Intense Extratropical Cyclone Research Articles

Changes of Intense Extratropical Cyclone Deepening Mechanisms in a Warmer Climate in Idealized Simulations

Abstract To understand how extratropical cyclones (ETCs) may change in a warmer climate, we conduct idealized baroclinic life cycle simulations using the Icosahedral Nonhydrostatic (ICON)–NWP model with varied initial conditions. With respect to a present-day climate, two experiments are highlighted: a 4-K uniform warming and a more realistic late twenty-first-century warming pattern projected by a Coupled Model Intercomparison Project, version 6 (CMIP6), model. Different ETC deepening mechanisms, especially by diabatic processes, are quantified via the pressure tendency equation analysis, and the horizontal model resolution dependency is examined by contrasting coarse-grid (80 km) and convection-permitting (2.5 km) simulations. While our simulated ETCs are primarily baroclinically driven, dominated by the horizontal warm-air advection in the air column above the surface low, such an effect only strengthens by 10% in both warming experiments. However, the direct contribution of diabatic heating to surface pressure drop almost doubles, which likely feeds back positively to horizontal warm-air advection. Although their combined response to warming is pronounced, it is largely offset by the strengthened adiabatic cooling (17%) due to enhanced upward motions in warmer and moister ETCs, leading to a marginal ETC deepening at maturity (lowers by ∼1.5–4 hPa). Nevertheless, the near-surface impacts strongly increase, particularly the local extreme precipitation (up to 56%). The convection-permitting and the coarse-grid simulations show qualitatively consistent ETC responses to global warming. We suggest that the systematically weaker ETCs (with higher central pressure) in 2.5-km compared to 80-km simulations might be primarily caused by model uncertainty in representing the convective-diabatic heating over the warm front near the cyclone core.

Present‐Day Regional Antarctic Sea Ice Response to Extratropical Cyclones

AbstractBoth atmospheric warming and poleward moisture transport increase the likelihood of sea ice surface melt. In the Southern Hemisphere, short‐lived extratropical cyclones (ETCs) are responsible for a bulk of total heat and moisture transport toward high latitudes. Although these storms form ubiquitously in the midlatitudes, moisture availability and temperature characteristics vary by source region. In this study, we assess atmospheric, oceanic, and sea ice concentration (SIC) anomalies associated with austral winter ETCs over different Antarctic regions using ERA5 reanalysis data. Between 1990 and 2019, we find a total of 514 ETCs, with greater storm frequency in the eastern hemisphere groups. Compared to the climatology, sea ice melts (grows) behind the warm (cold) front of each system and is negatively correlated with atmospheric poleward moisture transport, temperature, meridional winds, and sea surface temperature for all ETCs. We find that Bellingshausen storms move moisture and warm air furthest poleward over their lifespan. However, East Weddell and East Antarctic ETCs are responsible for greater absolute poleward moisture transport than Bellingshausen and Ross systems. More intense ETCs correspond to greater SIC through Day 1, suggesting that SIC impacts ETC strength, regardless of ETC region. From cyclogenesis to cyclolysis, sea ice extent declines underneath composite ETCs, trends are generally not significant. Overall, while sea ice response produced by ETC‐induced atmospheric and oceanic changes varies regionally, the long‐term impacts of ETCs on regional sea ice are negligible over the study period.

The relationship between extra-tropical cyclone intensity and precipitation in idealised current and future climates

Abstract. Extra-tropical cyclones (ETCs) are the main cause of precipitation in the mid-latitudes, and there is substantial evidence that ETC-related precipitation will increase in the future. However, little is known about how this will impact on the dynamical strength of ETCs, and whether the impact will differ for different types of ETCs. We quantify the linear relationship between maximum vorticity and ETC-related precipitation in the current and idealised future climates and determine how this relationship depends on the structure and characteristics of the ETC. Three 10-year-long aqua-planet simulations are performed with a state-of-the-art global model, OpenIFS, that differ in their specified sea surface temperature (SST) distributions. A control simulation, a uniform warming simulation, and a polar amplification simulation are performed. ETCs are objectively identified using the feature-tracking software TRACK, and k-means clustering is applied to the ETC precipitation field to group the ETCs into clusters with similar precipitation structures. In all experiments, ETCs with stronger maximum vorticity are associated with more precipitation. For all cyclones considered together, we find that the slope of the linear relationship between maximum cyclone vorticity and ETC precipitation is larger in the uniform warming and polar amplification simulations than in the control simulation. We hypothesise that if an increase in precipitation in warmer climates were to feed back, via diabatic heating and potential vorticity anomalies, onto the dynamical intensity of the ETCs, precipitation and vorticity would increase at similar rates, and hence the slope of the linear regression line between precipitation and vorticity would remain similar. Our results indicate either that there is no feedback or that the increase in vorticity due to diabatic heating is masked by the decrease in the Eady growth rate which occurs in both the uniform warming and polar amplification simulations compared to the control. The k-means clustering identifies four distinct and physically realistic types of ETCs which are present in all experiments meaning that the average precipitation patterns associated with ETCs are unlikely to change in the future. The strongest dependency between ETC maximum vorticity and precipitation occurs for ETCs that have the most precipitation associated with the warm front. ETCs with the heaviest precipitation along the cold front, which are the most intense storms in terms of maximum vorticity, also exhibit a strong dependency between precipitation and maximum vorticity, but this dependency is weaker and has a smaller correlation coefficient than the warm-front ETCs. Not all ETC types exhibit a strong dependency between precipitation and maximum vorticity. ETCs located at high latitudes with weak precipitation show little dependency due to the lack of moisture, whereas ETCs with the precipitation located mainly in the centre of the ETCs have the weakest linear regression slope, which is likely due to the lack of upper-level forcing. These results stress that despite small changes in the strength of the cyclones, the precipitation increases are large, indicating potential future increases in flooding associated with cyclones.

Long‐Term Trends of Northern Hemispheric Winter Cyclones in the Extended ERA5 Reanalysis

AbstractUnderstanding how historical extratropical cyclone tracks respond to global warming is crucial for predicting future storm activity. Thus, long reanalysis data sets are needed to investigate long‐term changes in storm features and their impacts. By using a systematic change‐point analysis, we provide evidence that the presatellite ERA5 data of the Backward Extension (covering 1950–1978) is highly compatible with the standard ERA5 (1979–2021) data set. The joint ERA5 data from 1950 to 2021 is consistent in all storm‐related quantities, consequently allowing long‐term studies. Our trend analysis suggests that the intensity of extratropical cyclones has increased significantly during 1950–2021. The number of North Pacific storms has increased, and while they have longer life cycles and travel larger distances, they grow more slowly. For 1979–2021, we find notable increases in wind gusts for North Atlantic storms and in precipitation extremes for North Pacific storms. Our results also indicate an increase in Northern Hemispheric storminess based on an index of an extreme percentile of the central geopotential. Finally, we show potential impact locations using the findings of our trend analysis.

Extreme Wind and Waves in U.S. East Coast Offshore Wind Energy Lease Areas

The Outer Continental Shelf along the U.S. east coast exhibits abundant wind resources and is now a geographic focus for offshore wind deployments. This analysis derives and presents expected extreme wind and wave conditions for the sixteen lease areas that are currently being developed. Using the homogeneous ERA5 reanalysis dataset it is shown that the fifty-year return period wind speed (U50) at 100 m a.s.l. in the lease areas ranges from 29.2 to 39.7 ms−1. After applying corrections to account for spectral smoothing and averaging period, the associated pseudo-point U50 estimates are 34 to 46 ms−1. The derived uncertainty in U50 estimates due to different distributional fitting is smaller than the uncertainty associated with under-sampling of the interannual variability in annual maximum wind speeds. It is shown that, in the northern lease areas, annual maximum wind speeds are generally associated with intense extratropical cyclones rather than cyclones of tropical origin. Extreme wave statistics are also presented and indicate that the 50-year return period maximum wave height may substantially exceed 15 m. From this analysis, there is evidence that annual maximum wind speeds and waves frequently derive from the same cyclone source and often occur within a 6 h time interval.

Enhanced, Climate‐Driven Sedimentation on Salt Marshes

AbstractWith Arctic amplification (enhanced polar warming) possibly increasing periods of intense winter freezing and global warming producing more powerful extratropical storms, winter sedimentation on northern salt marshes may likely increase in the future. Here, we show that a large ice‐rafting event in northern Massachusetts delivered the equivalent of 15 years of mineral deposition to the marsh surface in a single storm. During an intense extratropical cyclone in January 2018, sediment‐laden ice was rafted onto the Great Marsh, Massachusetts, by an ~1‐m storm surge coinciding with high astronomical tides. The muddy sand content combined with abundant shells indicates that the sediment originated from proximal bays and tidal flats. Sediment layers delivered by individual ice rafts averaged 3.19 cm in thickness, 12 times the combined organic and inorganic yearly vertical accretion rate on high marshes. This previously underappreciated vector for sediment deposition is likely to help marsh resiliency to sea‐level rise.

A Climatological Assessment of Intense Extratropical Cyclones from the Potential Vorticity Perspective

Extratropical cyclones (ETCs) are known to intensify due to three vertically interacting positive potential vorticity perturbations that are associated with potential temperature anomalies close to the surface (θB), condensational heating in the lower-level atmosphere (qsat), and stratospheric intrusion in the upper-level atmosphere (qtr). This study presents the first climatological assessment of how much each of these three mechanisms contributes to the intensity of extreme ETCs. Using relative vorticity at 850 hPa as a measure of ETC intensity, results show that in about half of all cases the largest contributions during maximum ETC intensity are associated with qsat (53% of all ETCs), followed by qtr (36%) and θB (11%). The relative frequency of storms that are dominated by qsat is higher 1) during warmer months (61% of all ETCs during warmer months) compared to colder months (50%) and 2) in the Pacific (56% of all ETCs in the Pacific) compared to the Atlantic (46%). The relative frequency of ETCs that are dominated by θB is larger 1) during colder months (13%) compared to warmer months (3%), 2) in the Atlantic (15%) compared to the Pacific (8%), and 3) in western (11%–20%) compared to eastern ocean basins (4%–9%). These findings are based on piecewise potential vorticity inversion conducted for intense ETCs that occurred from 1980 to 2016 in the Northern Hemisphere (3273 events; top 7%). The results may serve as a baseline for evaluating ETC biases and uncertainties in global climate models.

A first characterization of high winds that affect the energy distribution system of Uruguay and their related effects

UTE is the company that delivers electric power to all Uruguay, serving almost a million and a half clients and 99.3% of Uruguayan homes.When direct wind effects on energy distribution systems are simulated, usually studying the action of wind on sections of these systems, it is generally assumed that high winds are of synoptic type, as e.g. those generated by hurricanes or winter storms. In Uruguay, high wind events are sometimes generated by synoptic winds, usually due to the passage of intense extratropical cyclones, but non-synoptic winds generated by severe convective activity are more frequent and intense in comparison, and are the cause of many incidences and damage to different economic sectors in the country.This paper presents results from the first academic research conducted on wind effects on the energy distribution system of Uruguay. Its aims were to characterize the main outages that occur in this system at national and management levels and their associated meteorological conditions, to explore the relative importance of synoptic and non-synoptic winds as causes of these incidences, and the possibility that different regions of the country exhibit different behavior in relation to the related weather conditions.

Projected Change in Wintertime Precipitation in California Using Projected Changes in Extratropical Cyclone Activity

Abstract 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. The authors 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.

Regions Subject to Rainfall Oscillation in the 5–10 Year Band

The decadal oscillation of rainfall in Europe that has been observed since the end of the 20th century is a phenomenon well known to climatologists. Consequences are considerable because the succession of wet or dry years produces floods or, inversely, droughts. Moreover, much research has tried to answer the question about the possible link between the frequency and the intensity of extra-tropical cyclones, which are particularly devastating, and global warming. This work aims at providing an exhaustive description of the rainfall oscillation in the 5–10 year band during one century on a planetary scale. It is shown that the rainfall oscillation results from baroclinic instabilities over the oceans. For that, a joint analysis of the amplitude and the phase of sea surface temperature anomalies and rainfall anomalies is performed, which discloses the mechanisms leading to the alternation of high and low atmospheric pressure systems. For a prospective purpose, some milestones are suggested on a possible link with very long-period Rossby waves in the oceans.

Anomalias de Circulação Atmosférica em Baixos Níveis do Verão (2013-2014) Anomalamente Quente nas Regiões Sul e Sudeste do Brasil

In this study the anomalous behavior of air temperature on South and Southeast Brazilian regions during summer (2013-2014), as well as the low-levels atmospheric circulation anomalies, were analyzed employing MERRA reanalysis dataset. Such anomalies were obtained from 1979-2008 climatology for two quarters: DJF (December 2013, January and February 2014) and JFM (January, February and March 2014). It was verified that for the designation of “abnormally warm”, JFM was the most representative quarter. By streamline anomaly field in 850 hPa an intensification of northern meridional wind component, that advected moist and warm air mass from Amazonian region (causing a positively anomalous behavior of specific humidity in this level, as well as temperature in surface), was observed. Moreover, a probably greater frequency and/or intensity of extratropical cyclones in Southeast of South America was verified. However, the cold fronts associated with them did not reach the study area.

Estudo de Caso de Sistemas Meteorológicos em Altos Níveis Sobre a América do Sul: Análise Sinótica

The upper tropospheric cyclonic vortex (VCAN) over northeastern Brazil (NeB) and the Bolivian High (AB) are the South America typical upper level (250 hPa) pressure systems, especially in warmer seasons. Besides these, one can also consider the extratropical VCAN, which has transient feature and is found throughout the year. In this work is presented a synoptic analysis, based on the specialized literature, of a case observed in February 01, 2005, when the three systems occurred simultaneously. Some features found in literature were noted in the systems of this case. The temperature field most associated to the extratropical system was 500 hPa and not 250 hPa. The most significant vertical movements were only found in extratropical system: intense upward movements in the lee of the upper tropospheric trough axis. It has found no system specifically associated to VCAN of NeB, while it was observed the association between the continental low (or Chaco Low) and the Bolivian High and an intense extratropical cyclone associated to extratropical VCAN. Such surface cyclone carries on a southerly and consequent cooling of the layer, what could be observed by the amplitude of the upper tropospheric trough, which is unusual in this season.

SynthETC: A Statistical Model for Severe Winter Storm Hazard on Eastern North America.

We develop, evaluate, and apply "SynthETC," a statistical-stochastic model of winter extra-tropical cyclones (ETCs) over eastern North America. SynthETC simulates the life cycle of ETCs from formation to termination, and it can be used to estimate the probability of extreme ETC events beyond the historical record. Two modes of climate variability are used as independent covariates: El Niño/Southern Oscillation (ENSO) Niño3.4 and the monthly North Atlantic Oscillation (NAO). We use SynthETC to estimate the annual occurrence rate over sites in eastern North America of intense ETC passage in different ENSO and NAO states. Positive NAO is associated with increased rates over the North Atlantic, while negative NAO is associated with decreased rates over the North Atlantic and increased rates over northern Quebec. Positive ENSO is associated with decreased rates over the North Atlantic, Ontario, and the Canadian Maritime, while negative ENSO is associated with increased rates over those regions, as well as the Great Lakes region.

North Atlantic storm driving of extreme wave heights in the North Sea

AbstractThe relationship between storms and extreme ocean waves in the North Sea is assessed using a long‐period wave data set and storms identified in the Interim ECMWF Re‐Analysis (ERA‐Interim). An ensemble sensitivity analysis is used to provide information on the spatial and temporal forcing from mean sea‐level pressure and surface wind associated with extreme ocean wave height responses. Extreme ocean waves in the central North Sea arise due to intense extratropical cyclone winds from either the cold conveyor belt (northerly‐wind events) or the warm conveyor belt (southerly‐wind events). The largest wave heights are associated with northerly‐wind events which tend to have stronger wind speeds and occur as the cold conveyor belt wraps rearward round the cyclone to the cold side of the warm front. The northerly‐wind events provide a larger fetch to the central North Sea to aid wave growth. Southerly‐wind events are associated with the warm conveyor belts of intense extratropical cyclones that develop in the left upper tropospheric jet exit region. Ensemble sensitivity analysis can provide early warning of extreme wave events by demonstrating a relationship between wave height and high pressure to the west of the British Isles for northerly‐wind events 48 h prior. Southerly‐wind extreme events demonstrate sensitivity to low pressure to the west of the British Isles 36 h prior.

Probabilistic evaluation of decadal prediction skill regarding Northern Hemisphere winter storms

Winter wind storms related to intense extra-tropical cyclones are meteorological extreme events, often with major impacts on economy and human life, especially for Europe and the mid-latitudes. Hence, skillful decadal predictions regarding the frequency of their occurrence would be of great socio-economic value. The present paper extends the study of Kruschke et al. (2014) in several aspects. First, this study is situated in a more impact oriented context by analyzing the frequency of potentially damaging wind storm events instead of targeting at cyclones as general meteorological features which was done by Kruschke et al. (2014). Second, this study incorporates more data sets by analyzing five decadal hindcast experiments – 41 annual (1961–2001) initializations integrated for ten years each – set up with different initialization strategies. However, all experiments are based on the Max-Planck-Institute Earth System Model in a low-resolution configuration (MPI-ESM-LR). Differing combinations of these five experiments allow for more robust estimates of predictive skill (due to considerably larger ensemble size) and systematic comparisons of the underlying initialization strategies. Third, the hindcast experiments are corrected for model bias and potential drifts over lead time by means of a novel parametric approach, accounting for non-stationary model drifts. We analyze whether skillful probabilistic three-category forecasts (enhanced, normal or decreased) can be provided regarding winter (ONDJFM) wind storm frequencies over the Northern Hemisphere (NH). Skill is assessed by using climatological probabilities and uninitialized transient simulations as reference forecasts. It is shown that forecasts of average winter wind storm frequencies for winters 2–5 and winters 2–9 are skillful over large parts of the NH. However, most of this skill is associated with external forcing from transient greenhouse gas and aerosol concentrations, already included in the uninitialized simulations. Only over East Asia and the Northwest Pacific, the Northwest Atlantic as well as the Eastern Mediterranean the initialized hindcasts perform significantly better than the uninitialized simulations. While no significant differences are evident between anomaly- and full-field-initialization, initializing the model's ocean component from GECCO2-ocean-reanalysis yields slightly better results than from ORA-S4, especially over the Northeast Pacific. Additionally, it is shown that the novel parametric drift-correction approach – estimating potential cubic drifts with parameters linearly changing in time – is more appropriate than the standard procedure – estimating constant model drifts via the lead-time-dependent bias – and, hence, yields higher skill estimates.

Beach response to Australian East Coast Lows: A comparison between the 2007 and 2015 events, Narrabeen-Collaroy Beach

ABSTRACT Harley, M.D.; Turner, I.L.; Splinter, K.D.; Phillips, M.S., and Simmons, J.A., 2016. Beach response to Australian East Coast Lows: a comparison between the 2007 and 2015 events, Narrabeen-Collaroy Beach. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 388–392. Coconut Creek (Florida), ISSN 0749-0208. East Coast Lows (ECLs) are intense extratropical cyclones that form off the east Australian coastline and are known to cause significant beach erosion. This study presents analysis of the beach response to two severe ECLs that occurred in the Sydney region in June 2007 and April 2015 based on a three-dimensional coastal monitoring program undertaken at Narrabeen-Collaroy Beach. The results indicate considerable reductions in the subaerial sand volume for both storms, with an average (maximum) reduction of 78 m3 (124 m3) per alongshore meter f...

New perspectives on the collective risk of extratropical cyclones

In this study, the relationship between the frequency and intensity of extratropical cyclones over the North Atlantic is investigated. A cyclone track database of extended October–March winters was obtained from the National Centers for Environmental Prediction and National Center for Atmospheric Research (NCEP–NCAR) reanalysis. A large positive correlation is found between winter cyclone counts and local sample mean vorticity over the exit region of the North Atlantic storm track in this cyclone track database. Conversely, a negative correlation is found over the Gulf Stream. Possible causes for the dependence are investigated by regressing winter cyclone counts and local sample mean vorticity on teleconnection indices with Poisson and linear models. The indices for the Scandinavian pattern, North Atlantic Oscillation and East Atlantic Pattern are able to account for most of the observed positive correlation over the North Atlantic. To consider the implications of frequency intensity dependence for the insurance industry, an aggregate risk metric was used as a proxy for the annual aggregate insured loss. Here, the aggregate risk is defined as the sum of the intensities of all events occurring within a season. Assuming independence between the frequency and intensity results in large biases in the variance and the extremes of the aggregate risk, especially over Scandinavia. Therefore including frequency intensity dependence in extratropical cyclone loss models is necessary to model the risk of extreme losses.

Extratropical cyclones in a warmer, moister climate: A recent Atlantic analogue

AbstractCurrent climate model projections do not exhibit a large change in the intensity of extratropical cyclones. However, there are concerns that current models represent moist processes poorly, and this provides motivation for investigating observational evidence for how cyclones behave in warmer climates. In the North Atlantic in particular, recent decades provide a clear contrast between warm and cold climates due to Atlantic Multidecadal Variability. In this paper we investigate these periods as analogues which may provide a guide to future cyclone behavior. While temperature and moisture rise in recent warm periods as in the projections, differences in energetics and temperature gradients imply that these periods are only partial analogues. The main result from current reanalyses is that while increased cyclone‐associated precipitation is seen in the recent warm periods, there is no robust evidence of an increase in cyclone intensity by other measures, such as maximum wind speed or vorticity. A set of low‐ and high‐resolution model simulations are also studied, suggesting that changes in cyclone intensity may be different in higher‐resolution reanalyses.

The dynamical structure of intense Mediterranean cyclones

This paper presents and analyzes the three-dimensional dynamical structure of intense Mediterranean cyclones. The analysis is based on a composite approach of the 200 most intense cyclones during the period 1989–2008 that have been identified and tracked using the output of a coupled ocean–atmosphere regional simulation with 20 km horizontal grid spacing and 3-hourly output. It is shown that the most intense Mediterranean cyclones have a common baroclinic life cycle with a potential vorticity (PV) streamer associated with an upper-level cyclonic Rossby wave breaking, which precedes cyclogenesis in the region and triggers baroclinic instability. It is argued that this common baroclinic life cycle is due to the strongly horizontally sheared environment in the Mediterranean basin, on the poleward flank of the quasi-persistent subtropical jet. The composite life cycle of the cyclones is further analyzed considering the evolution of key atmospheric elements as potential temperature and PV, as well as the cyclones’ thermodynamic profiles and rainfall. It is shown that most intense Mediterranean cyclones are associated with warm conveyor belts and dry air intrusions, similar to those of other strong extratropical cyclones, but of rather small scale. Before cyclones reach their mature stage, the streamer’s role is crucial to advect moist and warm air towards the cyclones center. These dynamical characteristics, typical for very intense extratropical cyclones in the main storm track regions, are also valid for these Mediterranean cases that have features that are visually similar to tropical cyclones.

Distinguishing the Cold Conveyor Belt and Sting Jet Airstreams in an Intense Extratropical Cyclone

Abstract Strong winds equatorward and rearward of a cyclone core have often been associated with two phenomena: the cold conveyor belt (CCB) jet and sting jets. Here, detailed observations of the mesoscale structure in this region of an intense cyclone are analyzed. The in situ and dropsonde observations were obtained during two research flights through the cyclone during the Diabatic Influences on Mesoscale Structures in Extratropical Storms (DIAMET) field campaign. A numerical weather prediction model is used to link the strong wind regions with three types of “airstreams” or coherent ensembles of trajectories: two types are identified with the CCB, hooking around the cyclone center, while the third is identified with a sting jet, descending from the cloud head to the west of the cyclone. Chemical tracer observations show for the first time that the CCB and sting jet airstreams are distinct air masses even when the associated low-level wind maxima are not spatially distinct. In the model, the CCB experiences slow latent heating through weak-resolved ascent and convection, while the sting jet experiences weak cooling associated with microphysics during its subsaturated descent. Diagnosis of mesoscale instabilities in the model shows that the CCB passes through largely stable regions, while the sting jet spends relatively long periods in locations characterized by conditional symmetric instability (CSI). The relation of CSI to the observed mesoscale structure of the bent-back front and its possible role in the cloud banding is discussed.