29 July 2019

Investigating the Influence of Cloud Radiative Effects on the Extratropical Storm Tracks

Science

The role of cloud radiative feedbacks in extratropical storm tracks is directly investigated using the Community Earth System Model, version 2 (CESM2). The cloud radiative feedback is artificially disabled using a method called “cloud locking” in which an independent cloud dataset is used for radiative transfer, instead of the model’s predicted cloud fields. Removing the feedback between cloud radiative effects and the atmospheric state results in a slight but significant change in extratropical storm tracks. To understand this change, individual cyclones are detected and tracked using a high-frequency model output. Compiling the statistics of the individual cyclone characteristics shows that the strengthening of the storm track is the result of longer-lived, more intense storms. Results using the fully-coupled system (with dynamic ocean model) and the atmosphere model forced with prescribed sea-surface temperature are very similar, suggesting the change in storm characteristics originate within the atmosphere. The changes in storms are driven by a change in lower-tropospheric heating that changes the static stability and enhances vertical motion within cyclones.

Impact

The result of cloud-locking in CESM2 confirms the findings of another study, but the cloud-locking methodology provides additional clarity by specifically targeting the high-frequency co-variance of clouds and dynamical fields.  Doing so also provides the new insight that it is the longwave effects of clouds in the midlatitudes that act to damp storms through their impact on the static stability. These results illustrate that properly representing clouds – including their impact on radiation and coupling with their environment -- is important for simulating the extratropical storm tracks. Biases in cloud properties within extratropical storms have been previously documented. This study indicates that such biases may impact the storm track properties, and may play a role in the projected changes in extratropical storm tracks.

Summary

Cloud locking is a methodology that directly disables the feedback between cloud radiative effects and the atmospheric state. This study uses cloud locking in CESM2 to investigate the role of cloud radiative feedbacks on extratropical storm tracks. In the absence of coupling between clouds and the atmospheric state, storm track activity increases by 5-10%; storms are stronger and last longer, meaning that under normal circumstances in CESM2 cloud effects act to damp storm tracks.

Contact
Brian Medeiros
National Center for Atmospheric Research (NCAR)
Publications
Grise, K, B Medeiros, J Benedict, and J Olson.  2019.  "Investigating the Influence of Cloud Radiative Effects on the Extratropical Storm Tracks."  Geophysical Research Letters.  https://doi.org/10.1029/2019gl083542.