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We perform a climatological analysis of summer‐time Arctic cyclone structure in reanalysis data using a phase space approach. A classification scheme for Arctic cyclones is proposed, dependent on whether vorticity structure during development is low‐level‐dominant (LLD) or upper‐level‐dominant (ULD). During growth, LLD cyclones (65.5%) exhibit warm‐core asymmetric structures, whereas ULD cyclones (34.5%) have cold‐core asymmetric structures. LLD cyclones typically have greater thermal asymmetry during growth. However, a transition to a persistent cold‐core axisymmetric structure after maturity is characteristic of summer‐time Arctic cyclones, regardless of structure during growth. LLD cyclones are typically stronger and preferentially track on the Russian coastline where there is high baroclinicity, whereas ULD cyclones tend to be longer‐lived and preferentially track in the Pacific sector, where they can interact with tropopause polar vortices. This study provides a platform for further research into different classes of Arctic cyclones and associated hazardous weather, and ultimately for developing conceptual models. Plain Language Summary: We investigate the structure of cyclones, large‐scale low‐pressure systems, in the summer‐time Arctic, using observations‐based data of the atmosphere. A phase space is used to represent two aspects of cyclone structure, specifically (a) the low‐level horizontal temperature contrast (i.e., whether there is a distinct warm sector), and (b) the vertical change in wind. We classify Arctic cyclones based on their vertical structure as they develop, as either low‐level‐dominant (LLD), where winds decrease with height, or upper‐level‐dominant (ULD), where winds increase with height. During growth, LLD cyclones (65.5%) exhibit warm‐core structures whereas ULD cyclones (34.5%) have cold‐core structures, both with strong low‐level temperature contrasts. LLD cyclones tend to have a stronger warm sector contrast during development. However, after maximum intensity, when a cyclone matures and begins to weaken, both subsets of cyclones typically evolve to have long‐lasting cold‐core structures without a warm sector. LLD cyclones are typically stronger and preferentially track on the Russian coastline, whereas ULD cyclones tend to be longer‐lived, and preferentially track in the Pacific sector of the Arctic. This study provides a platform for further research into different classes of Arctic cyclones, and ultimately for developing conceptual models, which are key for anticipating associated hazardous weather. Key Points: Arctic cyclones in summer are classified by their vorticity structure during growth: low‐level‐dominant (LLD) or upper‐level‐dominant (ULD)LLD cyclones (65.5%) exhibit more warm‐core baroclinic structures, preferentially track on the Russian coastline, and are typically strongerULD cyclones (34.5%) exhibit more cold‐core axisymmetric structures, preferentially track in the Pacific sector, and tend to be longer‐lived [ABSTRACT FROM AUTHOR]

Arctic cyclones are the most energetic weather systems in the Arctic, producing strong winds and precipitation that present major weather hazards. In summer, when the sea ice cover is reduced and more mobile, Arctic cyclones can have large impacts on ocean waves and sea ice. While the development of mid-latitude cyclones is known to be dependent on boundary layer (BL) turbulent fluxes, the dynamics of summer-time Arctic cyclones and their dependence on surface exchange processes have not been investigated. The purpose of this study is to characterise the BL processes acting in summer-time Arctic cyclones and understand their influence on cyclone evolution. The study focuses on two cyclone case studies, each characterised by a different structure during growth in the Arctic: (A) low-level-dominant vorticity (warm-core) structure and (B) upper-level-dominant vorticity (cold-core) structure, linked with a tropopause polar vortex. A potential vorticity (PV) framework is used to diagnose the BL processes in model runs from the ECMWF Integrated Forecasting System model. Both cyclones are associated with frictional Ekman pumping and downward sensible heat fluxes over sea ice. However, a third process, the frictional baroclinic generation of PV, acts differently in A and B due to differences in their low-level temperature structures. Positive PV is generated in Cyclone A near the bent-back warm front, like in typical mid-latitude cyclones. However, the same process produces negative PV tendencies in B, shown to be a consequence of the vertically aligned axisymmetric cold-core structure. This frictional process also acts to cool the lower troposphere, reducing the warm-core anomaly in A and amplifying the cold-core anomaly in B. Both cyclones attain a vertically aligned cold-core structure that persists for several days after maximum intensity, which is consistent with cooling from frictional Ekman pumping, frictional baroclinic PV generation, and downward sensible heat fluxes. This may help to explain the longevity of isolated cold-core Arctic cyclones with columnar vorticity structure. [ABSTRACT FROM AUTHOR]

A novel Lagrangian framework is developed to attribute monthly precipitation variability to physical processes. Precipitation variability is partitioned into a combination of five factors: airmass origin location, origin surface temperature variation, ascent intensity, mass fraction of ascending air, and the number of 'wet' analysis times per month [>1 mm (6 h)−1]. Precipitation in a target region is linked to 'origin' locations of air masses where the water vapor mixing ratio was last set by boundary layer moistening and is a maximum along back trajectories. Applying the technique to the England and Wales region, the factors together account for 83%-89% of the observed summer precipitation variability. The dominant contributor is the number of wet analyses, which is shown to be associated with cyclone statistics. The wettest summer months are mainly associated with anomalous cyclone duration rather than the number of cyclones. In addition, surface temperature and saturation humidity at the origin locations are found to be below their climatological averages (1979-2013). Therefore, the direct thermodynamic effect of anomalous surface temperature on marine boundary layer humidity acts to reduce monthly precipitation anomalies. The decadal precipitation change between phases of the Atlantic multidecadal oscillation is approximately 20% of the interannual variability between summer months. Changes in cyclone statistics have an effect 6 times larger than the direct thermodynamic factor in both monthly and decadal precipitation variability. [ABSTRACT FROM AUTHOR]

The statistical properties and skill in predictions of objectively identified and tracked cyclonic features (frontal waves and cyclones) are examined in the 15-day version of the Met Office Global and Regional Ensemble Prediction System (MOGREPS-15). The number density of cyclonic features is found to decline with increasing lead time, with analysis fields containing weak features that are not sustained past the first day of the forecast. This loss of cyclonic features is associated with a decline in area-averaged enstrophy with increasing lead time. Both feature number density and area-averaged enstrophy saturate by around 7 days into the forecast. It is found that the feature number density and area-averaged enstrophy of forecasts produced using model versions that include stochastic energy backscatter saturate at higher values than forecasts produced without stochastic physics. The ability of MOGREPS-15 to predict the locations of cyclonic features of different strengths is evaluated at different spatial scales by examining the Brier skill (relative to the analysis climatology) of strike probability forecasts: the probability that a cyclonic feature center is located within a specified radius. The radius at which skill is maximized increases with lead time from 650 km at 12 h to 950 km at 7 days. The skill is greatest for the most intense features. Forecast skill remains above zero at these scales out to 14 days for the most intense cyclonic features, but only out to 8 days when all features are included irrespective of intensity. [ABSTRACT FROM AUTHOR]

Warm conveyor belts (WCBs) are the main ascending air masses within extratropical cyclones. They often exhibit strong condensation and precipitation, associated with ascent on large scales or embedded convection. Most of the air outflows into the upper troposphere as part of a ridge. Such ridges are an integral part of Rossby waves propagating along the tropopause and are identified with a negative potential vorticity (PV) anomaly and associated anticyclonic circulation. It has been argued that diabatic modification of PV in WCBs has an important influence on the extent of the ridge, propagation of Rossby waves and weather impacts downstream. Following the coherent ensemble of trajectories defining a WCB, PV is expected to increase with time while below the level of maximum latent heating and then decrease as trajectories ascend above the heating maximum. In models, it is found that the net change is approximately zero, so that the average PV of the WCB outflow is almost equal to the PV of its inflow. Here, the conditions necessary for this evolution are explored analytically using constraints arising from the conservation of circulation. It is argued that the net PV change is insensitive to the details of diabatic processes and the PV maximum midway along a WCB depends primarily on the net diabatic transport of mass from the inflow to the outflow layer. The main effect of diabatic processes within a WCB is to raise the isentropic level of the outflow, rather than to modify PV. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]