seem to be influenced non-linearly by mesoscale conditions.

 

The analysis made in the study are based on case-studies from NARVAL2, but future activities, in particular measurements from the recently concluded EUREC4A campaign provide more abundant sampling and richer data-streams. Thus, there are further opportunities to study interactions between clouds and their atmospheric environment at the mesoscale. The authors, along with many others, are now digging into these measurements to better understand the processes linking shallow cumulus clouds to circulation — something that has remained elusive and has made understanding cloud-feedback, i.e., response of clouds to a warming surface a major challenge.

 

Original publication:

George, G., Stevens, B., Bony, S., Klingebiel, M., & Vogel, R. (2021) Observed impact of meso-scale vertical motion on cloudiness, Journal of the Atmospheric Sciences, 78 (8), doi.org/10.1175/JAS-D-20-0335.1

 

Contact:
Geet George
Max Planck Institute for Meteorology
Email: geet.george@we dont want spammpimet.mpg.de