Stephan Pfahl

Quantifying the Relevance of Cyclones for Precipitation Extremes

AbstractOwing 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 sensitiv...

Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010). Part II: Moisture Origin and Relevance for Precipitation

AbstractThe 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 th...

Tropopause folds in ERA‐Interim: Global climatology and relation to extreme weather events

Tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics. They play an important role for stratosphere-troposphere exchange, the dynamical coupling of upper and lower tropospheric levels, and for generating severe weather events. This study presents a global climatology of tropopause folds using ERA-Interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3-D labeling and fold identification algorithm. This algorithm objectively separates stratospheric and tropospheric air in complex situations, e.g., in regions with strong low-level inversions, and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur. Three classes of tropopause folds are defined (shallow, medium, and deep), and their geographical distribution, vertical extent, and seasonal cycle are investigated. Most shallow folds occur along the subtropical jet stream, in agreement with previous studies. Hot spots of medium and deep tropopause folds are found west of Australia and along the coast of Antarctica in the Southern Hemisphere and around the east coast of North America in the Northern Hemisphere. Seasonal cycles show maxima in winter for all fold classes. Medium and deep folds are frequently associated with surface wind gust and precipitation extremes, as quantified for folds over the southern Indian Ocean. Wind gust extremes occur mainly in an elongated band upstream and equatorward of folds, whereas precipitation extremes occur mainly east and poleward of folds. Overall, in the considered region, about 20% of medium folds and 33% of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold.

How important is intensified evaporation for Mediterranean precipitation extremes

The moisture sources for heavy precipitation events in the northwestern Mediterranean are investigated with a Lagrangian moisture source diagnostic, which is based on the analysis of moisture changes during 6 h time intervals along backward trajectories, originating from the location of the heavy precipitation events. We consider the 50 strongest events in each season in the region 0–15°E, 42–46°N during the years 1989–2009. The results of the moisture source diagnostic indicate that during precipitation extremes the Mediterranean Sea surface is only one of several source regions in all seasons, and typically matched by moisture from the North Atlantic in autumn and winter and land evapotranspiration in summer. The event-to-event variability is large. Individual events can be dominated by land evapotranspiration, moisture transport from the North Atlantic, or an export of moisture from the tropics. The time of maximum moisture uptake varies between a few hours to more than a week before the precipitation event. The results from the moisture source analysis are used for investigating whether intense surface evaporation occurs prior to heavy precipitation. Surface evaporation anomalies at the moisture sources are positive over the North Atlantic and the European and African land surface, whereas no signal is present over the Mediterranean. Therefore, for the Mediterranean moisture contribution, convergence from the background moisture reservoir is essential, whereas for the remote sources anomalously intense surface evaporation is required to foster the moisture supply for Mediterranean heavy precipitation events.

Extratropical cyclones in idealized simulations of changed climates

AbstractCyclones 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...

A Climatology of Cold Air Outbreaks and Their Impact on Air–Sea Heat Fluxes in the High-Latitude South Pacific

AbstractA 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...

The Role of Extratropical Cyclones and Fronts for Southern Ocean Freshwater Fluxes

AbstractIn 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...

Isotope meteorology of cold front passages: A case study combining observations and modeling

This study investigates the role of below-cloud evaporation and evapotranspiration for the short-term variability of stable isotopes in near-surface water vapor and precipitation associated with central European cold fronts. To this end, a combination of observations with high temporal resolution and numerical sensitivity experiments with the isotope-enabled regional weather prediction model COSMOiso is used. The representation of the interaction between rain droplets and ambient vapor below the cloud is fundamental for adequately simulating precipitation isotopes ((p)) and total rainfall amount. Neglecting these effects leads to depletion biases of 20-40 in delta H-2(p) and 5-10% in delta O-18(p) and to an increase of 74% in rainfall amount. Isotope fractionation during soil evaporation is of primary importance for correctly simulating the variability of continental low-level vapor delta H-2(v) and delta O-18(v) and particularly of the secondary isotope parameter deuterium excess (d(v)). (Less)

Regional‐scale jet waviness modulates the occurrence of midlatitude weather extremes

Several studies have attributed the occurrence of recent weather extremes to an amplified waviness of the upper-tropospheric jet stream. Although trends in jet waviness are still under discussion, it is crucial to better understand the mechanisms through which jet waviness affects weather extremes. Here we show that variations in jet waviness on regional scales effectively modulate the occurrence of daily weather extremes, however, in regionally different ways. The jet waviness over the North Atlantic and the North Pacific mainly affects where wind, precipitation and cold extremes occur, while a wavy jet over Eurasia strongly favors the occurrence of hot extremes in summer. This is because regional variations of jet waviness are intrinsically linked to the occurrence and tracks of synoptic-scale weather systems, which can trigger the extremes. We conclude that potential jet waviness changes would affect the occurrence of weather extremes differently depending on where these changes occur.

Clustering of Regional-Scale Extreme Precipitation Events in Southern Switzerland

AbstractTemporal 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.Fo...