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August 1982 - present
Publications
Publications (150)
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In wintertime the earth's stratospheric polar vortex extends from a height of around 15 km to well above 50 km and equatorward to around 30° from the pole. At its edge, winds can be significantly in excess of 100 m s⁻¹ whilst the vortex's core has been viewed as an enclosed containment vessel partially insulated from exterior...
Eady edge waves (EEWs) propagate on surface baroclinic zones and they resemble synoptic and sub-synoptic scale frontal wave-trains. The wave-trains are frequently depicted on surface charts, and occur per se on a front with a local baroclinicity extremum. The possible linkage of EEWs to frontal waves is explored within the framework of so-called ‘s...
This monograph recounts the history and major developments of the teaching and research in the atmospheric sciences such as climatology, meteorology, glaciology, etc. from the foundation of the "Polytechnikum" (in 1855, later renamed into ETH) until the 21st century.
Tor Bergeron was a key member of the Bergen School of Meteorology that developed some of the most influential contributions to synoptic analysis in the 20th century: air- mass analysis, polar-front theory, and the Norwegian cyclone model. However, the eventual success of these so-called Bergen methods of synoptic analysis was not guaranteed. Concer...
The year 1919 was important in meteorology, not only because it was the year that the American Meteorological Society was founded, but also for two other reasons. One of the foundational papers in extratropical cyclone structure by Jakob Bjerknes was published in 1919, leading to what is now known as the Norwegian cyclone model. Also that year, a s...
This paper synthesizes and summarizes atmospheric variability on time scales from seconds to decades through a phenomenological census. We focus mainly on unforced variability in the troposphere, stratosphere, and mesosphere. In addition to atmosphere-only modes, our scope also includes coupled modes, in which the atmosphere interacts with the othe...
The Observing System Research and Predictability Experiment (THORPEX) was a 10-yr, international research program organized by the World Meteorological Organization’s World Weather Research Program. THORPEX was motivated by the need to accelerate the rate of improvement in the accuracy of 1-day to 2-week forecasts of high-impact weather for the ben...
Day-to-day weather forecasting has improved substantially over the past few decades. In contrast, progress in seasonal prediction outside the tropics has been meagre and mixed. On seasonal timescales, the constraining influence of the initial atmospheric state is weak, and the internal variability associated with transient weather systems tends to...
A two-component study is undertaken of the classical quasigeostrophic (QG) omega equation. First, a reappraisal is undertaken of extant formulations of the equation’s so-called forcing function. It pinpoints shortcomings of various formulations and prompts consideration of alternative forms. Particular consideration is given to the contribution of...
Synoptic- and larger-scale atmospheric flow in the extratropics is almost geostrophic in character, indicating an approximate balance between the Coriolis and pressure gradient forces. The concept of quasigeostrophy (QG) encapsulates elegantly and simply the dynamics whereby the flow retains this character, although both the maintenance of geostrop...
Consideration is given to the diagnosis and dynamics of synoptic and subsynoptic forecast error from a potential vorticity (PV) perspective. A depiction of the extratropical "forecast minus analysis" PV pattern on a cross-tropopause isentropic surface serves to illustrate characteristic features of the PV-error field, and these features relate both...
A suite of idealized mesoscale model simulations are conducted to examine the dynamic pathway to the genesis of short-scale, moist baroclinic disturbances. It is shown that in an initially subsaturated environment, two distinct stages of development are necessary before a preexisting surface-concentrated, warm-core vortex can begin to amplify. The...
A refinement is proposed to the conventional Hovmöller diagram. The rationale is set out for the refined approach, a description provided of its design, and examples presented to illustrate its value as a diagnostic tool. It is shown that it can depict the dynamical characteristics of synoptic-scale disturbances as they evolve on a meandering tropo...
A brief commentary is provided on a paper of Heinrich von Ficker that first appeared in the Meteorologische Zeitschrift in 1920. There are two main strands to Ficker's paper. First he recapitulates, elaborates, and refines his conception of an extra-tropical cyclone as a composite of upper-and lower-level components with the former assuming primacy...
Inspection of the potential vorticity (PV) distribution on isentropic surfaces in the lowermost stratosphere reveals the ubiquitous presence of numerous subsynoptic positive PV anomalies. To examine the space time characteristics of these anomalies, a combined "identification and tracking" tool is developed that can catalog each individual anomaly'...
The relationship between the vertical partitioning of the diabatic heating and the preferred mode of motion is explored for an Ekman-CISK model. First an unrealistic, but heuristically useful, sinusoidal heating distribution in the vertical is used to establish the link between the growth rate of unstable modes, the Rossby radius of deformation and...
Jet streams located on the dynamical tropopause are accompanied by coaligned bands of enhanced potential vorticity (PV) gradients, and these bands can serve as space-time evolvingwaveguides for synoptic and larger scale fiow. Consideration is given to the detection and examination of the along fiow extent and lateral connement of the waveguides on...
A study is undertaken of semi-geostrophic flow of a stratified inviscid atmosphere over various forms of infinite (and some forms of finite) length isentropic orographic features on an f-plane. Finite-amplitude, explicit solutions are derived under the stipulation that the velocity (U) and the Brunt Visl frequency (N) of the upstream flow are both...
The westerly Greenland tip jet is an intense, narrow and intermittent wind phenomenon located southeast of Cape Farewell that occurs frequently during the winter season. Using the ERA-40 reanalysis dataset, a catalogue of 586 objectively detected westerly tip jet events is compiled for the winters 1957-2002, and an analysis is undertaken of the cha...
The Greenland tip jet is an intense, narrow and intermittent westerly wind phenomenon southeast of Cape Farewell that often takes place in winter and plays an important role for both the circulation and seasonal evolution of the mixed layer in the Irminger Sea below. The orographically induced winds result from an interaction between a low pressure...
The Madden Julian Oscillation (MJO), a large-scale coupled pattern between tropical deep convection and atmospheric circulation, is known to be the dominant source of intraseasonal variability in the tropics. Its impacts, however, are not limited to tropical regions: latent heat release associated with MJO convection can force planetary scale Rossb...
THORPEX is an international research programme whose scope is global and
that aims to accelerate improvements in the accuracy of 1-day to 2-week
high impact weather forecasts. The programme's priorities are :- to
address the global-to-regional influences on the evolution and
predictability of weather systems (including the exploitation of
ensemble...
Both sudden stratospheric warming (SSW) events and blocking events are
major atmospheric flow phenomena. An SSW event severely disrupts the
stratospheric polar night jet, as the polar vortex is either displaced
equatorwards and sheared into a comma shape (a displacement SSW) or torn
into two individual vortices (a splitting SSW). Similarly, blockin...
A three-part study of the anomalously cold European winter of 2005/06 is undertaken. Climatological analysis indicates that the dominant pattern of climate variability in the Euro–Atlantic sector during this winter was not a negative phase of the North Atlantic Oscillation (NAO), but a pattern with a ‘‘blocklike’’ center located immediately upstrea...
1] A composite analysis of 43 Kona lows in conjunction with a case study of a particularly damaging Kona low indicate that downstream development is dynamically important to the subtropical cyclogenesis. It takes the form of eastward propagating, statistically significant upstream potential vorticity (PV) anomalies with accompanying meridional wind...
The link between atmospheric blocking and propagating and breaking synoptic-scale Rossby waves (termed PV streamers) are explored for the climatological period 1958–2002, using the ERA-40 re-analysis data. To this end, potential vorticity (PV) based climatologies of blocking and breaking waves are used, and features of the propagating waves is extr...
The link between atmospheric blocking and propagating and breaking synoptic-scale Rossby waves (termed PV streamers) are explored for the climatological period 1958–2002, using the ERA-40 re-analysis data. To this end, potential vorticity (PV) based climatologies of blocking and breaking waves are used, and features of the propagating waves is extr...
On 24-25 February 2005, a significant East Coast cyclone deposited from 4 to nearly 12 in. (!10-30 cm) of snow on parts of the northeastern United States. The heaviest snowfall and most rapid deepening of the cyclone coincided with the favorable positioning of an upper-level, short-wave trough immediately up- stream of a preexisting surface cyclone...
This study examines the upper-level flow pattern that foreshadows the comparatively frequent co-occurrence of heavy precipitation events on the Alpine south-side together with potential vorticity (PV) streamers aloft over western Europe. It is hypothesized that each streamer is itself preceded by a dynamically distinctive and a thermodynamically ac...
In 1912, the polar explorer and scientist Fridtjof Nansen published an article entitled “Bottom Water and the Cooling of the Ocean”, in which he discussed the origin of the deep water in the North Atlantic south of the Greenland–Scotland Ridge (Nansen 1912). It was known at the time that dense water spills over the ridge system, both through Denmar...
In the Northern hemisphere, regions characterized by an enhanced frequency of atmospheric blocking overlap significantly with
those associated with the major extra-tropical patterns of large-scale climate variability—namely the North Atlantic Oscillation
(NAO) and the Pacific North American (PNA) pattern. There is likewise an overlap in the tempora...
Breaking waves on the tropopause are viewed as potential vorticity (PV) streamers on middle-world isentropic levels. A Northern Hemisphere winter climatology of the streamers’ spatial distribution and meridional orientation is derived from the 40-yr ECMWF Re-Analysis (ERA-40) dataset, and used to assess the nature and frequency of occurrence of bre...
Consideration is given to the impact of both synoptic-scale flow features and different model configurations upon the performance of an Alpine-encompassing regional NWP model. The so-called regional LM forecast model is used to simulate two events selected from the Mesoscale Alpine Programme (MAP), and simulations are undertaken with: different dom...
One focus of the Mesoscale Alpine Programme (MAP) was the study of upper-tropospheric potential vorticity (PV) anomalies that take the form of narrow meridionally elongated troughs termed ‘PV streamers’. A systematic effort was undertaken within the MAP framework to: establish a streamer climatology, develop appropriate instrumentation and undertak...
A dynamically based climatology is derived for Northern Hemisphere atmospheric blocking events. Blocks are viewed as large amplitude, long-lasting, and negative potential vorticity (PV) anomalies located beneath the dynamical tropopause. The derived climatology [based on the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analys...
An upper bound is placed upon the amplification rate of an arbitrary perturbation imposed upon the observed, initial state of a barotropic flow and a two-level, quasi-geostrophic baroclinic flow. The implied lower bound to the predictability of the barotropic flow is a function of the mean energy and the mean enstrophy of the initial perturbation,...
One focus of the Mesoscale Alpine Programme (MAP) was the study of upper-tropospheric potential vorticity (PV) anomalies that take the form of narrow meridionally elongated troughs termed ‘PV streamers’. A systematic
effort was undertaken within the MAP framework to: establish a streamer climatology, develop appropriate instrumentation and undertak...
During the autumn there are frequent events of heavy precipitation on the southern slopes of the meso‐α‐scale Alpine chain. Moreover, the accompanying climatology exhibits a distinctive spatial distribution with some meso‐β‐scale regions of enhanced precipitation such as the ‘wet spot’ in the vicinity of Lago Maggiore.
Possible mechanisms to accoun...
There are several extant techniques for diagnosing the dynamics and structure of synoptic-scale flow. The essence of three of these approaches (the Q-field diagnosis, Lagrangian airstreams and the PV-perspective) is outlined, and their relative characteristics illustrated by examining two synoptic cases. It is shown that the three approaches can in...
An examination is undertaken of the diurnal surface-pressure pattern across the main east-west ridge of the European Alps. A case study for clear-sky, quiescent synoptic conditions reveals a pronounced across-ridge asymmetry—to the north the primary feature is an approximate 0.5 hPa semi-diurnal oscillation whilst on the southern side there is an a...
The conventional formulation of the wave‐CISK mechanism involves a quasi‐equilibrium assumption whereby the instantaneous large‐scale diabatic heating that is induced by cloud‐scale motions is assumed to be related to the instantaneous distribution of the large‐scale flow, and in particular to the low‐level moisture convergence of the large‐scale f...
A variant of the classical deformation-induced surface frontogenesis problem is studied in the semigeostrophic limit. The particular flow system examined is the response of a baroclinic, semi-infinite, stratified, uniform potential vorticity atmosphere to an imposed deformation flow field.
This flow system permits the derivation of: a class of simp...
A brief assessment is provided of both the case against and the case for assigning priority to research on large-scale weather
systems (LSWS). The three-fold case against is based upon: the emergence of new overarching themes in environmental science;
the fresh emphasis upon other sub-disciplines of the atmospheric science; and the mature state of...
The tasks of providing multi-decadal climate projections and seasonal plus sub-seasonal climate predictions are of significant societal interest and pose major scientific challenges. An outline is presented of the challenges posed by, and the approaches adopted to, tracing the possible evolution of the climate system on these various time-scales. F...
The tasks of providing multi-decadal climate projections and seasonal plus subseasonal climate predictions are of significant societal interest and pose major scientific challenges. An outline is presented of the challenges posed by, and the approaches adopted to, tracing the possible evolution of the climate system on these various time-scales. Fi...
This study discusses the role of stratospheric intrusions (determined as potential vorticity (PV) streamers) as upper-level instigators of heavy precipitation along the Swiss Alpine south side (AS) on a climatological timescale. A climatology of streamers is used compiled on the basis of the ECMWF 40-year re-analysis data set (ERA-40). Days of extr...
A novel climatology is developed for upper-tropospheric jet streams, which is complementary to and an alternative for the traditional depictions of the time-mean jets. It entails identifying the occurrence of a jet event at a given location and then compiling the spatial frequency distribution of such events. The resulting climatology, derived usin...
ABSTRACTA refinement is proposed to the conventional Hovmöller diagram. The rationale is set out for the refined approach, a description provided of its design, and examples presented to illustrate its value as a diagnostic tool. It is shown that it can depict the dynamical characteristics of synoptic-scale disturbances as they evolve on a meanderi...
Conformity to observations takes on a particular significance in relation to weather prediction. Day-to-day weather changes constitute the temporal evolution of a complex and highly chaotic system, and yet the accuracy of weather forecasts can be, and are, readily and regularly assessed by both practitioners and the general public. Here, the develo...
The tasks of providing multi-decadal climate projections and seasonal plus sub-seasonal climate predictions are of significant societal interest and pose major scientific challenges. An overview is presented of the challenges presented by, and the approaches adopted to, tracing the possible evolution of the climate system on these various time-scal...
Numerical weather prediction models will in the near future be operationally run at resolutions of the order of 1-3km even at the Alpine scale. It is the purpose of this study to investigate the potential benefits and also the problems involved with this increase in resolution. To this end high resolution simulations (at 2km) with the Lokal Modell...
One focus of the Mesoscale Alpine Programme (MAP) was the study of upper-level features that take the form of narrow meridionally elongated troughs called 'PV streamers'. One motivation was the recognition that the passage of such a streamer toward and over the Alps during autumn is often accompanied by the sequential occurrence of most of the majo...
The distribution of extratropical potential vorticity (PV) on isentropic
surfaces that transect the tropopause is a key feature of
planetary-scale teleconnection patterns, synoptic-scale weather systems,
and mesoscale stratosphere-troposphere exchange. Here a Northern
Hemisphere January and July climatology is presented for the mean and
variability...
A theoretical study is undertaken of the relative influence of intermediate-level, near-surface, and tropopause-level perturbations upon flow development and cyclogenesis. The study is based upon two complementary quasigeostrophic models with consideration given to different types of perturbations of Eady-like basic states.First, the standard linea...
A novel dynamically-based approach is introduced to identify, describe and diagnose atmospheric blocking events. The approach is based upon the potential vorticity perspective and takes into account the three-dimensional structure of the phenomenon. It is argued that the essence of a blocking anomaly is located in the upper troposphere, just below...
Consideration of well-established teleconnection patterns account for only a moderate part of European winter variations. Here January interannual variability is examined from a potential vorticity perspective and this yields a reappraisal of the pattern structure and link to European winter weather. The influence of a novel hemispheric pattern in...
Interrannual flow variations in the extratropics are reappraised from a potential vorticity (PV) perspective. A statistical study of the patterns of PV variability on a tropopause-transecting isentropic surface provides a compact description of, and dynamical insight into, the Pacific-North American teleconnection and the North Atlantic Oscillation...
The potential vorticity (PV) pattern in the vicinity of the jet stream takes the form of a narrow tube of enhanced PV gradient on the in situ isentropic surfaces. It is asserted that this distinctive structure can serve as a waveguide and a seat for trapped Rossby waves and that a neighboring vortexlike anomaly can trigger such waves and/or interac...
Storms occurring in the natural environment are distinguished by their rarity, unpredictability and severity, and are notable for their diversity and complexity. Their study is characterized by the acquistion and analysis of empirical data, consideration of the pertinent natural processes, identification of the key contributory factors, and the dev...
An alternative approach is set out for the study of the difference between the forecast and contemporaneous analysis fields of a weather prediction system. Illustrations and interpretations are proffered of this difference when viewed in terms of potential vorticity (PV) for both a case study analysis and a single winter's climatology. The results...
South Alpine heavy precipitation episodes are very often characterized by the occurence of a nar- row meridionally elongated potential vorticity (PV) fllament over western Europe and by the formation of a low-level cyclone over the Mediterranean. These synoptic features are responsible for a southerly ∞ow that carries large amounts of moisture to t...
Mesoscale polar stratospheric clouds (PSCs) are usually regarded as ``mountain wave PSCs'' that are generated by vertically propagating, orographically induced inertia-gravity waves over high latitude topography (e.g. over Scandinavia and Greenland). Recently, it has been proposed that gravity waves emitted from highly unbalanced jet streams at the...
A model-based study is undertaken of a synoptic-scale vortex's encounter with Greenland-scale topog-raphy. The ambient setting is a barotropic westerly flow of uniform potential vorticity. This flow is both incident upon and induces an anticyclone over the topography, and it also serves to advect the vortex toward the mountain. In a simulated evolu...
A study is undertaken of the substructure of an upper‐level potential‐vorticity streamer that approached the European Alps during the field phase of the Mesoscale Alpine Programme (MAP). The diagnosis is based upon a Lagrangian Forward Projection (LFP) technique which can provide a spatial refinement of the operational European Centre for Medium‐Ra...
During the Special Observation Period of the Mesoscale Alpine Program (MAP) the Canadian Mesoscale Compressible Community Model (MC2) has been run in real time at a horizontal resolution of 3 km on a computational domain of 350X300X50 grid points, covering the whole of the Alpine region. An overview of the model configuration and performance is pre...
With the development of a version of the Canadian non-hydrostatic MC2 model optimized for massively parallel processors, it
has become possible to solve very large weather forecast problems in a time sufficiently short to envision a realtime daily
calculation over a domain covering the entire Alps. MAP is a large multinational research program that...
1. INTRODUCTION Greenland (GL) is the third-largest orographic feature on the Northern Hemisphere and is located contiguous to the North Atlantic storm track. Both factors imply that it has the potential to exert a significant impact upon planetary-and synoptic-scale flow. The ambient flow response to a topographic obstacle is influenced by the sca...
The climatological zonal mean zonal wind distribution shows a single maximum, the so-called subtropical jet that coincides with the climatological tropopause break. This is in contrast to daily upper-air charts which, in addition to the subtropical jet stream, also reveal (i) intense jet streams in mid-latitudes associated with transient synoptic-...
The juxtaposition of a localized potential vorticity (PV) anomaly with the extratropical jet-stream is a frequent feature of upper-tropospheric flow. The mutual interaction between these two upper-level flow structures is investigated in idealized and real settings and is related to the in-situ surface development. First, the essence of the upper-l...
Past changes and possible future variations in the nature of extreme precipitation and flood events in Central Europe and
the Alpine region are examined from a physical standpoint. An overview is given of the following key contributory physical
processes: (1) the variability of the large-scale atmospheric flow and the associated changes of the Nort...
The covariation of the mid-tropospheric flow and the sea surface temperature (SST) of the extratropical North Atlantic is studied for the period 1962 to 1992. A statistical approach is adopted and the variables selected for examination, in addition to the SST, are the 500 hPa fields of the geopotential height and the dynamic storm track. Singular V...
Rainstorms that occur on the south side of the European Alps during spring and fall are usually accompanied at the tropopause level by a single elongated filament of intruded stratospheric air. This study examines the dependency of one such storm (the Piedmont event of 5-6 November 1994) upon the filament's structure. The focus is on quantitative p...
The surface low of a mature extra-tropical cyclone is often surmounted by a troposphere-spanning column of anomalously high potential vorticity (PV). In this study the growth and decay of such a PV-tower is traced for one major North Atlantic frontal-wave cyclone using the ECMWF analysis fields and adopting both Eulerian and Lagrangian frameworks....
Extratropical cyclogenesis is usually accompanied by both an upper-level trough and a surface front with its cloud-diabatic accoutrements. In the present study these distinctive structures are interpreted as salient elements of the atmosphere's potential vorticity (PV) distribution, and their role examined for one particular event of frontal-wave c...
Extratropical cyclogenesis is usually accompanied by both an upper-level trough and a surface front with its cloud-diabatic accoutrements. In the present study these distinctive structures are interpreted as salient elements of the atmosphere's potential vorticity (PV) distribution, and their role examined for one particular event of frontal-wave c...
A three-part study is undertaken of the evolution of an upper-level potential vorticity (PV) streamer as it approaches the European Alps. First an examination is performed of one such event that was accompanied by severe precipitation on the Alpine southside. Second a sequence of model simulations is performed to explore the separate and combined i...
A review is presented of theoretical frontogenesis studies. The framework for the study is that of quasi- and semigeostrophic dynamics, and the approach is to consider idealized models that incorporate some presumed quintessential frontogenetic process(es). The focus of the study is the temporal development and nature of surface fronts.
Insight is...
Upper-tropospheric fronts and frontogenesis are viewed from a potental vorticity (PV) perspective. The rudiments of this approach are to regard such a front as a zone of strong PV gradient on isentropic surfaces, and to treat the accompanying frontogenesis as the process whereby this gradient is enhanced on tropopause-transcending isentropic surfac...
Climate simulations have suggested that a greenhouse-gas induced global warming would also lead to a moistening of the atmosphere and an intensification of the mean hydrological cycle. Here we study possible at-tendant effects upon the frequency of heavy precipitation events. For this purpose simulations with a regional climate model are conducted,...
Analyses of four separate autumnal events of very heavy precipitation on the southern slopes of the European Alps indicate that each was accompanied by a strikingly similar anomaly at upper-tropospheric levels. It took the form of a narrow (∼ 500 km), deep (∼4 km) and elongated (∼ 2000 km) streamer of intruded stratospheric air [sic. high potential...