A new method to objectively classify extratropical cyclones for climate studies: Testing in the Southwest Pacific region

This is the author accepted manuscript. The final version is available from American Meteorological Society via the DOI in this recordExtratropical cyclones can vary widely in their configuration during cyclogenesis, development mechanisms, spatial and temporal characteristics, and impacts. An automated method to classify extratropical cyclones identified in ERA-Interim data from 1979 to 2010 in the Australia and New Zealand region has been developed. The technique uses K-means clustering on two upper-tropospheric flow fields at the time of cyclogenesis and identifies four distinct clusters. Composites of these clusters are investigated, along with their life cycles and their spatial and temporal variability. The four clusters are similar to a previous manual classification. Cluster 1 develops in the equatorward entrance region of the subtropical jet, clusters 2 and 4 develop in the poleward exit region of the subtropical jet but with different relative positions of the upper-level trough and jet streak, and cluster 3 resembles secondary cyclogenesis on a preexisting front far poleward of the subtropical jet. The clusters have different impacts in terms of their precipitation (cluster 1 has the highest average precipitation), different seasonal cycles, and different preferred genesis locations. Features of the composite cyclones resemble extratropical cyclones from other regions, indicating the utility of the method over larger regions. The method has been developed to be easily applied to climate model output in order to evaluate the ability of models to represent the full range of observed extratropical cyclones.This work was funded by the Australian Research Council (ARC) through a Discovery Early Career Research Award (DE140101305) and supported by the Centre of Excellence for Climate Systems Science (CE110001028). Thanks to Julian Quinting and Duncan Ackerley for comments on an earlier version of the manuscript. Thanks also to Matt Hawcroft (University of Exeter) for use of his precipitation data. ERA-Interim data are available online (http://apps.ecmwf.int/datasets/). The author acknowledges with thanks the valuable comments and suggestions from a number of anonymous reviewers

Extratropical cyclone classification and its use in climate studies

This is the final version of the article. Available from American Geophysical Union via the DOI in this record.Extratropical cyclones have long been known to be important for midlatitude weather. It is therefore important that our current state‐of‐the‐art climate models are able to realistically represent these features, in order that we can have confidence in how they are projected to change in a warming climate. Despite the observation that these cyclones are extremely variable in their structure and features, there have, over the years, been numerous attempts to classify or group them. Such classifications can provide insight into the different cloud structures, airflows, and dynamical forcing mechanisms within the different cyclone types. This review collects and details as many classification techniques as possible, and may therefore act as a reference guide to classifications. These classifications offer the opportunity to improve the way extratropical cyclone evaluation in climate models is currently done by giving more insight into the dynamical and physical processes that occur in climate models (rather than just evaluating the mean state over a broad region as is often done). Examples of where these ideas have been used, or could be used, are reviewed. Finally, the potential impacts of future climate changes on extratropical cyclones are detailed. The ways in which the classification techniques could improve our understanding of future changes in extratropical cyclones and their impacts are given.I gratefully acknowledge the very helpful comments and suggestions of the Editor and two anonymous reviewers, which improved the manuscript. I would like to express my immense gratitude to Duncan Ackerley for reading and commenting on earlier versions of this review and for the very helpful discussions during the writing process. This work was supported by the Australian Research Council project DE140101305

Extreme weather caused by concurrent cyclone, front and thunderstorm occurrences

This is the final version of the article. Available from Springer Nature via the DOI in this recordPhenomena such as cyclones, fronts and thunderstorms can cause extreme weather in various regions throughout the world. Although these phenomena have been examined in numerous studies, they have not all been systematically examined in combination with each other, including in relation to extreme precipitation and extreme winds throughout the world. Consequently, the combined influence of these phenomena represents a substantial gap in the current understanding of the causes of extreme weather events. Here we present a systematic analysis of cyclones, fronts and thunderstorms in combination with each other, as represented by seven different types of storm combinations. Our results highlight the storm combinations that most frequently cause extreme weather in various regions of the world. The highest risk of extreme precipitation and extreme wind speeds is found to be associated with a triple storm type characterized by concurrent cyclone, front and thunderstorm occurrences. Our findings reveal new insight on the relationships between cyclones, fronts and thunderstorms and clearly demonstrate the importance of concurrent phenomena in causing extreme weather.This project is supported through funding from the Australian Government’s National Environmental Science Programme (NESP)

The importance of fronts for extreme precipitation

This is the final version. Available from AGU via the DOI in this recordExtratropical cyclones and their associated frontal systems are well known to be related to heavy precipitation events. Here an objective method is used to directly link extreme precipitation events with atmospheric fronts, identified using European Centre for Medium‐Range Weather Forecasts Interim Reanalysis data, to quantify the importance of fronts for precipitation extremes globally. In some parts of the major midlatitude storm track regions, over 90% of precipitation extremes are associated with fronts, with slightly more events associated with warm fronts than cold fronts. On average, 51% of global precipitation extremes are associated with fronts, with 75% in the midlatitudes and 31% in the tropics. A large proportion of extreme precipitation events occur in the presence of both a cyclone and a front, but remote fronts are responsible for many of the “front‐only” events. The fronts producing extreme precipitation events are found to have up to 35% stronger frontal gradients than other fronts, potentially providing some improved forecasting capabilities for extreme precipitation events.This study was supported by the Australian Research Council through the Linkage Project grant LP0883961, and the Discovery Project grant DP0877417

Climatology and dynamics of the link between dry intrusions and cold fronts during winter, Part II: Front-centred perspective

This is the author accepted manuscript. The final version is available from Springer via the DOI in this record.The conceptual picture of an extratropical cyclone typically includes a cold front and a dry intrusion (DI) behind it. By objectively identifying fronts and DIs in ECMWF ERA-Interim data for 1979–2014, Part I quantified the climatological relationship between cold fronts and DIs. Driven by the finding that front intensity and frontal precipitation are enhanced in the presence of DIs, here we employ a front-centred perspective to focus on the dynamical and thermodynamical environment of cold fronts with and without DIs in the Northern Hemisphere winter. Distinguishing between trailing fronts (that connect to a parent cyclone) and isolated fronts, examples of DIs behind each type illustrate the baroclinic environment of the trailing front, and the lack of strong temperature gradients across the isolated front. Composite analyses of North Atlantic and North Pacific fronts outline the major differences in the presence of DIs, compared to similar fronts but without DIs in their vicinity. The magnitude and spatial structure of the modification by DIs depends on the front intensity. Yet, generally with DIs, trailing fronts occur with stronger SLP dipole, deeper upper-tropospheric trough, stronger 10-m wind gusts, enhanced ocean sensible and latent heat fluxes in the cyclone cold sector and heavier precipitation. Isolated weak fronts exhibit similar behaviour, with different spatial structure. This study highlights the central role of DIs for shaping the variability of fronts and their associated environment and impact.Australian Research Council DECR

Climatology and dynamics of the link between dry intrusions and cold fronts during winter. Part I: global climatology

This is the final version. Available on open access from Springer via the DOI in this recordData availability: ERA-Interim data are available online (http://apps.ecmwf.int/datasets/).Cold fronts are a primary feature of the day-to-day variability of weather in the midlatitudes, and feature in conceptual extratropical cyclone models alongside the dry intrusion airstream. Here the climatological frequency and spatial distribution of the co-occurrence of these two features are quantified, and the differences in cold front characteristics (intensity, size, and precipitation) when a dry intrusion is present or not are calculated. Fronts are objectively identified in the ECMWF ERA-Interim dataset for the winter seasons in each hemisphere and split into 3 sub-types: central fronts (within a cyclone area); trailing fronts (outwith the cyclone area but connected to a central front); and isolated fronts (not connected to a cyclone). These are then associated with dry intrusions identified using Lagrangian trajectory analysis. Trailing fronts are most likely to be associated with a DI in both hemispheres, and this occurs more frequently in the western parts of the major storm track regions. Isolated fronts are linked to DIs more frequently on the eastern ends of the storm tracks, and in the subtropics. All front types, when co-occurring with a DI, are stronger in terms of their temperature gradient, are much larger in area, and typically have higher average precipitation. Therefore, climatologically the link with DIs increases the impact of cold fronts. There are some differences in the statistics of the precipitation for trailing and isolated fronts that are further investigated in Part II of this study.Australian Research CouncilSwiss National Science FoundationBenoziyo Endowment Fund for the Advancement of Scienc

Projected change in wintertime precipitation in California using projected changes in extratropical cyclone activity

This is the final version of the article. Available from the American Meteorological Society via the DOI in this record.Wintertime extratropical cyclones in the east Pacific region are the source of much of the precipitation over California. There is a lot of uncertainty in future projections of Californian precipitation associated with predicted changes in the jet stream and the midlatitude storm tracks. The question this work seeks to answer is how the changes in the frequency and the intensity of extratropical cyclones in the Pacific storm track influence future changes in Californian precipitation. We used an objective cyclone identification method applied to 25 CMIP5 models for the historical and RCP8.5 simulations and investigated the changing relationships between storm frequency, intensity and precipitation. Cyclone data from the historical simulations and differences between the historical and RCP8.5 simulations were used to “predict” the modeled precipitation in the RCP8.5 simulations. In all models, the precipitation predicted using historical relationships gives a lower future precipitation change than the direct model output. In the future, the relationship between track density and precipitation indicates that for the same number of tracks, more precipitation is received. The relationship between track intensity and precipitation (which is quite weak in the historical simulations) does not change in the future. This suggests that other sources, likely enhanced moisture availability, are more important than changes in the intensity of cyclones for the rainfall associated with the storm tracks.This work was funded by the Australian Research Council (ARC) through a Discovery Early Career Research Award (DE140101305), and the Centre of Excellence for Climate Systems Science (CE110001028)

The intensity and motion of hybrid cyclones in the Australian region in a composite potential vorticity framework

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this recordHybrid cyclones (HCs) in the Australian region typically reach their peak intensity in an amplified flow comprising upper‐tropospheric ridges upstream and downstream of the cyclone and a north–south elongated trough. Nonetheless, there is considerable case‐to‐case variability. Taking a composite viewpoint, the present study investigates how such variations in the upper‐tropospheric potential vorticity (PV) anomalies affect the subsequent intensity and motion of HCs in the Australian region. First, cyclones are grouped into four clusters with structurally‐similar environments through a k‐means clustering of the 315‐K PV anomaly. The clusters reveal that HCs can be associated with a north–south elongated trough (Cluster 1), a PV cut‐off (Cluster 2), and cyclonically breaking troughs (Clusters 3 and 4). Second, the effect of these features on the intensity and tracks is quantified using piecewise PV inversion. The maximum intensity of cyclones in Cluster 1 is largely determined by their upper‐tropospheric cyclonic PV anomaly. Conversely, diabatically generated lower‐tropospheric PV anomalies dominate the intensity of cyclones in Clusters 3 and 4. In these two clusters, the cyclonically breaking trough and a downstream ridge induce an anomalous northeasterly low‐level flow across the cyclone centre. The downstream ridge is most pronounced in Cluster 4, leading to the greatest poleward cyclone displacement compared to the other clusters. In Clusters 1 and 2, the upper‐level PV anomaly primarily slows the eastward motion of the cyclones. In agreement with recent idealised studies, the analysis suggests that the effect of upper‐tropospheric PV anomalies on the poleward motion of HCs is analogous to the beta‐gyres that influence the motion of tropical cyclones.Helmholtz-AssociationAustralian Research Council Centre of Excellence for Climate ExtremesAustralian Research Counci

Synoptic climatology of hybrid cyclones in the Australian region

This is the author accepted manuscript. The final version is available from Wiley via the DOI in this record In May and September 2016, two intense hybrid cyclones (HCs) developed over the Great Australian Bight damaging infrastructure and causing a state‐wide power outage in South Australia. These two cyclones motivate the compilation of the first synoptic climatology of HCs in the Australian region, including an analysis of their importance for wind and precipitation extremes, and a composite view of the large‐scale flow in which they develop. HCs are identified in ERA‐Interim data from 1979 to 2010 using an objective feature tracking method and a cyclone phase space diagnostic. HCs exhibit a pronounced seasonal cycle with most of them occurring from May to September. During these months, HCs are most frequent over the Tasman Sea and the Great Australian Bight where they account for 50% of all cyclones. A common characteristic of all HCs is that the strongest precipitation, which is locally extreme in 91% of all HCs, falls in the warm‐sector and along a bent‐back warm front on the poleward side of the cyclones. Moreover, the area affected by extreme precipitation and the maximum precipitation in HCs are no different from non‐hybrid cyclones (NHCs). In contrast, the area affected by extreme wind gusts is significantly larger in HCs than for NHCs. In both HCs and NHCs the strongest near‐surface wind gusts typically occur in the cold air mass in the wake of the cyclones, especially in those over the Great Australian Bight. The upper‐tropospheric structure of HCs is characterised by an elongated cyclonic potential vorticity anomaly embedded between two ridges that eventually cuts off. In contrast, NHCs are characterised by a zonal flow upstream and upper‐tropospheric cyclonic wave breaking.Helmholtz-AssociationAustralian Research Council Centre of Excellence for Climate ExtremesAustralian Research Counci

Future changes in the extratropical storm tracks and cyclone intensity, wind speed, and structure

This is the final version. Available on open access from Copernicus Publications via the DOI in this record. Data availability. ERA5 reanalysis is available from the Copernicus Climate Change Service Climate Data Store (https://doi.org/10.24381/cds.bd0915c6, Hersbach et al., 2018). CMIP6 data are publicly available through the Earth System Grid Federation (Eyring et al., 2016).Code availability. Code is available at the request of the author. The cyclone tracking and compositing algorithm TRACK is available at the request of Kevin Hodges from https://gitlab.act.reading.ac.uk/track/track (Hodges, 2022, 1994, 1995, 1999)Future changes in extratropical cyclones and the associated storm tracks are uncertain. Using the new CMIP6 models, we investigate changes to seasonal mean storm tracks and composite wind speeds at different levels of the troposphere for the winter and summer seasons in both the Northern Hemisphere (NH) and Southern Hemisphere (SH). Changes are assessed across four different climate scenarios. The seasonal mean storm tracks are predicted to shift polewards in the SH and also in the North Pacific, with an extension into Europe for the North Atlantic storm track. Overall, the number of cyclones will decrease by ∼5 % by the end of the 21st century, although the number of extreme cyclones will increase by 4 % in NH winter. Cyclone wind speeds are projected to strengthen throughout the troposphere in the winter seasons and also summer in the SH, with a weakening projected in NH summer, although there are minimal changes in the maximum wind speed in the lower troposphere. Changes in wind speeds are concentrated in the warm sector of cyclones, and the area of extreme winds may be up to 40 % larger by the end of the century. The largest changes are seen for the SSP5-85 scenario, although a large amount of change can be mitigated by restricting warming to that seen in the SSP1-26 and 2-45 scenarios. Extreme cyclones show larger increases in wind speed and peak vorticity than the average-strength cyclones, with the extreme cyclones showing a larger increase in wind speed in the warm sector.Natural Environment Research Council (NERC