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The influence of adiabatic and diabatic processes on the midlatitude circulation is a formidable research question, especially considering their projected changes under global warming. This study presents the prospects, merits, and caveats of a potential vorticity (PV) gradient perspective as a means to disentangle the contributions of adiabatic and diabatic processes affecting the midlatitude circulation. Theoretical considerations reassess the link between the PV gradient and the jet stream. They reveal that the maximum isentropic PV gradient is consistently located on the stratospheric side of the jet, whereas the gradient of ln(PV)$$ \ln \left(\mathrm{PV}\right) $$ is shifted to the tropospheric side but, in general, is better aligned with the jet axis. The stratospheric shift of the PV gradient results from variations in stability across the tropopause, whereas the tropospheric shift of the ln(PV)$$ \ln \left(\mathrm{PV}\right) $$ gradient results from variations in vorticity. Regions of high PV gradient may serve as a proxy for the curvature of the wind field in the case of sufficiently small variations in stability. Otherwise, they depict variations in both wind and thermal stratification along tropopause‐intersecting isentropic surfaces. Lagrangian "PV gradient thinking" is demonstrated in two case studies of jet streak evolution in a simulation with 1.1 km grid spacing performed with the graphics‐processing‐unit‐enabled numerical weather prediction model Consortium for Small‐Scale Modelling featuring on‐line air parcel trajectories. Dry deformation drives the Lagrangian evolution of the PV gradient in the first case, whereas there is a pronounced influence of diabatic modification in the second case. The Lagrangian PV gradient perspective presented offers fresh insight into adiabatic and diabatic processes underlying the midlatitude circulation variability and change. [ABSTRACT FROM AUTHOR]

The ambient conditions during cyclogenesis on fronts in the North Atlantic (NA) are characterised climatologically in the ERA-Interim dataset. These 'secondary' cyclones, which grow from frontal waves, are typically small in scale, shallow and deepen rapidly, and pose a challenge to forecasters and numerical weather prediction models. Frontal-wave development has been studied intensively in the past based on observations (e.g. FASTEX), real-case studies or analytical models. In this study, frontal-wave cyclogenesis is studied from a climatological perspective and is detected by a combination of two automated identification methods for the period between 1979 and 2014. A climatology of frontal-wave cyclogenesis is presented. Horizontal composites provide an estimate of the climatological mean of conditions during frontal-wave cyclogenesis which may prove useful in the assessment of the realism of proposed idealized models of frontogenesis. The findings are also compared with assumptions made in earlier idealized studies of frontal waves. The composites reveal coherent structures of diabatically generated low-level PV in the lower troposphere, neutral moist static stability at cyclogenesis and an ambient lower troposphere close to saturation. The vertical PV structure in the east NA is more vertically aligned than in the west NA and PV anomalies from the upper and lower levels reach deeper into the mid-troposphere, which indicates an enhanced phase-lock between the two anomalies. Previous studies suggest the existence of an upper limit of along-frontal stretching above which frontal-wave development is strongly suppressed. The presented multi-decadal analysis confirms its existence in a range between 0.6 and 0.8 × 10−5 s−1. Finally a climatology of along-frontal stretching for the NA is presented and compared to the fraction of cyclogenesis forming on frontal waves. [ABSTRACT FROM AUTHOR]

This study continues the investigation of airstreams in idealized moist baroclinic waves and addresses the formation of the cold conveyor belt (CCB), its linkage to the warm conveyor belt (WCB), and their impact on the development of a midlatitude cyclone. The CCB is identified as a coherent bundle of trajectories, characterized by weak ascent and a strong increase of potential vorticity (PV) along the flow, in contrast to the WCB, defined as the trajectories with maximum ascent. The authors illuminate the role of the two conveyor belts in the formation of two strong PV anomalies that form in the upper (WCB, negative PV anomaly) and lower troposphere (CCB, positive PV anomaly), respectively, and thereby establish a link between these airstreams and relevant aspects of the dynamics of extratropical cyclones. The CCB moves close to the surface along the colder side of the bent-back front and experiences a PV increase as it passes below a region of maximum latent heat release at midtropospheric levels. Accordingly, it arrives with high PV values at the tail of the bent-back front where the most intense low-level winds occur. The WCB, which rises above the bent-back front, causes the formation of the midtropospheric heating rate maximum and thereby not only influences the upper-level downstream development, but also drives the increase of PV along the CCB and, in consequence, indirectly drives the formation of the low-level jet at the tail of the bent-back front. [ABSTRACT FROM AUTHOR]

This idealized modeling study of moist baroclinic waves addresses the formation of moist ascending airstreams, so-called warm conveyor belts (WCBs), their characteristics, and their significance for the downstream flow evolution. Baroclinic wave simulations are performed on the f plane, growing from a finite-amplitude upper-level potential vorticity (PV) perturbation on a zonally uniform jet stream. This nonmodal approach allows for dispersive upstream and downstream development and for studying WCBs in the primary cyclone and the downstream cyclone. A saturation adjustment scheme is used as the only difference between the dry and moist simulations, which are systematically compared using a cyclone-tracking algorithm, with an eddy kinetic energy budget analysis, and from a PV perspective. Using trajectories and a selection criterion of maximum ascent, forward- and rearward-sloping WCBs in the moist simulation are identified. No WCB is identified in the dry simulation. Forward-sloping WCBs originate in the warm sector, move into the frontal fracture region, and ascend over the bent-back front, where maximum latent heating occurs in this simulation. The outflow of these WCBs is located at altitudes with prevailing zonal winds; they hence flow anticyclonically ('forward') into the downstream ridge. In case of a slightly weaker ascent, WCBs curve cyclonically ('rearward') above the cyclone center. A detailed analysis of the PV evolution along the WCBs reveals PV production in the lower troposphere and destruction in the upper troposphere. Consequently, WCBs transport low-PV air into their outflow region, which contributes to the formation of distinct negative PV anomalies. They, in turn, affect the downstream flow and enhance downstream cyclogenesis. [ABSTRACT FROM AUTHOR]