The Role of Cloud Diabatic Processes in the Life Cycle of Atlantic-European Large-scale Flow Regimes

The variability of the atmospheric circulation in the Atlantic-European sector at sub-seasonal time scales is strongly modulated by quasi-stationary and recurrent large-scale flow patterns, so-called weather regimes (Vautard, 1990). From a forecast perspective the onset, persistence, and transition of weather regimes present a severe challenge in current numerical weather prediction models (Ferranti et al., 2015). Three out of the four European weather regimes classically described in literature involve blocking anticyclones. Recently, Pfahl et al. (2015) showed that, via a net transport of air with low potential vorticity (PV) to the tropopause, latent heat release in ascending air streams is a first order process in blocking onset and maintenance.

This study investigates the role of diabatic outflow due to latent heat release in synoptic-scale weather systems in European weather regime life cycles. Weather regimes are identified in 36 years of ERA-Interim reanalysis data using standard EOF analysis and k-means clustering. Based on the EOF-clustering individual weather regime life cycles are identified which newly allows to objectively define important life cycle stages such as the regime onset, maximum, decay, or transition. The role of latent heat release at these weather regime life cycle stages is assessed based on the warm conveyor belt (WCB) climatology of Madonna et al. (2014).

Results indicate that the period prior to regime onset is characterized by important changes in location and frequency of WCB occurrence. These changes persist during the early stage of a regime life cycle and weaken thereafter. Most importantly, prior to the onset of the "Greenland Blocking" (NAO-) and "European Blocking" (BL) regimes, a statistically significant increase in WCB frequency occurs upstream of the evolving blocking anticyclone and before blocking is detectable. This suggests that WCB outflow helps to establish the upper-level negative PV anomaly associated with the involved blocking anticyclone. For both the NAO- and BL regimes WCB outflow helps maintaining the blocking anticyclone later during the life cycle. In contrast, for the "Atlantic Ridge" (AR) regime no WCB precursor is evident. Still WCB outflow is important in maintaining the AR regime. Finally, a case study for a recent severe European forecast bust, demonstrates the importance of correctly representing diabatic outflow in numerical weather prediction (NWP) systems for  predicting the large-scale circulation in the medium-range (5-15 days ahead).

With a focus on European weather regime life cycles, this study corroborates that diabatic outflow establishes an important link from smaller scale processes in synoptic-scale weather systems to the large-scale circulation. This underpins the need for a seamless NWP approach in order to predict the large-scale flow accurately.

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Pfahl, S., C. Schwierz, M. Croci-Maspoli, C.M. Grams, and H. Wernli, 2015: Importance of latent heat release in ascending air streams for atmospheric blocking. Nature Geosci., 8, 610–614, doi:10.1038/ngeo2487.

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