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Stephan Pfahl
and
Heini Wernli

Abstract

Owing to the huge potential impact of precipitation extremes on society, it is important to better understand the mechanisms causing these events, and their variations with respect to a changing climate. In this study, the importance of a particular category of weather systems, namely cyclones, for the occurrence of regional-scale precipitation extremes is quantified globally using the ECMWF Interim reanalysis (ERA-Interim) dataset. Such an event-based climatological approach complements previous case studies, which established the physical relationship between cyclones and heavy precipitation. A high percentage of precipitation extremes is found to be directly related to cyclones. Regional hot spots are identified where this percentage of cyclone-induced precipitation extremes exceeds 80% (e.g., in the Mediterranean region, Newfoundland, near Japan, and over the South China Sea). The results suggest that in these regions changes of heavy precipitation with global warming are specifically sensitive to variations in the dynamical forcing, for example, related to shifts of the storm tracks. Furthermore, properties of cyclones causing extreme precipitation are investigated. In the exit regions of the Northern Hemisphere storm tracks, these cyclones are on average slightly more intense than low pressure systems not associated with precipitation extremes, but no differences with respect to minimum core pressure are found in most other parts of the midlatitudes. The fundamental linkage between cyclones and precipitation extremes may thus provide guidance to forecasters involved in flood prediction, but it is unlikely that forecasting rules based on simple cyclone properties can be established.

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Dominik Büeler
and
Stephan Pfahl

Abstract

Extratropical cyclones develop because of baroclinic instability, but their intensification is often substantially amplified by diabatic processes, most importantly, latent heating (LH) through cloud formation. Although this amplification is well understood for individual cyclones, there is still need for a systematic and quantitative investigation of how LH affects cyclone intensification in different, particularly warmer and moister, climates. For this purpose, the authors introduce a simple diagnostic to quantify the contribution of LH to cyclone intensification within the potential vorticity (PV) framework. The two leading terms in the PV tendency equation, diabatic PV modification and vertical advection, are used to derive a diagnostic equation to explicitly calculate the fraction of a cyclone’s positive lower-tropospheric PV anomaly caused by LH. The strength of this anomaly is strongly coupled to cyclone intensity and the associated impacts in terms of surface weather. To evaluate the performance of the diagnostic, sensitivity simulations of 12 Northern Hemisphere cyclones with artificially modified LH are carried out with a numerical weather prediction model. Based on these simulations, it is demonstrated that the PV diagnostic captures the mean sensitivity of the cyclones’ PV structure to LH as well as parts of the strong case-to-case variability. The simple and versatile PV diagnostic will be the basis for future climatological studies of LH effects on cyclone intensification.

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Dominik Büeler
and
Stephan Pfahl

Abstract

It is still debated how enhanced cloud-condensational latent heating (LH) in a warmer and moister climate may affect the dynamics of extratropical cyclones. In this study, a diagnostic method that explicitly quantifies the contribution of LH to the lower-tropospheric cyclonic potential vorticity (PV) anomaly is used to investigate the effects of stronger LH on the dynamics, intensity, and impacts of cyclones in two conceptually different sets of idealized climate change simulations. A first set of regional surrogate climate change simulations of individual moderate to intense Northern Hemisphere cyclones in a spatially homogeneously 4-K-warmer climate reveals that enhanced LH can largely but not exclusively explain the substantially varying increase in intensity and impacts of most of these cyclones. A second set of idealized aquaplanet GCM simulations demonstrates that the role of enhanced LH becomes multifaceted for large ensembles of cyclones if climate warming is additionally accompanied by changes in the horizontal and vertical temperature structure: cyclone intensity increases with warming due to the continuous increase in LH, reaches a maximum in climates warmer than present day, and decreases beyond a certain warming once the increase of LH is overcompensated by the counteracting reduction in mean available potential energy. Because of their substantially stronger increase in LH, the most intense cyclones reach their maximum intensity in warmer climates than moderately intense cyclones with weaker LH. This suggests that future projections of the extreme tail of the storm tracks might be particularly sensitive to a correct representation of LH.

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Lukas Papritz
and
Stephan Pfahl

Abstract

In this study the dynamical mechanisms shaping the evolution of a marine cold air outbreak (CAO) that occurred over the Ross, Amundsen, and Bellingshausen Seas in June 2010 are investigated in an isentropic framework. The drainage of cold air from West Antarctica into the interior Ross Sea, its subsequent export, and the formation of a dome of cold air off the sea ice edge are shown to be intimately linked to a lower-tropospheric cyclone, as well as the cyclonic breaking of an upper-level potential vorticity trough. The dome formation is accompanied by an extreme deepening of the boundary layer, whose top reaches to the height of the low-lying tropopause within the trough, potentially allowing for deep stratosphere–troposphere exchange. A crucial finding of this study is that the decay of the CAO is essentially driven by the circulation associated with a train of mesocyclones and the release of latent heat in their warm sectors. Sensitivity experiments with switched off fluxes of sensible and latent heat reveal that the erosion of the CAO air mass depends critically on the moistening by latent heat fluxes, whereby the synergistic effects of sensible heat fluxes and moist processes amplify the erosion. Within the CAO air mass, the erosion is inhibited by cloud-top radiative cooling and the dissolution of clouds by the entrainment of dryer air. These findings potentially have implications for the representation of CAOs in coarse-resolution climate models.

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Lukas Papritz
,
Stephan Pfahl
,
Harald Sodemann
, and
Heini Wernli

Abstract

A climatology of cold air outbreaks (CAOs) in the high latitudes of the South Pacific and an analysis of the dynamical mechanisms leading to their formation are presented. Two major and distinct regions with frequent CAOs from autumn to spring are identified: one in the Ross Sea and another in the Amundsen and Bellingshausen Seas. Using an objective method to attribute CAOs to extratropical cyclones, it is shown that about 80% of the CAOs occur in association with the cyclonic flow induced by the passage of extratropical cyclones. Based on kinematic backward trajectories it is quantified that more than 40% of the air masses leading to CAOs originate from Antarctica and descend substantially, with the Ross Ice Shelf corridor as the major pathway. CAO trajectories descending from Antarctica differ from those originating over sea ice by a much lower specific humidity, stronger diabatic cooling, and much more intense adiabatic warming, while potential vorticity evolves similarly in both categories. In winter, CAOs are the major contributor to the net turbulent heat flux off the sea ice edge and CAO frequency strongly determines its interannual variation. Wintertime variations of the frequency of extratropical cyclones are strongly imprinted on the frequency of CAOs and the net turbulent heat and freshwater fluxes. In particular, much of the precipitation associated with the passage of extratropical cyclones is compensated by intense evaporation in cyclone-induced CAOs. This highlights the dominant role of the extratropical storm track in determining the variability of the buoyancy flux forcing of the Southern Ocean.

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Rupert Klein
,
Lisa Schielicke
,
Stephan Pfahl
, and
Boualem Khouider

Abstract

Quasigeostrophic (QG) theory describes the dynamics of synoptic-scale flows in the troposphere that are balanced with respect to both acoustic and internal gravity waves. Within this framework, effects of (turbulent) friction near the ground are usually represented by Ekman layer theory. The troposphere covers roughly the lowest 10 km of the atmosphere while Ekman layer heights are typically just a few hundred meters. However, this two-layer asymptotic theory does not explicitly account for substantial changes of the potential temperature stratification due to diabatic heating associated with cloud formation or with radiative and turbulent heat fluxes which can be significant in about the lowest 3 km and in the middle latitudes. To address this deficiency, this paper extends the classical QG–Ekman layer model by introducing an intermediate dynamically and thermodynamically active layer, called the “diabatic layer” (DL) from here on. The flow in this layer is also in acoustic, hydrostatic, and geostrophic balance but, in contrast to QG flow, variations of potential temperature are not restricted to small deviations from a stable and time-independent background stratification. Instead, within the DL diabatic processes are allowed to affect the leading-order stratification. As a consequence, this layer modifies the pressure field at the top of the Ekman layer, and with it the intensity of Ekman pumping seen by the quasigeostrophic bulk flow. The result is the proposed extended quasigeostrophic three-layer QG–DL–Ekman model for midlatitude dynamics.

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Stephan Pfahl
,
Erica Madonna
,
Maxi Boettcher
,
Hanna Joos
, and
Heini Wernli

Abstract

The role of moisture for extratropical atmospheric dynamics is particularly pronounced within warm conveyor belts (WCBs), which are characterized by intense latent heat release and precipitation formation. Based on the WCB climatology for the period 1979–2010 presented in Part I, two important aspects of the WCB moisture cycle are investigated: the evaporative moisture sources and the relevance of WCBs for total and extreme precipitation. The most important WCB moisture source regions are the western North Atlantic and North Pacific in boreal winter and the South Pacific and western South Atlantic in boreal summer. The strongest continental moisture source is South America. During winter, source locations are mostly local and over the ocean, and the associated surface evaporation occurs primarily during 5 days prior to the start of the WCB ascent. Long-range transport and continental moisture recycling are much more important in summer, when a substantial fraction of the evaporation occurs more than 10 days before the ascent. In many extratropical regions, WCB moisture supply is related to anomalously strong surface evaporation, enforced by low relative humidity and high winds over the ocean. WCBs are highly relevant for total and extreme precipitation in many parts of the extratropics. For instance, the percentage of precipitation extremes directly associated with a WCB is higher than 70%–80% over southeastern North America, Japan, and large parts of southern South America. A proper representation of WCBs in weather forecast and climate models is thus essential for the correct prediction of extreme precipitation events.

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Stephan Pfahl
,
Paul A. O’Gorman
, and
Martin S. Singh

Abstract

Cyclones are a key element of extratropical weather and frequently lead to extreme events like wind storms and heavy precipitation. Understanding potential changes of cyclone frequency and intensity is thus essential for a proper assessment of climate change impacts. Here the behavior of extratropical cyclones under strongly varying climate conditions is investigated using idealized climate model simulations in an aquaplanet setup. A cyclone tracking algorithm is applied to assess various statistics of cyclone properties such as intensity, size, lifetime, displacement velocity, and deepening rates. In addition, a composite analysis of intense cyclones is performed. In general, the structure of extratropical cyclones in the idealized simulations is very robust, and changes in major cyclone characteristics are relatively small. Median cyclone intensity, measured in terms of minimum sea level pressure and lower-tropospheric relative vorticity, has a maximum in simulations with global mean temperature slightly warmer than present-day Earth, broadly consistent with the behavior of the eddy kinetic energy analyzed in previous studies. Maximum deepening rates along cyclone tracks behave similarly and are in agreement with linear quasigeostrophic growth rates if the effect of latent heat release on the stratification is taken into account. In contrast to moderate cyclones, the relative vorticity of intense cyclones continues to increase with warming to substantially higher temperatures, and this is associated with enhanced lower-tropospheric potential vorticity anomalies likely caused by increased diabatic heating. Moist processes may, therefore, lead to the further strengthening of intense cyclones in warmer climates even if cyclones weaken on average.

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Lukas Papritz
,
Stephan Pfahl
,
Irina Rudeva
,
Ian Simmonds
,
Harald Sodemann
, and
Heini Wernli

Abstract

In this study, the important role of extratropical cyclones and fronts for the atmospheric freshwater flux over the Southern Ocean is analyzed. Based on the Interim ECMWF Re-Analysis (ERA-Interim), the freshwater flux associated with cyclones is quantified and it is revealed that the structure of the Southern Hemispheric storm track is strongly imprinted on the climatological freshwater flux. In particular, during austral winter the spiraliform shape of the storm track leads to a band of negative freshwater flux bending toward and around Antarctica, complemented by a strong freshwater input into the midlatitude Pacific, associated with the split storm track. The interannual variability of the wintertime high-latitude freshwater flux is shown to be largely determined by the variability of strong precipitation (>75th percentile). Using a novel and comprehensive method to attribute strong precipitation uniquely to cyclones and fronts, it is demonstrated that over the Southern Ocean between 60% and 90% of the strong precipitation events are due to these synoptic systems. Cyclones are the dominant cause of strong precipitation around Antarctica and in the midlatitudes of the Atlantic and the Pacific, while in the south Indian Ocean and the eastern Atlantic fronts bring most of the strong precipitation. A detailed analysis of the spatial variations of intense front and cyclone precipitation associated with the interannual variability of the wintertime frequency of cyclones in the midlatitude and high-latitude branches of the Pacific storm track underpins the importance of considering both fronts and cyclones in the analysis of the interannual variability of freshwater fluxes.

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Yannick Barton
,
Paraskevi Giannakaki
,
Harald von Waldow
,
Clément Chevalier
,
Stephan Pfahl
, and
Olivia Martius

Abstract

Temporal clustering of extreme precipitation events on subseasonal time scales is of crucial importance for the formation of large-scale flood events. Here, the temporal clustering of regional-scale extreme precipitation events in southern Switzerland is studied. These precipitation events are relevant for the flooding of lakes in southern Switzerland and northern Italy. This research determines whether temporal clustering is present and then identifies the dynamics that are responsible for the clustering.

An observation-based gridded precipitation dataset of Swiss daily rainfall sums and ECMWF reanalysis datasets are used. Also used is a modified version of Ripley’s K function, which determines the average number of extreme events in a time period, to characterize temporal clustering on subseasonal time scales and to determine the statistical significance of the clustering. Significant clustering of regional-scale precipitation extremes is found on subseasonal time scales during the fall season.

Four high-impact clustering episodes are then selected and the dynamics responsible for the clustering are examined. During the four clustering episodes, all heavy precipitation events were associated with an upper-level breaking Rossby wave over western Europe and in most cases strong diabatic processes upstream over the Atlantic played a role in the amplification of these breaking waves. Atmospheric blocking downstream over eastern Europe supported this wave breaking during two of the clustering episodes. During one of the clustering periods, several extratropical transitions of tropical cyclones in the Atlantic contributed to the formation of high-amplitude ridges over the Atlantic basin and downstream wave breaking. During another event, blocking over Alaska assisted the phase locking of the Rossby waves downstream over the Atlantic.

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