Sebastian Schemm

Warm Conveyor Belts in Idealized Moist Baroclinic Wave Simulations

AbstractThis 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 originat...

The Linkage between the Warm and the Cold Conveyor Belts in an Idealized Extratropical Cyclone

AbstractThis 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 level...

Increase in the number of extremely strong fronts over Europe? A study based on ERA-Interim reanalysis (1979-2014)

Evidence is presented that the frequency of extremely strong fronts, which occur mainly in summer, has increased over Europe in ERA-Interim reanalyses data (1979–2014). Fronts are defined using a common detection scheme based on gradients of equivalent potential temperature (θe) at 850 hPa. The frequency increase is due to increasing atmospheric humidity, which in turn is reported as statistically significant over Europe in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5). There is no trend in the frequency of extremely strong fronts in North America where humidity trends are, according to the IPCC AR5, close to zero. Because frontal precipitation increases with frontal strength, measured by the θe gradient, the increase in the number of extremely strong fronts may help explain regional patterns of longer-term trends in strong precipitation events.

Development of an idealised downstream cyclone: Eulerian and Lagrangian perspective on the kinetic energy

ABSTRACT In this idealised modelling study, the development of a downstream cyclone, which closely follows the life-cycle of a Shapiro-Keyser cyclone, is addressed from a quasi-geostrophic kinetic energy perspective. To this end a simulation of a dry, highly idealised, dispersive baroclinic wave, developing a primary and a downstream cyclone, is performed. Kinetic energy and processes contributing to its tendency – in particular baroclinic conversion and ageostrophic geopotential fluxes – are investigated in three dimensions both in an Eulerian and a Lagrangian framework from the genesis of the downstream cyclone as an upper-level kinetic energy centre, over frontal fracture to the fully developed cyclone showing the characteristic T-bone surface frontal structure, with a strong low-level jet along the bent-back front. Initially the downstream cyclone grows by the convergence of ageostrophic geopotential fluxes from the primary cyclone, but as vertical motions intensify this process is replaced by baroclinic conversion in the warm sector. We show that kinetic energy released in the warm sector is radiated away at all levels by ageostrophic geopotential fluxes: in the upper troposphere they are directed downstream, while in the lower troposphere they radiate kinetic energy to the rear of the cyclone. Thereby, vertical ageostrophic geopotential fluxes, their location and divergence, are identified to play a major role in the intensification of the cyclone in the lower troposphere and for the formation of the low-level jet. Low-level rearward ageostrophic geopotential fluxes converging along the bent-back front are shown to be a general characteristic of an eastward propagating baroclinic wave.

Global Climatologies of Eulerian and Lagrangian Flow Features based on ERA-Interim

AbstractThis paper introduces a newly compiled set of feature-based climatologies identified from ERA-Interim (1979–2014). Two categories of flow features are considered: (i) Eulerian climatologies of jet streams, tropopause folds, surface fronts, cyclones and anticyclones, blocks, and potential vorticity streamers and cutoffs and (ii) Lagrangian climatologies, based on a large ensemble of air parcel trajectories, of stratosphere–troposphere exchange, warm conveyor belts, and tropical moisture exports. Monthly means of these feature climatologies are openly available at the ETH Zurich web page (http://eraiclim.ethz.ch) and are annually updated. Datasets at higher resolution can be obtained from the authors on request. These feature climatologies allow studying the frequency, variability, and trend of atmospheric phenomena and their interrelationships across temporal scales. To illustrate the potential of this dataset, boreal winter climatologies of selected features are presented and, as a first applicati...

Influence of Tropical Pacific Sea Surface Temperature on the Genesis of Gulf Stream Cyclones

AbstractThis study investigates the relationship between tropical Pacific sea surface temperature (SST) variability and cyclogenesis over the Gulf Stream region of the North Atlantic. A cyclone identification scheme and Lagrangian trajectories are used to compare preferred cyclogenesis locations and precyclogenesis flow paths associated with three patterns of tropical Pacific SST variability: eastern Pacific (EP) El Nino, central Pacific (CP) El Nino, and La Nina. During EP El Nino and La Nina winters, the upper-level precyclogenesis flow takes a subtropical path over North America and Gulf Stream cyclogenesis predominantly occurs under the North Atlantic jet entrance, which is the climatologically preferred location. In contrast, during CP El Nino winters, when the warmest SST anomalies occur in the central tropical Pacific, the precyclogenesis flow takes a northern path across North America and Gulf Stream cyclogenesis tends to occur farther north under the jet exit. The shift in preferred cyclogenesis ...

Nowcasting Foehn Wind Events Using the AdaBoost Machine Learning Algorithm

AbstractThe south foehn is a characteristic downslope windstorm in the valleys of the northern Alps in Europe that demands reliable forecasts because of its substantial economic and societal impacts. Traditionally, a foehn is predicted based on pressure differences and tendencies across the Alpine ridge. Here, a new objective method for foehn prediction is proposed based on a machine learning algorithm (called AdaBoost, short for adaptive boosting). Three years (2000–02) of hourly simulations of the Consortium for Small-Scale Modeling’s (COSMO) numerical weather prediction (NWP) model and corresponding foehn wind observations are used to train the algorithm to distinguish between foehn and nonfoehn events. The predictors (133 in total) are subjectively extracted from the 7-km COSMO reanalysis dataset based on the main characteristics of foehn flows. The performance of the algorithm is then assessed with a validation dataset based on a contingency table that concisely summarizes the cooccurrence of observe...

The Ocean Version of the Lagrangian Analysis Tool LAGRANTO

AbstractThe Lagrangian Analysis Tool (LAGRANTO) is adopted and applied to ECMWF’s latest ocean reanalysis. The primary motivation behind this study is to introduce and document LAGRANTO Ocean (LAGRANTO.ocean) and explore its capabilities in combination with an eddy-permitting ocean reanalysis. The tool allows for flexibly defining starting points, within circles, cylinders, or any user-defined region or volume. LAGRANTO.ocean also offers a sophisticated way to refine a set of computed trajectories according to a wide range of mathematical operations that can be combined into a single refinement criterion. Tools for calculating—for example, along-trajectory cross sections or trajectory densities—are further provided. After introducing the tool, three case studies are presented, which were chosen to reflect a selection of phenomena on different spatial and temporal scales. The case studies also serve as hands-on examples. For the first case study, at the mesoscale, ocean trajectories are computed during the...

When during their life cycle are extratropical cyclones attended by fronts

AbstractFor nearly a century, the study of atmospheric dynamics in the midlatitudes has presented dichotomic perspectives on one of its focal points: the birth and life cycle of cyclones. In particular, the role of fronts has driven much of the historical discourse on cyclogenesis. In the 1910s–20s, the Bergen School of Meteorology postulated that cyclogenesis occurs on a preexisting front. This concept was later replaced by the baroclinic instability paradigm, which describes the development of a surface front as a consequence of the growing cyclone rather than its cause. However, there is ample observational evidence for cyclogenesis on well-marked fronts (frontal-wave cyclones) as well as for cyclogenesis in the absence of fronts in broader baroclinic zones. Thus, after a century of research on the link between extratropical cyclones and fronts, this study has the objective of climatologically quantifying their relationship. By combining identification schemes for cyclones and fronts, the fraction of c...

Extratropical Cyclogenesis Changes in Connection with Tropospheric ENSO Teleconnections to the North Atlantic: Role of Stationary and Transient Waves

AbstractThis study investigates mechanisms for changes in wintertime extratropical cyclogenesis over North America and the North Atlantic during different phases of El Nino–Southern Oscillation (EN...