Rossby wave packets (RWPs) have been associated with increased atmospheric predictability but also with the growth and propagation of forecast uncertainty. To address the important question of under which conditions RWPs imply high and low predictability, a potential vorticity–potential temperature (PV–θ) framework is introduced to diagnose RWP dynamics. Finite-amplitude RWPs along the midlatitude waveguide are considered and are represented by the synoptic-scale, wavelike undulations of the tropopause. The evolution of RWPs is examined by the amplitude evolution of the individual troughs and ridges. Troughs and ridges are identified as PV anomalies on θ levels intersecting the midlatitude tropopause. By partitioning the PV-tendency equation, individual contributions to the amplitude evolution are identified. A novel aspect is that the important role of the divergent flow and the diabatic PV modification is quantified explicitly. Arguably, prominent upper-tropospheric divergent flow is associated to a large extent with latent-heat release below and can thus be considered as an indirect diabatic impact. A case study of an RWP evolution over 7 days illustrates the PV–θ diagnostic. In general, baroclinic coupling and, important, the divergent flow make contributions to the amplitude evolution of individual troughs and ridges that are comparable in magnitude to the wave’s group propagation. Diabatic PV modification makes a subordinate contribution to the evolution. The relative importance of the different processes exhibits considerable variability between individual troughs and ridges. A discussion of the results in light of recent studies on forecast errors and predictability concludes the paper.

Synoptic-scale error growth near the tropopause is investigated from a process-based perspective. Following previous work, a potential vorticity (PV) error tendency equation is derived and partitioned into individual contributions to yield insight into the processes governing error growth near the tropopause. Importantly, we focus here on the further amplification of preexisting errors and not on the origin of errors. The individual contributions to error growth are quantified in a case study of a 6-day forecast. In this case, localized mesoscale error maxima have formed by forecast day 2. These maxima organize into a wavelike pattern and reach the Rossby wave scale around forecast day 6. Error growth occurs most prominently within the Atlantic and Pacific Rossby wave patterns. In our PV framework, the error growth is dominated by the contribution of upper-level, near-tropopause PV anomalies (near-tropopause dynamics). Significant contributions from upper-tropospheric divergent flow (prominently associated with latent heat release below) and lower-tropospheric anomalies [tropospheric-deep (i.e., baroclinic) interaction] are associated with a misrepresentation of the surface cyclone development in the forecast. These contributions are, in general, of smaller importance to error growth than near-tropopause dynamics. This result indicates that the mesoscale errors generated near the tropopause do not primarily project on differences in the subsequent baroclinic growth, but instead directly project on the tropopause evolution and amplify because of differences in the nonlinear Rossby wave dynamics.

Transport of Saharan dust over the Atlantic to the Americas is a relevant process since dust is a nutrient for marine and terrestrial ecosystems. It is therefore important to better quantify the frequency and amount of transatlantic dust transport, its preferred altitude and duration, and the regions of dust origin. This study uses a novel combination of Eulerian and Lagrangian diagnostics, applied to a previously validated 5 year simulation of the fifth generation European Centre for Medium Range Weather Forecast‐Hamburg‐model (ECHAM5)/Modular Earth Submodel System (MESSy) Atmospheric Chemistry model, to quantify these dust transport characteristics and their seasonal variations. Results confirm the previously found preferred transatlantic dust pathways: in boreal winter and spring, African dust is mainly transported below 800 hPa toward South America, whereas in summer and autumn the preferred pathway is to the Caribbean and occurs in a layer up to 500 hPa. The averaged transport duration from dust emission to deposition is 10 days in winter for deposition in the Amazon region and almost 12 days in summer for deposition in the Caribbean. These estimates were obtained by combining correlation analyses of Eulerian dust fluxes and trajectory calculations. The latter were also essential to identify the main source regions of transatlantic dust transport, which were found in all seasons in northwestern Africa (Algeria, Mali, and Mauritania) but not farther east, e.g., in the Bodélé Depression. A specific Lagrangian analysis for this dust emission hot spot suggests that wet deposition associated with the Intertropical Convergence Zone in winter and the African monsoon in summer inhibits Bodélé dust to leave the African continent.

Abstract. Rossby wave packets (RWPs) are fundamental to midlatitude dynamics and govern weather systems from their individual life cycles to their climatological distributions. Renewed interest in RWPs as precursors to high-impact weather events and in the context of atmospheric predictability motivates this study to revisit the dynamics of RWPs. A quantitative potential-vorticity (PV) framework is employed. Based on the well-established PV thinking of midlatitude dynamics, the processes governing RWP amplitude evolution comprise group propagation of Rossby waves, baroclinic interaction, the impact of upper-tropospheric divergent flow, and direct diabatic PV modification by nonconservative processes. An advantage of the PV framework is that the impact of moist processes is more directly diagnosed than in alternative, established frameworks for RWP dynamics. The mean dynamics of more than 6000 RWPs from 1979–2017 are presented using ERA5 data, complemented with nonconservative tendencies from the Year of Tropical Convection data (available 2008–2010). Confirming a pre-existing model of RWP dynamics, group propagation within RWPs is consistent with linear barotropic theory, and baroclinic and divergent amplifications occur most prominently during the mature stage and towards the trailing edge of RWPs. Refining the pre-existing model, the maximum of divergent amplification occurs in advance of maximum baroclinic growth, and baroclinic interaction tends to weaken RWP amplitude towards the leading edge. “Downstream baroclinic development” is confirmed to provide a valid description of RWP dynamics in both summer and winter, although baroclinic growth is substantially smaller (about 50 %) in summer. Longwave radiative cooling makes a first-order contribution to ridge and trough amplitude, with the potential that this contribution is partly associated with cloud-radiative effects. The direct impact of other nonconservative tendencies, including latent heat release, is an order of magnitude smaller than longwave radiative cooling. Arguably, latent heat release still has a substantial impact on RWPs by invigorating upper-tropospheric divergence. The divergent flow amplifies ridges and weakens troughs. This impact is of leading order and larger than that of baroclinic growth. To the extent that divergence is associated with latent heat release below, our results show that moist processes contribute to the well-known asymmetry in the spatial scale of troughs and ridges. For ridges, divergent amplification is strongly coupled to baroclinic growth and enhanced latent heat release. We thus propose that the life cycle of ridges is best described in terms of downstream moist-baroclinic development. Consistent with theories of moist-baroclinic instability, both the amplitude and the relative location of latent heat release within the developing wave pattern depend on the state of the baroclinic development. Taking this “phasing” aspect into account, we provide some evidence that variability in the strength of divergent ridge amplification can predominantly be attributed to variability in latent heat release below rather than to secondary circulations associated with the dry dynamics of a baroclinic wave.

Abstract. Atmospheric blocking describes a situation in which a stationary and persistent anticyclone blocks the eastward propagation of weather systems in the midlatitudes and can lead to extreme weather events. In the North Atlantic–European region, blocking contributes to life cycles of weather regimes which are recurrent, quasi-stationary, and persistent patterns of the large-scale circulation. Despite progress in blocking theory over the last decades, we are still lacking a comprehensive, process-based conceptual understanding of blocking dynamics. Here we combine three different perspectives on so-called “blocked” weather regimes, namely the commonly used Eulerian and Lagrangian perspectives, complemented by a novel quasi-Lagrangian perspective. Within the established framework of midlatitude potential vorticity (PV) thinking, the joint consideration of the three perspectives enables a comprehensive picture of the dynamics and quantifies the importance of dry and moist processes during a blocked weather regime life cycle. We apply the diagnostic framework to a European blocking weather regime life cycle in March 2016, which was associated with a severe forecast bust in the North Atlantic–European region. The three perspectives highlight the importance of moist processes during the onset or maintenance of the blocked weather regime. The Eulerian perspective, which identifies the processes contributing to the onset and decay of the regime, indicates that dry quasi-barotropic wave dynamics and especially the eastward advection of PV anomalies (PVAs) into the North Atlantic–European region dominate the onset of the regime pattern. By tracking the negative upper-tropospheric PVA associated with the “block”, the quasi-Lagrangian view reveals, for the same period, abrupt amplification due to moist processes. This is in good agreement with the Lagrangian perspective indicating that a large fraction of air parcels that end up in the negative PVA experience diabatic heating. Overall, the study shows that important contributions to the development take place outside of the region in which the blocked weather regime eventually establishes, and that a joint consideration of different perspectives is important in order not to miss processes, in particular moist-baroclinic dynamics, contributing to a blocked regime life cycle.