Synoptic climatologies

The availability of several novel and temporally-extended reanalysis data sets (e.g. ERA-Interim (Dee et al. 2009), Modern Era Retrospective Analysis for Research and Applications (MERRA) (Rienecker et al. 2011), 20^th-Century Reanalysis (Compo et al. 2011)) has been very useful for compiling a large number of synoptic climatologies for a diversity of atmospheric flow features on the global and regional scale. Classical approaches (e.g. composites) and new technical

approaches have been developed and applied. Only a few of them are mentioned here as important examples.

Several global climatologies of extratropical cyclones were produced during the last decade (e.g.

Hoskins and Hodges, 2002; Jung et al. 2006; Trigo, 2006; Wernli and Schwierz, 2006; Inatsu, 2009;

Hewson and Titley, 2010; Hodges et al. 2011), using different cyclone identification and tracking algorithms. Raible et al. (2008) compared three of these techniques and found that for trend analyses, results are sensitive to both the choice of the detection and tracking scheme and the reanalysis dataset. This was an important motivation for starting a major cyclone tracking intercomparison project, which identified the robust and more sensitive aspects of cyclone

climatologies produced with different algorithms (Neu et al. 2012). Some of these techniques have also been applied to investigate the occurrence of cyclones in simulations of the present and future climate (e.g. Lionello et al. 2002; Löptien et al. 2008; Bengtsson et al. 2009; Raible et al. 2010;

Ulbrich et al. 2012).

Other climatological cyclone studies looked in more detail at specific characteristics or categories of extratropical cylcones, e.g. dynamical forcing mechanisms (Gray and Dacre, 2006), extreme North Atlantic cyclones (Pinto et al. 2009), explosive cyclones (Allen et al. 2010), the vertical PV structure of cyclones (Campa and Wernli, 2012), and the specific category of diabatic Rossby waves

(Boettcher and Wernli, 2013). In addition, climatologies have been compiled of extratropical transition events in the North Pacific (Archambault et al. 2013; Wood and Ritchie, 2014).

Other novel synoptic climatologies focused on surface fronts (Berry et al. 2011; Simmonds et al.

2012), atmospheric blockings (Pelly and Hoskins, 2003; Croci-Maspoli et al. 2007),

upper-tropospheric jet streams (Koch et al. 2006; Schiemann et al. 2009; Limbach et al. 2012;

Manney et al. 2014), near-surface barrier jets (Harden et al. 2011), Rossby wave packets (Souders et al. 2014) and Rossby wave breakings (Peters and Waugh, 2003; Waugh and Funatsu, 2003;

Wernli and Sprenger, 2007; Martius et al. 2007), spectral properties of midlatitudes waves

(dell’Aquila et al. 2005, 2007) and processes leading to heavy precipitation events (e.g. Reale and Lionello, 2013; Lavers and Villarini, 2013; Viale and Garreaud, 2014; Collins et al. 2014; Winschall et al. 2014). It is important to note that a broad range of methodological concepts have been used in these studies, including the shape of contours, PV anomalies, and region growing algorithms for 4-dimensional feature detection - illustrating that progress in this area is also related to

methodological innovation^.

Open questions:

• How well do reanalysis datasets represent extreme events?

• Do systematic climatologies provide a basis for novel and more meaningful classifications of weather systems?

• How can synoptic climatologies be used for a better process understanding and forecast performance analysis of midlatitude weather systems?

5.3 CONCLUSION

Although far from being comprehensive, this overview provides evidence of the impressive progress in research on midlatitude weather systems and their prediction during the THORPEX decade. Advances, due to more sophisticated reanalysis datasets and the emergence of ensemble systems and convection-permitting models, led to unprecedented increases in understanding and predictive skill of the complex interplay of atmospheric processes relevant for midlatitude weather systems. Special consideration has been given to interactions between dynamics and physics and their representation in numerical models for the understanding and prediction of high-impact weather systems. Researchers have continued to amalgamate theoretical concepts of atmospheric dynamics (e.g. the PV framework and Rossby wave dynamics) and aspects related to the atmospheric water cycle (e.g. latent heating and cooling in clouds), leading to improved insight into the mechanisms determining the evolution of weather systems and a set of novel unresolved research questions. Research in this area had a first boom at the time of the ERICA (Experiment on Rapidly Intensifying Cyclones over the Atlantic) field experiment in 1989 and remains crucial for further improving numerical weather prediction in the next decades. It will likely profit from (i) increased cooperation between weather services and academia (as established during THORPEX), (ii) a continuation of international field experiments, (iii) novel modelling capabilities with a more realistic representation of moist convection, and (iv) an emphasis on dynamically motivated

feature-based analyses of forecast errors. Finally, in addition to these specific challenges, an essential task is to further bridge the gap between weather and climate research. This will be achieved by establishing the seamless modelling approach for time scales from hours to centuries and, even more challenging, by using the weather system perspective and dynamical expertise for an in-depth evaluation of the enormous amount of data from simulations of the current and future climate.

5.4 ACKNOWLEDGEMENTS

We thank Oscar Martínez-Alvarado, Neil Hart, Christian Grams, and the anonymous reviewers for their very helpful input to earlier versions of this overview article.

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