Norwegian Cyclone Model Research Articles

Analysis of meteorological parameters triggering rainfall-induced landslide: a review of 70 years in Valtellina

Abstract. This paper presents an extended reanalysis of the rainfall-induced geo-hydrological events that have occurred in the last 70 years in the alpine area of the Lombardy region, Italy. The work is focused on the description of the major meteorological triggering factors that have caused diffuse episodes of shallow landslides and debris flow. The aim of this reanalysis was to try to evaluate their magnitude quantitatively. The triggering factors were studied following two approaches. The first one started from the conventional analysis of the rainfall intensity (I) and duration (D) considering local rain gauge data and applying the I–D threshold methodology integrated with an estimation of the events' return period. We then extended this analysis and proposed a new index for the magnitude assessment (magnitude index, MI) based on frequency–magnitude theory. The MI was defined considering both the return period and the spatial extent of each rainfall episode. The second approach is based on a regional-scale analysis of meteorological triggers. In particular, the strength of the extratropical cyclone (EC) structure associated with the precipitation events was assessed through the sea level pressure tendency (SLPT) meteorological index. The latter has been estimated from the Norwegian cyclone model (NCM) theory. Both indexes have shown an agreement in ranking the event's magnitude (R2=0.88), giving a similar interpretation of the severity that was also found to be in accordance with the information reported in historical databases. This back analysis of 70 years in Valtellina identifies the MI and the SLPT as good magnitude indicators of the event, confirming that a strong cause–effect relationship exists among the EC intensity and the local rainfall recorded on the ground. In respect of the conventional I–D threshold methodology, which is limited to a binary estimate of the likelihood of landslide occurrence, the evaluation of the MI and the SLPT indexes allows quantifying the magnitude of a rainfall episode capable of generating severe geo-hydrological hazards.

Antecedents for the Shapiro–Keyser Cyclone Model in the Bergen School Literature

AbstractTwo widely accepted conceptual models of extratropical cyclone structure and evolution exist: the Norwegian and Shapiro–Keyser cyclone models. The Norwegian cyclone model was developed around 1920 by the Bergen School meteorologists. This model has come to feature an acute angle between the cold and warm fronts, with the reduction in the area of the warm sector during the evolution of the cyclone corresponding to the formation of an occluded front. The Shapiro–Keyser cyclone model was developed around 1990 and was motivated by the recognition of alternative frontal structures depicted in model simulations and observations of rapidly developing extratropical cyclones. This model features a right angle between the cold and warm fronts (T-bone), a weakening of the poleward portion of the cold front (frontal fracture), an extension of the warm or occluded front to the rear of and around the cyclone (bent-back front), and the wrapping around of the bent-back front to form a warm-core seclusion of post-cold-frontal air. Although the Norwegian cyclone model preceded the Shapiro–Keyser cyclone model by 70 years, antecedents of features of the Shapiro–Keyser cyclone model were apparent in observations, analyses, and conceptual models presented by the Bergen School meteorologists, their adherents, and their progeny. These “lost” antecedents are collected here for the first time to show that the Bergen School meteorologists were aware of them, although not all of the antecedents survived until their reintroduction into the Shapiro–Keyser cyclone model in 1990. Thus, the Shapiro–Keyser cyclone model can be viewed as a synthesis of various elements of cyclone structure and evolution recognized by the Bergen School meteorologists.

Defender and Expositor of the Bergen Methods of Synoptic Analysis: Significance, History, and Translation of Bergeron’s (1928) “Three-Dimensionally Combining Synoptic Analysis”

AbstractTor Bergeron was a key member of the Bergen School of Meteorology that developed some of the most influential contributions to synoptic analysis in the twentieth century: airmass 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. Concerns and criticisms of the methods—in part from the lack of referencing to prior studies, overly simplified conceptual models, and lack of real data in papers by J. Bjerknes and Solberg—were inhibiting worldwide adoption. Bergeron’s research output in the 1920s was aimed at addressing these concerns. His doctoral thesis, written in German, was published as a journal article in Geofysiske Publikasjoner in 1928. Here, an accessible and annotated English translation is provided along with a succinct overview of this seminal study. Major interlaced themes of Bergeron’s study were the first comprehensive description of the Bergen methods: a vigorous defense of cyclogenesis as primarily a lower-tropospheric process as opposed to an upper-tropospheric–lower-stratospheric one; a nuanced explanation of the assertion that meteorology constituted a distinct and special scientific discipline; and, very understandably, a thorough account of Bergeron’s own contributions to the Bergen School. His contributions included identifying how deformation results in frontogenesis and frontolysis, classifying the influence of aerosols on visibility, and explaining the role of the ambient conditions in the onset of drizzle as opposed to rain showers—a distinction that led the formulation of the Wegener–Bergeron–Findeisen process.

Extratropical Cyclones: A Century of Research on Meteorology’s Centerpiece

AbstractThe 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 series of meetings was held that led to the formation of organizations that promoted the international collaboration and scientific exchange required for extratropical cyclone research, which by necessity involves spatial scales spanning national borders. This chapter describes the history of scientific inquiry into the structure, evolution, and dynamics of extratropical cyclones, their constituent fronts, and their attendant jet streams and storm tracks. We refer to these phenomena collectively as the centerpiece of meteorology because of their central role in fostering meteorological research during this century. This extremely productive period in extratropical cyclone research has been possible because of 1) the need to address practical challenges of poor forecasts that had large socioeconomic consequences, 2) the intermingling of theory, observations, and diagnosis (including dynamical modeling) to provide improved physical understanding and conceptual models, and 3) strong international cooperation. Conceptual frameworks for cyclones arise from a desire to classify and understand cyclones; they include the Norwegian cyclone model and its sister the Shapiro–Keyser cyclone model. The challenge of understanding the dynamics of cyclones led to such theoretical frameworks as quasigeostrophy, baroclinic instability, semigeostrophy, and frontogenesis. The challenge of predicting explosive extratropical cyclones in particular led to new theoretical developments such as potential-vorticity thinking and downstream development. Deeper appreciation of the limits of predictability has resulted from an evolution from determinism to chaos. Last, observational insights led to detailed cyclone and frontal structure, storm tracks, and rainbands.

Cold‐conveyor‐belt jet, sting jet and slantwise circulations in idealized simulations of extratropical cyclones

The present study investigates the formation of sting‐jet cyclones using an idealized configuration of a high‐resolution mesoscale numerical model. The initial set‐up is composed of axisymmetric finite‐amplitude synoptic‐scale cyclones added to a baroclinic zonal jet. When the surface cyclone is initialized on the warm‐air side of the zonal jet, a strong bent‐back warm front forms, which is a favourable condition for sting jets. A rather high vertical resolution of about 200 m in the lower troposphere is required to obtain well‐formed slantwise circulations near the cloud head and to make the distinction between cold‐conveyor‐belt jet and sting jet. In contrast, when the surface cyclone is initialized on the jet axis, the surface cyclone is more representative of the Norwegian cyclone model and the bent‐back warm front is almost non‐existent.Sting‐jet mechanisms are analyzed using backward Lagrangian trajectories. A minority group of air parcels entering the sting‐jet region undergoes diabatic cooling due to sublimation of snow and graupel. The majority group does not present such a diabatic cooling and corresponds to descending air masses from the cloud‐head region in a near‐neutral environment with respect to conditional symmetric instability. Indeed, the saturated moist potential vorticity is near zero and the momentum surfaces and saturated moist isentropes are quasi‐parallel in the main slantwise descent. The latter descent is shown to be geostrophically forced by the strong divergent component of the along‐front Q vector.Boundary‐layer processes are also analyzed. In most areas, the static stability being strong, there is no penetration of descents coming from the sting jet into the boundary layer and the peaks in surface wind gusts are connected to intrinsic boundary‐layer convective cells and parametrized turbulence. Such a penetration occurs in some areas but is not related to the strongest wind gusts.

Overview of advances in synoptic meteorology: Four stages of development in conceptual models of frontal cyclones

The advance of fluid dynamics may be divided into four stages: ancient, classical, neoteric, and contemporary. A similar four-stage framework can be used to describe the advance of synoptic meteorology, such as the conceptual models of extratropical cyclones and their attendant fronts. The first conceptual model of extratropical cyclones was proposed by Admiral FitzRoy in 1863. Based purely on Admiral FitzRoy’s personal experience (although it does contain some scientific essentials), this model represents the “ancient” stage of synoptic meteorology. The Norwegian cyclone model was developed based on Newtonian mechanics about 100 yr ago, and represents the classical stage of synoptic meteorology. This model was based on the idea that weather changes are primarily caused by baroclinicity, but contain some serious flaws. In particular, the Norwegian model regards fronts as zeroth-order discontinuities in density, which is inconsistent with the continuity principle of fluid dynamics. The Chicago three-dimensional conceptual model of fronts and cyclones, which was developed approximately 50 yr ago by using quasi-geostrophic theory, can be thought of as representing the neoteric stage of synoptic meteorology. The Chicago model was replaced in the late 20th century by a model of extratropical cyclones characterized by back-bent and wrap-up warm fronts. This model has been developed with massive numerical calculations, and represents the contemporary stage of synoptic meteorology In the era of large data, contemporary synoptic meteorology should be careful to maintain and make full use of the profound physical understanding generated during the neoteric stage of synoptic meteorology.

Searching for the Elusive Cold-Type Occluded Front

Abstract According to the Norwegian cyclone model, whether a warm-type or cold-type occluded front forms depends upon which cold air mass is colder: the prewarm-frontal air mass or the postcold-frontal air mass. For example, a cold-type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm-frontal air mass is warmer than the postcold-frontal air mass. This temperature difference and the resulting occluded-frontal structure in the Norwegian cyclone model is part of what is called the temperature rule. Paradoxically, no clear example of a rearward-sloping, cold-type occluded front has been found in the literature, even though the required temperature difference has been documented in several cases. This article presents the first documented, rearward-sloping, cold-type occluded front. This occluded front forms in a cyclone over the North Atlantic Ocean on 3–5 January 2003 and is documented in model output from the European Centre for Medium-Range Weather Forecasts. Cross sections through the evolving cyclone show the occluded front forms as the less statically stable warm-frontal zone ascends over the more stable cold-frontal zone. Such a stability difference between the cold- and warm-frontal zones is consistent with a previously published hypothesis that the less stable air is lifted by the more stable air to form occluded fronts, in disagreement with the temperature rule. Because warm-frontal zones and the cold air underneath tend to be more stable than cold-frontal zones and the postcold-frontal air, warm-type occluded fronts are much more common than cold-type occluded fronts, explaining why well-defined, rearward-sloping, cold-type occluded fronts are not common in the meteorological literature.

Occluded Fronts and the Occlusion Process: A Fresh Look at Conventional Wisdom

Traditionally, the formation of an occluded front during the occlusion process in extratropical cyclones has been viewed as the catch-up of a faster-moving cold front to a slower-moving warm front separating the warm-sector air from the low center, as first described in the Norwegian cyclone model over 90 yr ago. In this article, the conventional wisdom, or the commonly held beliefs originating from the Norwegian cyclone model, about occluded fronts and the occlusion process are critically examined. The following four tenets of this conventional wisdom are addressed. First, the occlusion process is better described not by catch-up, but by the wrapping up and lengthening of the warm-air tongue as a result of deformation and rotation around the low center. Second, the merger of the cold front and warm front does not result in the frontal zone with the warmer air ascending over the other frontal zone. Instead, the occluded frontal zone tilts over the more statically stable frontal zone. Because a warmfrontal zone tends to be more stable than a cold-frontal zone, this process usually produces a warm-type occlusion, confirming that cold-type occlusions are less common than warm-type occlusions. Third, occlusion does not mean that the cyclone has stopped deepening, because many cyclones continue to deepen 10–30 mb for 12–36 h after the formation of the occluded front. Fourth, clouds and precipitation associated with occluded fronts differ from their widespread stratiform depiction in textbooks. Embedded precipitation bands may be parallel to the front, and little relationship may exist between the fronts and the cloud mass. These four tenets help to explain anomalies in the Norwegian cyclone model, such as how occluded fronts that spiral around the low center do not require catch-up to form, how Shapiro–Keyser cyclones undergo occlusion, why some cyclones do not form occluded fronts, how some cyclones deepen after occlusion, why few cold-type occlusions have been observed, and why occluded cyclones are often associated with heavy precipitation. This reexamination of conventional wisdom leads to a new paradigm for occluded fronts and occluded cyclones. A supplement to this article is available online: DOI: 10.1175/2010BAMS3057.2

An assessment of a Greenland lee cyclone during the Greenland Flow Distortion experiment: An observational approach

AbstractOn 3 March 2007 between 0920 and 1545 UTC, the mesoscale structure of a mature lee cyclone southeast of Greenland was successfully captured during a flight with a research aircraft during the Greenland Flow Distortion experiment (GFDex). The cyclone was accompanied by hazardous weather conditions such as extensive icing and low‐level wind speeds exceeding 34 m s−1. Calculations based on atmospheric soundings from Egedesminde on the west coast of Greenland indicate that conditions were favourable for flow splitting and hence cyclogenesis in the lee of Greenland during the formation of the cyclone. PV inversion carried out on an upper‐level southward‐moving potential vorticity (PV) anomaly indicates that upper‐level forcing as well as orographic forcing had an important role in the cyclone development. The frontal seclusion of the cyclone's warm core was consistent with both the Norwegian cyclone model and Shapiro and Keyser's model. However, it was not possible to match this cyclone precisely with any of these models, which is probably a consequence of the strong influence from Greenland on the cyclone development. In situ measurements showing a combination of warm dry air with relatively high ozone concentrations in the centre of the cyclone together with trajectory calculations indicate that this air had experienced considerable descent, probably induced by Greenland's orography. The observations documented the cyclone's mesoscale structure, and clearly revealed a deep tropopause fold. This tropopause fold is a manifestation of the forcing on the cyclone from the upper‐level PV anomaly. Copyright © 2009 Royal Meteorological Society

CLOUDSAT: Adding a New Dimension to a Classical View of Extratropical Cyclones

Research into extratropical cyclones has a long and rich history. Indeed, much of modern meteorology has grown out of the study of these weather systems, aided by the development of conceptual models of cyclone structure and evolution. Early versions of these models derived from a combination of physical understanding and collections of observations, and today, results from numerical models are used to augment this process. Though observations have played a key role in understanding extratropical cyclone evolution, global observations of the internal structure of the clouds that accompany these storms have been lacking. The launch of Cloudsat in 2006 introduced a new tool for observing the details of clouds and precipitation produced by midlatitude weather systems. In this paper, we examine recent measurements of frontal clouds and precipitation from Cloudsat, and reflect back on the early view of these systems as represented in the Norwegian Cyclone Model. In the process, we describe how Cloudsat observations can be used as a novel component in the research process to obtain unique information on the distribution and characteristics of clouds and precipitation in midlatitude low pressure systems. We present three cases in which Cloudsat views the clouds and precipitation associated with a frontal system, and point out the unique features evident in the observed clouds. In the process, we also demonstrate how Cloudsat data can be used to assess the performance of a numerical representation of the atmosphere. We finish with a suggestion of Cloudsat's anticipated role in the long chain of research into extratropical cyclone structure and evolution.

Baroclinic development within zonally‐varying flows

AbstractPrevious idealized‐modelling studies have shown the importance of across‐jet barotropic shear to the resulting evolution of cyclones, anticyclones, surface‐based fronts, and upper‐level fronts. Meanwhile, many observational studies of cyclones have shown the importance of along‐jet variations in the horizontal wind speed (i.e. confluence and diffluence). This study investigates the importance of these along‐jet (zonal, for zonally‐oriented jets) variations in the horizontal wind speed to the resulting structures and evolutions of baroclinic waves, using idealized models of growing baroclinic waves. An idealized primitive‐equation channel model is configured with growing baroclinic perturbations embedded within confluent and diffluent background flows. When the baroclinic perturbations are placed in background confluence, the lower‐tropospheric frontal structure and evolution initially resemble the Shapiro–Keyser cyclone model, with a zonally‐oriented cyclone, strong warm front, and bent‐back warm front. Later, as the baroclinic wave is amplified in the stronger downstream baroclinicity, the warm sector of the cyclone narrows, becoming more reminiscent of the Norwegian cyclone model. The upper‐level frontal structure develops with a southwest–northeast orientation, and becomes strongest at the base of the trough, where geostrophic cold advection is occurring. In contrast, when the baroclinic perturbations are placed in background diffluence, the lower‐tropospheric frontal structure and evolution resemble the Norwegian cyclone model, with a meridionally‐oriented cyclone, strong cold front, and occluded front. The upper‐level frontal structure is initially oriented northwest–southeast on the western side of the trough, before becoming zonally oriented. Weak geostrophic temperature advection occurs along its length. These results are compared to those from previous observational and idealized‐modelling studies. Copyright © 2007 Royal Meteorological Society

The Structure and Evolution of Extratropical Cyclones, Fronts, Jet Streams, and the Tropopause in the GEOS General Circulation Model

The realism of extratropical cyclones, fronts, jet streams, and the tropopause in the Goddard Earth Observing System (GEOS) general circulation model (GCM), implemented in assimilation and simulation modes, is evaluated from climatological and case-study perspectives using the GEOS-1 reanalysis climatology and applicable conceptual models as benchmarks for comparison. The latitude-longitude grid spacing of the datasets derived from the GEOS GCM ranges from 2° × 2.5° to 0.5° × 0.5°. Frontal systems in the higher-resolution datasets are characterized by horizontal potential temperature gradients that are narrower in scale and larger in magnitude than their lower-resolution counterparts, and various structural features in the Shapiro–Keyser cyclone model are replicated with reasonable fidelity at 1° × 1° resolution. The remainder of the evaluation focuses on a 3-month Northern Hemisphere winter simulation of the GEOS GCM at 1° × 1° resolution. The simulation realistically reproduces various large-scale circulation features related to the North Pacific and Atlantic jet streams when compared with the GEOS-1 reanalysis climatology, and conforms closely to a conceptualization of the zonally averaged troposphere and stratosphere proposed originally by Napier Shaw and revised by Hoskins. An extratropical cyclone that developed over the North Atlantic Ocean in the simulation features surface and tropopause evolutions corresponding to the Norwegian cyclone model and to the LC2 life cycle proposed by Thorncroft et al., respectively. These evolutions are related to the position of the developing cyclone with respect to upper-level jets identified in the time-mean and instantaneous flow fields. This article concludes with the enumeration of several research opportunities that may be addressed through the use of state-of-the-art GCMs possessing sufficient resolution to represent mesoscale phenomena and processes explicitly.

Classification and Composite Diagnosis of Extratropical Cyclogenesis Events in the Southwest Pacific

Characteristic patterns of cyclogenesis in the southwest Pacific region are identified from a sample of 40 developing cyclones during 1990–94. Cases were chosen objectively to ensure a realistic sampling of typical rather than “ideal” events. A subjective classification based on synoptic-scale upper-tropospheric flow signatures prior to cyclone intensification suggested four classes into which all but three cases fell. Three categories, each containing about a quarter of the population, involved direct coupling with the upper jet. They represent cyclone formation beneath (i) the poleward exit region of a 300-hPa jet upstream from a diffluent trough (class U), (ii) the confluent equatorward entrance region of the upper wind maximum (E), and (iii) the upper jet exit region where the jet is downstream from the upper trough (class D). These are analogous to previously identified categories for the western North Atlantic region. A fourth class involved cyclones forming beneath a preexisting intense upper-level trough (class T) located poleward of the upper-level jet. Class U cyclones, forming within diffluent airflow, exhibited strong cold fronts, weak warm fronts, and a meridional configuration while class E cyclones forming in confluent flow attained a more zonally elongated structure marked by stronger warm fronts and weak cold fronts. Class U cyclones featured frontal evolution similar to the Norwegian cyclone model while class E and D cyclones exhibited characteristics of the Shapiro–Keyser model. These results provide further observational support for the emerging paradigm of contrasting frontal and cyclone structure resulting from confluent versus diffluent large-scale flow.

The Effect of Large-Scale Flow on Low-Level Frontal Structure and Evolution in Midlatitude Cyclones

Abstract Observational and modeling studies documented in the literature indicate that the large-scale flow has an important effect on the structure and evolution of low-level fronts in midlatitude cyclones. The purpose of this paper is to address the role of the large-scale flow on low-level cyclone/frontal structure and evolution through a combined observational and idealized modeling approach. Analyses of two observed cyclone cases embedded in large-scale diffluence and confluence, respectively, are presented to illustrate two possible cyclone/frontal structures and evolutions. Specifically, the cyclone moving into a diffluent, high-amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front. The cold front rotates into the warm front, forming an occluded front in the manner of the Norwegian cyclone model, as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center. In contrast, the cyclone moving i...

The Frontal Structure of an Explosive Oceanic Cyclone: Airbone Radar Observations of ERICA IOP 4

Observation taken during the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA) have permitted analyses of explosive oceanic cyclogenesis of unprecedented detail. The most intense of the cyclones that occurred during this experiment was that of intensive observing period 4 (IOP 4). This storm not only contained the lowest sea level pressure ever observed south of 40°N in an extratropical cyclone over the Atlantic but was well-sampled by the specially deployed observing systems (aircraft, airbone Doppler radar, dropwindsondes, and buoys. This paper presents detailed analysis of this case. The primary issues addressed here are 1) the finescale structure of the fronts, and 2) the structure and organization of the associated precipitating features. Both of these issues have previously been investigated primarily through numerical simulation of various cases of intense cyclogenesis. Analysis of the resulting model output has indicated a structural evolution of such cyclones that departs significantly from that described by the Norwegian cyclone model. Diagnosis of the output has indicated that latent heat release plays a significant role, both in the cyclone intensification and in the evolution of the associated fronts. The detailed in situ observations in the present case allow for observational evaluation of the attendant conclusions of these prior modeling studies. Principal findings include: 1) Confirmation of the existence of a “bent-back%rdquo; warm front wrapping to the west around the cyclone center; the frontal structure is very different from that of the occluded front that would here by analyzed according to the Norwegian model. 2) The presence of an extremely sharp warm front, with Kelvin-Helmholtz waves and an intense line of convection found along the front. 3) A continuous extension of convective activity along the cold front to the point of intersection with the warm front, with no evident "fracture” zone. 4) The presence of only scattered convective cells along and to the north of the bent-back warm front. 5) A significant displacement between the cold front and the main cloud band. The cold front lay along a narrow line of intense convection well to the rear of the main comma-shaped cloud mass evident in the satellite imagery.

Cold Fronts Aloft and the Forecasting of Precipitation and Severe Weather East of the Rocky Mountains

Abstract Brief descriptions are given of four cases that illustrate the important role that cold fronts aloft (CFA) can play in producing significant weather cast of the Rocky Mountains. In all four cases, the CFA, and its associated short wave, were located ∼200–300 km ahead of a surface trough. Precipitation (and in some cases severe weather) developed along the leading edge of the CFA. The nested grid model generally did a good job of locating the CFA. Analysis of absolute momentum confirms that these features were fronts, in a dynamic, as well as a thermodynamic sense. A conceptual model for CFA is presented. In the cases examined, this model provides not only a useful picture of the distribution of clouds and precipitation associated with CFA, but also means for locating them. It also helps to define a major class of systems that do not fit the Norwegian cyclone model. Therefore, it should help in the identification of CFA and in improving the forecasting of precipitation and severe weather associate...

Cyclone Development in Polar Air Streams over the Wintertime Continent

Abstract A polar air stream cyclogenesis over the wintertime North American Continent is documented. Numerous similarities are found between this land case and oceanic polar lows studied Harrold and Browning (1969), Reed (1979) and Mullen (1979). The main difference between the land case and oceanic polar lows is that the land cyclone develops in an air mass with negligible water vapor content whereas oceanic ones clearly do not since they are characterized by enhanced cumulus activity during their early stage of development Hence, it is concluded that cumulus convection and its associated latent heat release are not essential for cyclone development in a polar air stream. It is also shown that the surface occlusion which forms from the merger of the polar trough with the main polar front does not follow the typical surface occlusion process as described by the Norwegian cyclone model.

Formation of Cloud Bands Associated with the American Subtropical Jet Stream and Their Interaction with Midlatitude Synoptic Disturbances Reaching Europe

This study describes the evolution of very large cloud bands originating from the eastern subtropical Pacific Ocean, and their relation to the American subtropical jet stream. In many cases, these cloud bands interact with polar lows and then give rise to a very large typical midlatitude cyclone. The cloud pattern associated with this cyclone is very similar to that described by the Norwegian cyclone model, although its origin is quite different.