scholarly journals Changes in Winter North Atlantic Extratropical Cyclones in High-Resolution Regional Pseudo–Global Warming Simulations

2017 ◽  
Vol 30 (17) ◽  
pp. 6905-6925 ◽  
Author(s):  
Allison C. Michaelis ◽  
Jeff Willison ◽  
Gary M. Lackmann ◽  
Walter A. Robinson
Keyword(s):  
High Resolution ◽  
North Atlantic ◽  
Grid Point ◽  
Potential Temperature ◽  
Storm Track ◽  
Extratropical Cyclones ◽  
Precipitation Strengthen ◽  
Cyclone Activity ◽  
Near Surface ◽  
Eddy Heat Flux

The present study investigates changes in the location, frequency, intensity, and dynamical processes of North Atlantic extratropical cyclones with warming consistent with the IPCC Fifth Assessment Report (AR5) representative concentration pathway 8.5 (RCP8.5) scenario. The modeling, analysis, and prediction (MAP) climatology of midlatitude storminess (MCMS) feature-tracking algorithm was utilized to analyze 10 cold-season high-resolution atmospheric simulations over the North Atlantic region in current and future climates. Enhanced extratropical cyclone activity is most evident in the northeast North Atlantic and off the U.S. East Coast. These changes in cyclone activity are offset from changes in eddy kinetic energy and eddy heat flux. Investigation of the minimum SLP reached at each grid point reveals a lack of correspondence between the strongest events in the current and future simulations, indicating the future simulations produced a different population of storms. Examination of the percent change of storms in the storm-track region shows a reduction in the number of strong storms (i.e., those reaching a minimum SLP perturbation of at least −51 hPa). Storm-relative composites of strong and moderate storms show an increase in precipitation, associated with enhanced latent heat release and strengthening of the 900–700-hPa layer-average potential vorticity (PV). Other structural changes found for cyclones in a future climate include weakened upper-level PV for strong storms and a weakened near-surface potential temperature anomaly for moderate storms, demonstrating a change in storm dynamics. Furthermore, the impacts associated with extratropical cyclones, such as strong near-surface winds and heavy precipitation, strengthen and become more frequent with warming.

scholarly journals Dominant Anomaly Patterns in the Near-Surface Baroclinicity and Accompanying Anomalies in the Atmosphere and Oceans. Part I: North Atlantic Basin

2009 ◽  
Vol 22 (4) ◽  
pp. 880-904 ◽  
Author(s):  
Mototaka Nakamura ◽  
Shozo Yamane
Keyword(s):  
Heat Flux ◽  
North Atlantic ◽  
Spatial Scales ◽  
Storm Track ◽  
Lower Troposphere ◽  
Mean Flow ◽  
Atlantic Basin ◽  
Near Surface ◽  
North Atlantic Basin ◽  
Eddy Heat Flux

Abstract Variability in the monthly mean flow and storm track in the North Atlantic basin is examined with a focus on the near-surface baroclinicity, B = Bxi + Byj. Dominant patterns of anomalous B found from empirical orthogonal function (EOF) analyses generally show patterns of shift and changes in the strength of B. Composited anomalies in the monthly mean wind at various pressure levels based on the signals in the EOFs display robust accompanying anomalies in the mean flow up to 50 hPa in the winter and up to 100 hPa in other seasons. Anomalous eddy fields accompanying the anomalous Bx patterns exhibit, broadly speaking, structures anticipated from linear theories of baroclinic instabilities and suggest a tendency for anomalous wave fluxes to accelerate/decelerate the surface westerly accordingly. Atmospheric anomalies accompanying By anomalies have patterns different from those that accompany Bx anomalies but are as large as those found for Bx. Anomalies in the sea surface temperature (SST) found for the anomalous patterns of Bx often show large values of small spatial scales along the Gulf Stream (GS), indicating that a meridional shift in the position of the GS and/or changes in the heat transport by the GS may be responsible for the anomalous Bx and concomitant tropospheric and lower-stratospheric anomalies. Anomalies in the net surface heat flux, SST in preceding months, and meridional eddy heat flux in the lower troposphere support this interpretation.

scholarly journals Identifying mesoscale high-wind features within extratropical cyclones

2021 ◽  
Author(s):  
Lea Eisenstein ◽  
Peter Knippertz ◽  
Joaquim G. Pinto
Keyword(s):  
Forecast Error ◽  
Grid Point ◽  
Potential Temperature ◽  
Conveyor Belt ◽  
Extratropical Cyclones ◽  
Forecast Errors ◽  
Wind Gust ◽  
High Wind ◽  
Exclusion Criteria ◽  
Surface Observations

<p>Extratropical cyclones cause strong winds and heavy precipitation events and are therefore one of the most dangerous natural hazards in Europe. The strongest winds within these cyclones are mostly connected to four mesoscale dynamical features: the warm (conveyor belt) jet (WJ), the cold (conveyor belt) jet (CJ), cold-frontal convective features (CFC) and the sting jet (SJ). While all four have high wind gust speeds in common, the timing, location and some further characteristics typically differ and hence likely also the forecast errors occurring in association with them.</p><p>Here we present an objective identification approach for the four features named above based on their most important characteristics in wind, rainfall, pressure and temperature evolution. The main motivations for this are to generate a climatology for Central Europe, to analyse forecast error specific to individual features, and to ultimately improve forecasts of high wind events through feature-dependent statistical post-processing. To achieve, we ideally want to be able to identify the features in surface observations and in forecasts in a consistent way.</p><p>Based on a dataset of hourly observations over Europe and nine windstorm cases during the winter seasons 2017/18, 2018/19 and 2019/20, it became apparent that mean sea-level pressure tendency, potential temperature tendency, change in wind direction and precipitation (all one-hourly) are most important for the distinction between the WJ and CFC. Further adding the time (relative to storm evolution) and location (relative to the storm centre) of occurrence helps to identify the CJ. Ultimately, the identification of each feature is based on a score on a scale from 0 to 10 that reflects the various criteria for a station or grid point. Additionally, exclusion criteria for each feature are defined to rule out locations that meet some criteria (and thus have a positive score) but strongly violate others. Finally, smooth contours are drawn around each feature to define their spatial extent.</p><p>While the distinction between WJ and CFC seems to work reliably, the identification of CJ remains ambiguous and needs further parameters and exclusion criteria to avoid too large areas and overlap with other features. Furthermore, SJ and CJ are very difficult to distinguish based on surface observations alone and are therefore taken together for this preliminary analysis. Once the definition of criteria is finalised, a climatology will be compiled based on observations and the German COSMO model and forecast errors analysed for said model.</p>

scholarly journals The Life Cycle of the North Atlantic Storm Track*

2015 ◽  
Vol 72 (2) ◽  
pp. 821-833 ◽  
Author(s):  
Lenka Novak ◽  
Maarten H. P. Ambaum ◽  
Rémi Tailleux
Keyword(s):  
Heat Flux ◽  
Life Cycle ◽  
North Atlantic ◽  
Storm Track ◽  
High Heat ◽  
High Heat Flux ◽  
Dominant Type ◽  
The North ◽  
Eddy Heat Flux ◽  
The North Atlantic

Abstract The North Atlantic eddy-driven jet exhibits latitudinal variability with evidence of three preferred latitudinal locations: south, middle, and north. Here the authors examine the drivers of this variability and the variability of the associated storm track. The authors investigate the changes in the storm-track characteristics for the three jet locations and propose a mechanism by which enhanced storm-track activity, as measured by upstream heat flux, is responsible for cyclical downstream latitudinal shifts in the jet. This mechanism is based on a nonlinear oscillator relationship between the enhanced meridional temperature gradient (and thus baroclinicity) and the meridional high-frequency (periods of shorter than 10 days) eddy heat flux. Such oscillations in baroclinicity and heat flux induce variability in eddy anisotropy, which is associated with the changes in the dominant type of wave breaking and a different latitudinal deflection of the jet. The authors’ results suggest that high heat flux is conducive to a northward deflection of the jet, whereas low heat flux is conducive to a more zonal jet. This jet-deflecting effect was found to operate most prominently downstream of the storm-track maximum, while the storm track and the jet remain anchored at a fixed latitudinal location at the beginning of the storm track. These cyclical changes in storm-track characteristics can be viewed as different stages of the storm track’s spatiotemporal life cycle.

Sources and transport pathways of precipitating waters in cold-season deep North Atlantic cyclones

2021 ◽  
Author(s):  
Lukas Papritz ◽  
Franziska Aemisegger ◽  
Heini Wernli
Keyword(s):  
North Atlantic ◽  
Gulf Stream ◽  
Water Cycle ◽  
Storm Track ◽  
Cold Season ◽  
Surface Fluxes ◽  
Extratropical Cyclones ◽  
Transport Pathways ◽  
The North ◽  
Cyclone Tracking

AbstractExtratropical cyclones are responsible for a large share of precipitation at mid-latitudes and they profoundly impact the characteristics of the water cycle. In this study, we use the ERA5 reanalysis and a cyclone tracking scheme combined with a Lagrangian diagnostic to identify the sources of moisture precipitating close to the center of 676 deep North Atlantic cyclones in winters 1979 to 2018. Moisture uptakes occur pre-dominantly in originally cold and dry air heated over the North Atlantic, in particular, over the warm waters of the Gulf Stream, whereas more remote sources from land or the subtropics are less important. Analysing the dynamical environment of moisture uptakes, we find that moisture precipitating during the cyclone intensification phase originates in the pre-cyclone environment in the cold sectors of preceding cyclones and the cyclone-anticyclone interaction zone. These moisture sources are linked to the cyclone’s ascent regions via the so-called feeder airstream, a north-easterly cyclone-relative flow that arises due to the cyclone propagation exceeding the advection by the low-level background flow. During the decay phase more and more of the moisture originates in the cyclone’s own cold sector. Consequently, the residence time of precipitating waters in cyclones is short (median of ≈ 2 days) and transport distances are typically less than the distance travelled by the cyclone itself. These findings emphasize the importance of pre-conditioning by surface fluxes in the pre-cyclone environment for the formation of precipitation in cyclones, and suggest an important role for the hand-over of moisture from one cyclone to the next within a storm track.

scholarly journals A Multimodel Assessment of Future Projections of North Atlantic and European Extratropical Cyclones in the CMIP5 Climate Models*

2013 ◽  
Vol 26 (16) ◽  
pp. 5846-5862 ◽  
Author(s):  
Giuseppe Zappa ◽  
Len C. Shaffrey ◽  
Kevin I. Hodges ◽  
Phil G. Sansom ◽  
David B. Stephenson
Keyword(s):  
Central Europe ◽  
North Atlantic ◽  
Climate Models ◽  
Storm Track ◽  
Coupled Model ◽  
Extratropical Cyclones ◽  
Strong Wind ◽  
Wind Speeds ◽  
High Emission ◽  
Strong Winds

Abstract The response of North Atlantic and European extratropical cyclones to climate change is investigated in the climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). In contrast to previous multimodel studies, a feature-tracking algorithm is here applied to separately quantify the responses in the number, the wind intensity, and the precipitation intensity of extratropical cyclones. Moreover, a statistical framework is employed to formally assess the uncertainties in the multimodel projections. Under the midrange representative concentration pathway (RCP4.5) emission scenario, the December–February (DJF) response is characterized by a tripolar pattern over Europe, with an increase in the number of cyclones in central Europe and a decreased number in the Norwegian and Mediterranean Seas. The June–August (JJA) response is characterized by a reduction in the number of North Atlantic cyclones along the southern flank of the storm track. The total number of cyclones decreases in both DJF (−4%) and JJA (−2%). Classifying cyclones according to their intensity indicates a slight basinwide reduction in the number of cyclones associated with strong winds, but an increase in those associated with strong precipitation. However, in DJF, a slight increase in the number and intensity of cyclones associated with strong wind speeds is found over the United Kingdom and central Europe. The results are confirmed under the high-emission RCP8.5 scenario, where the signals tend to be larger. The sources of uncertainty in these projections are discussed.

SST fronts along the Gulf Stream and Kuroshio affect the atmospheric winter climatology primarily in the absence of storms

2021 ◽  
Author(s):  
Thomas Spengler ◽  
Leonidas Tsopouridis ◽  
Clemens Spensberger
Keyword(s):  
North Atlantic ◽  
Gulf Stream ◽  
Storm Track ◽  
Heat Fluxes ◽  
Cyclone Activity ◽  
Surface Heat Fluxes ◽  
The North ◽  
Upper Level ◽  
Upper Level Jet ◽  
Kuroshio Region

<p>The Gulf Stream and Kuroshio regions feature strong sea surface temperature (SST) gradients that influence cyclone development and the storm track. Smoothing the SSTs in either the North Atlantic or North Pacific has been shown to yield a reduction in cyclone activity, surface heat fluxes, and precipitation, as well as a southward shift of the storm track and the upper-level jet. To what extent these changes are attributable to changes in individual cyclone behaviour, however, remains unclear. Comparing simulations with realistic and smoothed SSTs in the atmospheric general circulation model AFES, we find that the intensification of individual cyclones in the Gulf Stream or Kuroshio region is only marginally affected by reducing the SST gradient. In contrast, we observe considerable changes in the climatological mean state, with a reduced cyclone activity in the North Atlantic and North Pacific storm tracks that are also shifted equator-ward in both basins. The upper-level jet in the Atlantic also shifts equator-ward, while the jet in the Pacific strengthens in its climatological position and extends further east. Surface heat fluxes, specific humidity, and precipitation also respond strongly to the smoothing of the SST, with a considerable decrease of their mean values on the warm side of the SST front. This decrease is more pronounced in the Gulf Stream than in the Kuroshio region, due to the amplified decrease in SST along the Gulf Stream SST front.  Considering the pertinent variables occurring within different radii of cyclones in each basin over their entire lifetime, we find cyclones to play only a secondary role in explaining the mean states differences between smoothed and realistic SST experiments.</p>

Using ‘heat tagging’ to understand the remote influence of atmospheric diabatic heating through long-range transport

2021 ◽  
Author(s):  
Robert Fajber ◽  
Paul J. Kushner
Keyword(s):  
Heat Transport ◽  
Diabatic Heating ◽  
Hydrological Cycle ◽  
Structural Characteristics ◽  
Potential Temperature ◽  
Storm Track ◽  
Local Heat ◽  
Radiative Heating ◽  
Latent Heating ◽  
Near Surface

AbstractIn the circulating atmosphere, diabatic heating influences the potential temperature content of air masses far from where the heating occurs. Budgets that balance local diabatic sources with local heat divergence and storage do not retain information about this remote influence, which requires air-mass tracking. In this study, a process based, passive-tracer diagnostic, called heat tagging, is introduced. Heat tagging locally decomposes the potential temperature into contributions from the distinctive diabatic processes that generate them, wherever they occur. The distribution, variability and transport of atmospheric heat tags are studied in the relatively simple setting of an idealized aquaplanet model. Heat tags from latent heating are generated in the deep tropics and the midlatitude storm track and then transported throughout the troposphere. By contrast dry sensible heat tags are enhanced near the surface, and radiative tags are mainly confined to the stratosphere. As a result, local heat transport, variability of potential temperature and global poleward heat transport are dominated by heat tags related to latent heating, with heat tags from sensible and radiative heating only making contributions in the polar near surface and the stratosphere respectively. Heat tagging thus quantifies how water vapor and latent heating link the structural characteristics of the atmosphere and illustrates the importance of the hydrological cycle in poleward energy transport.

Potential Temperature and Potential Vorticity Inversion: Complementary Approaches

2010 ◽  
Vol 67 (12) ◽  
pp. 4001-4016 ◽  
Author(s):  
Joseph Egger ◽  
Klaus-Peter Hoinka
Keyword(s):  
North Atlantic ◽  
Potential Vorticity ◽  
Potential Temperature ◽  
Storm Track ◽  
Temperature Inversion ◽  
Weather Forecasts ◽  
The North ◽  
Potential Vorticity Inversion ◽  
Medium Range ◽  
Balanced Flow

Abstract Given the distribution of one atmospheric variable, that of nearly all others can be derived in balanced flow. In particular, potential vorticity inversion (PVI) selects potential vorticity (PV) to derive pressure, winds, and potential temperature θ. Potential temperature inversion (PTI) starts from available θ fields to derive pressure, winds, and PV. While PVI has been applied extensively, PTI has hardly been used as a research tool although the related technical steps are well known and simpler than those needed in PVI. Two idealized examples of PTI and PVI are compared. The 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) datasets are used to determine typical anomalies of PV and θ in the North Atlantic storm-track region. Statistical forms of PVI and PTI are applied to these anomalies. The inversions are equivalent but the results of PTI are generally easier to understand than those of PVI. The issues of attribution and piecewise inversion are discussed.

scholarly journals Climatology and Changes of Extratropical Cyclone Activity: Comparison of ERA-40 with NCEP–NCAR Reanalysis for 1958–2001

2006 ◽  
Vol 19 (13) ◽  
pp. 3145-3166 ◽  
Author(s):  
Xiaolan L. Wang ◽  
Val R. Swail ◽  
Francis W. Zwiers
Keyword(s):  
North Atlantic ◽  
Storm Track ◽  
Northern Europe ◽  
Cyclone Activity ◽  
Historical Trends ◽  
Sea Level Pressure ◽  
The North ◽  
Abrupt Changes ◽  
Increasing Trend ◽  
Activity Comparison

Abstract In this study, a cyclone detection/tracking algorithm was used to identify cyclones from two gridded 6-hourly mean sea level pressure datasets: the 40-yr ECMWF Re-Analysis (ERA-40) and the NCEP–NCAR reanalysis (NNR) for 1958–2001. The cyclone activity climatology and changes inferred from the two reanalyses are intercompared. The cyclone climatologies and trends are found to be in reasonably good agreement with each other over northern Europe and eastern North America, while ERA-40 shows systematically stronger cyclone activity over the boreal extratropical oceans than does NNR. However, significant differences between ERA-40 and NNR are seen over the austral extratropics. In particular, ERA-40 shows significantly greater strong-cyclone activity and less weak-cyclone activity over all oceanic areas south of 40°S in all seasons, while it shows significantly stronger cyclone activity over most areas of the austral subtropics in the warm seasons. The most notable historical trends in cyclone activity are found to be associated with strong-cyclone activity. Over the boreal extratropics, both ERA-40 and NNR show a significant increasing trend in January–March (JFM) strong-cyclone activity over the high-latitude North Atlantic and over the midlatitude North Pacific, with a significant decreasing trend over the midlatitude North Atlantic and a small increasing trend over northern Europe. The JFM changes over the North Atlantic are associated with the mean position of the storm track shifting about 181 km northward. Importantly, there is no evidence of abrupt changes identified for the boreal extratropics, although previous studies have suggested that the upward trend found in the NNR data could be biased high. However, there exist a few abrupt changes over the austral extratropics, which appear to be attributable to the increasing availability of observations assimilated in the reanalyses. After diminishing the effects of these abrupt changes, strong-cyclone activity over the austral circumpolar oceanic region is identified to have an increasing trend in October–December (OND) and July–September (JAS), with a decreasing trend over the 40°–60°S zone in JAS.

scholarly journals On the Significance of the Sensible Heat Supply from the Ocean in the Maintenance of the Mean Baroclinicity along Storm Tracks

2011 ◽  
Vol 24 (13) ◽  
pp. 3377-3401 ◽  
Author(s):  
Daisuke Hotta ◽  
Hisashi Nakamura
Keyword(s):  
Heat Supply ◽  
Planetary Wave ◽  
Storm Track ◽  
Storm Tracks ◽  
Substantial Contribution ◽  
Latent Heating ◽  
Sensible Heat ◽  
Near Surface ◽  
Eddy Heat Flux ◽  
Wave Response

Abstract The relative importance between the sensible heat supply from the ocean and latent heating is assessed for the maintenance of near-surface mean baroclinicity in the major storm-track regions, by analyzing steady linear responses of a planetary wave model to individual components of zonally asymmetric thermal forcing taken from a global reanalysis dataset. The model experiments carried out separately for the North Atlantic, North Pacific, and south Indian Oceans indicate that distinct local maxima of near-surface baroclinicity observed along the storm tracks can be reinforced most efficiently as a response to the near-surface sensible heating. The result suggests the particular importance of the differential sensible heat supply from the ocean across an oceanic frontal zone for the efficient restoration of surface baroclinicity, which acts against the relaxing effect by poleward eddy heat transport, setting up conditions favorable for the recurrent development of transient eddies to anchor a storm track. Unlike what has been suggested, the corresponding reinforcement of the near-surface baroclinicity along a storm track as the response to the latent heating due either to cumulus convection or large-scale condensation is found less efficient. As is well known, poleward eddy heat flux convergence acts as the primary contributor to the reinforcement of the surface westerlies, especially in the core of a storm track. In its exit region, a substantial contribution to the reinforcement arises also from a planetary wave response to the sensible heat supply from the ocean. In contrast, the surface wind acceleration as a planetary wave response to the latent heating is found to contribute negatively to the maintenance of the surface westerlies along any of the major storm tracks.

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