Potential vorticity anomalies related to clouds and precipitation in extratropical cyclones

• Main Author: Crezee, Bas
• Other Authors: Wernli, Heini, Joos, Hanna, Rivière, Gwendal
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: ETH Zurich 2017
• Subjects: info:eu-repo/classification/ddc/550; Earth sciences; Bering Sea
• Online Access: https://hdl.handle.net/20.500.11850/204404; https://doi.org/10.3929/ethz-b-000204404

Different microphysical processes influence the dynamics of cyclones by their associated latent heating or cooling, which modifies potential vorticity (PV) from the boundary layer to slightly above the tropopause level. A new method is developed based on integrating diabatic PV changes along backward trajectories. This method allows us to decompose the Lagrangian PV change over a certain time interval into contributions from different microphysical processes, including the below-cloud processes snow sublimation, snow melting and rain evaporation. The method is first applied to an extratropical cyclone in an idealized baroclinic channel setup. The mesoscale PV structure along the bent-back front results from a complex combination of microphysical processes. The microphysical contributions to the different positive and negative anomalies are analyzed in detail. It is found that for each anomaly, typically one specific microphysical process takes the leading role in its diabatic generation. A large but rather weak low and mid-level positive anomaly is produced by depositional growth of ice and snow. Two smaller but stronger positive anomalies at lower levels are generated mainly by in-cloud condensational heating at the warm front, and below-cloud rain evaporation and snow melting 200 km further north. In addition, near-surface negative anomalies are produced by snow melting and snow sublimation, respectively. The same method is applied to a particularly strong maritime extratropical cyclone, occurring over the Bering Sea on 12-14 December 2015. The warm-frontal structure is characterized by one strong positive PV anomaly stretching from the surface almost up to the tropopause. For this anomaly, condensation and below-cloud rain evaporation contribute most in the lower part, whereas deposition of ice and snow are more important in the upper part. Snow melting contributes very strongly in a localized region at the surface front. Snow sublimation leads to a weak negative anomaly on the cold side of the front. The cold frontal structure was found to be characterized by two strong low-level positive PV anomalies. The one on the warm side is produced due to condensation and below-cloud rain evaporation, the one on the cold side, located outside the clouds, has contributions from almost all microphysical processes. The time evolution of the anomalies showed that positive anomalies vary most over time, whereas negative anomalies stay relatively constant. The transition of the cyclone to a situation where it is embedded in a colder environment is marked by an increasing relative importance of the ice-phase processes compared to the liquid phase processes. The general findings of this study are: (a) a complex combination of microphysical processes dictates the strength and structure of diabatic PV anomalies along the fronts; (b) below-cloud processes are relevant for both the positive and negative anomalies throughout most of the troposphere; and (c) the Lagrangian approach proved meaningful for this detailed process-based analysis of dynamically relevant mesoscale flow structures. With regard to the below-cloud process, we note their strong contributions up to 1.0 PVU to the positive low-level anomalies, their relevance for anomalies in both the warm sector (mainly rain evaporation) and the cold sector (mainly rain evaporation and snow sublimation), and the importance of snow melting in localized regions close to the surface front. Furthermore, it is shown that in the mid troposphere both the integrated diabatic PV tendencies (Lagrangian anomalies) and consideration of cross-isentropic transport in a background with a vertical PV gradient are important to understand the resulting PV anomalies. Idealized modelling has the potential to further distinguish the inherently linked effects of Lagrangian PV modification and cross-isentropic transport of low-PV air. Finally, it is shown how our novel method could lead to a better understanding of the pathways of stratosphere-troposphere exchange and the involved physical processes.