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Sigbjørn Grønås

Other affiliations: University of Bergen
Bio: Sigbjørn Grønås is an academic researcher from Geophysical Institute, University of Bergen. The author has contributed to research in topics: Extratropical cyclone & Storm. The author has an hindex of 9, co-authored 10 publications receiving 440 citations. Previous affiliations of Sigbjørn Grønås include University of Bergen.

Papers
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BookDOI
01 Jan 1999
TL;DR: The most recent edition of the International Symposium on the Life Cycle of Extratropical Cyclones, held in Bergen, Norway, 27 June-1 July 1994, coincided with the 75th anniversary of the introduction of the frontal-cyclone model presented in his seminal article "On the Structure of Moving Cyclones".
Abstract: This monograph contains expanded versions of the invited papers presented at the International Symposium on the Life Cycles of Extratropical Cyclones, held in Bergen, Norway, 27 June-1 July 1994. The symposium coincided with the 75th anniversary of the introduction of Jack Bjerknes's frontal-cyclone model presented in his seminal article "On the Structure of Moving Cyclones." The first nine chapters present a historical overview of extratropical cyclone research and forecasting from the early eighteenth century into the mid-twentieth century. The succeeding chapters review and present contemporary research on the theory, observations, analysis, diagnosis, and prediction of extratropical cyclones. The sequence of presentations transcends the planetary scale to mesoscale scales of motion. This monograph should be of interest to historians of meteorology, researchers and forecasters. It contains material appropriate for teaching courses in advanced undergraduate and graduate meteorology. The chapter bibliographies provide a valuable source for key references on many aspects of extratropical cyclones.

86 citations

Journal ArticleDOI
01 Jan 1993-Tellus A
TL;DR: In this paper, the effect of nonlinearity on the formation of mountain-wave induced stagnation points is examined using the scaling laws for ideal hydrostatic flow and a series of runs with decelerating winds in a numerical model.
Abstract: The effect of non-linearity on the formation of mountain-wave induced stagnation points is examined using the scaling laws for ideal hydrostatic flow and a series of runs with decelerating winds in a numerical model. In the limit of small deceleration rate (i.e., near steady state) runs with a variety of mountain heights and widths give similar results; i.e., the speed extrema values in the 3-D wave fields collapse onto “universal curves”. For a Gaussian hill with circular contours, stagnation first occurs at a point above the lee slope. This result contradicts the result of linear theory that stagnation begins on the windward slope. The critical value of ĥfor stagnation above a Gaussian hill is ĥ crit = 1.1 ± 0.1. For a 3/2-power hill, the critical height is slightly higher, ĥ crit = 1.2 ± 0.2. These values are significantly larger than the value for a ridge (ĥ crit = 0.85), due to dispersion of wave energy aloft. The application of Sheppard's rule and the vorticity near the stagnation point are discussed. As expected from linear theory, the presence of positive windshear suppresses stagnation aloft. With Richardson number = 20 for example, stagnation first begins at the ground at a value of ĥ= 1.6 ± 0.2. When a stagnation point first forms aloft in the unsheared case, the flow field begins to evolve in the time domain and the scaling laws are violated. We interpret these events as a wave-breaking induced bifurcation which leads to stagnation on the windward slope and the formation of a wake. DOI: 10.1034/j.1600-0870.1993.00003.x

82 citations

Journal ArticleDOI
01 Oct 1995-Tellus A
TL;DR: In this paper, the authors investigated four cases satisfying the necessary conditions for development of Arctic outbreak polar lows from numerical simulations and found that simply the height between the tropopause and the top of the convective planetary boundary layer is a good indicator of the risk of polar low formation.
Abstract: Four cases satisfying the necessary conditions for development of Arctic outbreak polar lows have been investigated from numerical simulations. Characteristic synoptic conditions are a mature extratropical cyclone situated over Scandinavia and a northerly, baroclinic flow in the Norwegian Sea. This flow has a convective planetary boundary layer (PBL) caused by heat fluxes from the warm ocean in cold Arctic air masses coming from the sea ice areas to the north and west. The height of the PBL may reach 700 hPa and even more. The synoptic situation is also characterized by a dry intrusion of stratospheric air west of the cyclones. When the tropopause is defined by the surface of potential vorticity (PV) equal 2 PVU (1 PVU = 10 −6 m 2 s −1 K kg −1 ), the intrusion lowers the tropopause typically down to levels between 450 and 750 hPa, bringing large positive anomalies of PV to levels normally in mid-troposphere. These synoptic conditions are ideal for cyclogenesis from mutual interaction of positive upper air and boundary layer PV anomalies. The Rossby height is high, which means that small-scale polar lows may form. It is found that simply the height between the tropopause and the top of the convective PBL is a good indicator of the risk of polar low formation. This height was found to be 2500 m or more in two of the cases when no polar lows were observed. When polar lows developed in the other two cases, heights of 1000 m or less were measured (in numerical simulations). The simulated development of these two polar lows has been investigated. The polar low cyclogenesis was found to be caused by the baroclinic instability in agreement with the conceptual model of Montgomery and Farrel. Diabatic intensification is important and seems to be necessary to seclude the warm core disturbance which is characteristic of Arctic outbreak polar lows. The seclusion process implies that warm air is being surrounded by cold air. DOI: 10.1034/j.1600-0870.1995.00121.xv

64 citations

Journal ArticleDOI
01 Oct 1995-Tellus A
TL;DR: In this paper, a simulation of a small synoptic-scale (∼ 1000 km), fast-moving extratropical cyclone ( ∼ 25 m/s−1) is presented.
Abstract: The Bergen School meteorologists realized that not all cyclones follow their conceptual cyclone model. In particular they found cases with re-generation of occluded cyclones. In their literature, for instance as found in Godske and co-workers, 2 kinds of re-generation were considered challenging to weather forecasting: a thermodynamic intensification with similarities to the development of tropical cyclones, and intensification of what was called the non-frontal trough, or the back-bent occlusion, of strong cyclones. It has been believed that the latter kind is characteristic of the strongest surface winds observed in the Northwestern Atlantic. In this paper such a case, resulting in the strongest cyclone landfall on the Norwegian west coast this century, has been investigated from a simulation of a small synoptic-scale (∼ 1000 km), fast-moving extratropical cyclone (∼ 25 m/s−1). It is found that the cyclone evolves as the conceptual frontal model of Shapiro and Keyser (1990), and that the strong winds are developed by a secondary, mesoscale (∼ 500 km) cyclogenesis closely linked to the seclusion process. The time scale of the intensification is 12 h, starting with what is called the seclusion trough at the tip of the back-bent warm front. As the cold air secludes the warm core, the disturbance develops into a separate low, here called the seclusion low. Release of latent heat connected to the back-bent warm front is found to play an important role in forming the seclusion. A part of the generated potential vorticity (PV) remains within the warm air in the seclusion process. Inversion of the low-level PV anomalies results in a low-level jet along the outer side of the seclusion trough. The strong winds are observed when the seclusion trough develops into the seclusion low and the low-level jet becomes parallel to the large scale westerly flow. A positive PV anomaly streamer, formed from a larger scale upper PV anomaly in phase with the surface low, takes part in this stage of the development. DOI: 10.1034/j.1600-0870.1995.00116.x

63 citations

Journal ArticleDOI
01 Mar 2005-Tellus A
TL;DR: In this article, a mesoscale numerical model with 10 km between the grid points horizontally is used for simulation of ideal flows passing southern Norway, where the large-scale wind direction was between south and west and the wind speed between 10 and 22.5 m s −1.
Abstract: Mesoscale structures have been identified and studied for simulations of ideal flows passing southern Norway. The large-scale wind direction was between south and west and the wind speed between 10 and 22.5 m s −1 . Flow data have been provided from simulations with a mesoscale numerical model with 10 km between the grid points horizontally. The results are found to be qualitatively in accordance with observational findings, including old forecasting rules for southern Norway. As expected, the influence of rotation is considerable. Accordingly, the flows are characterized by a jet on the left side of the mountains and a minimum on the right upstream side. In addition, a wind shadow extends far downstream of the main mountains, with signs of increased winds on the right side of the wind shadow. The wind shadow is connected to an inertio-gravity wave with downstream signatures caused by rotation. When the background wind direction was turned, the alignment of the structures was turned accordingly. For flows in the sector 200–270°, the action of the Coriolis force gave an efficiently narrower mountain (than without rotation). A similar action for southerly flows, on the other hand, resulted in an efficiently wider mountain. Different mountain widths resulted in different shape of the gravity waves and different acceleration of the jet on the left side. When the wind speed is increased, the amplitudes of the mesoscale structures are decreased with no abrupt change in the character of the flow.

56 citations


Cited by
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01 Jan 1989
TL;DR: In this article, a two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea.
Abstract: Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

3,813 citations

MonographDOI
01 Nov 2006

1,443 citations

Book
06 Nov 2006
TL;DR: A comprehensive unified treatment of atmospheric and oceanic fluid dynamics is provided in this paper, including rotation and stratification, vorticity, scaling and approximations, and wave-mean flow interactions and turbulence.
Abstract: Fluid dynamics is fundamental to our understanding of the atmosphere and oceans. Although many of the same principles of fluid dynamics apply to both the atmosphere and oceans, textbooks tend to concentrate on the atmosphere, the ocean, or the theory of geophysical fluid dynamics (GFD). This textbook provides a comprehensive unified treatment of atmospheric and oceanic fluid dynamics. The book introduces the fundamentals of geophysical fluid dynamics, including rotation and stratification, vorticity and potential vorticity, and scaling and approximations. It discusses baroclinic and barotropic instabilities, wave-mean flow interactions and turbulence, and the general circulation of the atmosphere and ocean. Student problems and exercises are included at the end of each chapter. Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation will be an invaluable graduate textbook on advanced courses in GFD, meteorology, atmospheric science and oceanography, and an excellent review volume for researchers. Additional resources are available at www.cambridge.org/9780521849692.

1,022 citations

Journal ArticleDOI
TL;DR: In this article, the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response).
Abstract: A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition ...

481 citations

Book
28 May 2007
TL;DR: In this article, the authors present a list of principal symbols and abbreviations for parameterization schemes and their application in the terrestrial environment, including land surface-atmosphere parameterizations, water-surface-layer and turbulence parameterizations.
Abstract: Preface List of principal symbols and abbreviations 1. Why study parameterization schemes? 2. Land surface-atmosphere parameterizations 3. Soil-vegetation-atmosphere parameterizations 4. Water-atmosphere parameterizations 5. Planetary boundary layer and turbulence parameterizations 6. Convective parameterizations 7. Microphysics parameterizations 8. Radiation parameterizations 9. Cloud cover and cloudy sky radiation parameterizations 10. Orographic drag parameterizations 11. Thoughts on the future 12. References Index.

452 citations