| 1. |
- Hanley, John, 1982-, et al.
(författare)
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Objective identification and tracking of multi-centre cyclones in the ERA-Interim reanalysis dataset
- 2012
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 138:664, s. 612-625
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Tidskriftsartikel (refereegranskat)abstract
- We present a novel cyclone identification and tracking method that explicitly recognizes multicentre cyclones (MCCs), defined as a cyclonic system with two or three sea-level pressure minima within its outermost contour. The method allows for the recognition of cyclone merger and splitting events in a natural way, and provides a consistent measure of the cyclone extent. Using the ERA-Interim reanalysis dataset, we compute a climatology using this method and show that MCCs occur in about 32 of all cyclone tracks and are much more prevalent in more intense storms. We also show that the method permits reconnection of tracks that would have been spuriously split using a conventional method. We present spatial maps of cyclone mergers, splitting, genesis and lysis using the method and also compute statistics of precipitation falling within cyclones, showing that it is strongly concentrated in the most intense cyclones.
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| 2. |
- Hannachi, Abdelwaheb, et al.
(författare)
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The North Atlantic jet stream : a look at preferred positions, paths and transitions
- 2012
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 138:665, s. 862-877
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Tidskriftsartikel (refereegranskat)abstract
- Preferred jet stream positions and their link to regional circulation patterns over the winter North Atlantic/European sector are investigated to corroborate findings of multimodal behaviour of the jet positions and to analyse patterns of preferred paths and transition probabilities between jet regimes using ERA-40 data. Besides the multivariate Gaussian mixture model, hierarchical clustering and data image techniques are used for this purpose. The different approaches all yield circulation patterns that correspond to the preferred jet regimes, namely the southern, central and the northern positions associated respectively with the Greenland anticyclone or blocking, and two opposite phases of an East Atlantic-like flow pattern. Growth and decay patterns as well as preferred paths of the system trajectory are studied using the mixture model within the delay space. The analysis shows that the most preferred paths are associated with central to north and north to south jet stream transitions with a typical time-scale of about 5 days, and with life cycles of 12 weeks. The transition paths are found to be consistent with transition probabilities. The analysis also shows that wave breaking seems to be the dominant mechanism behind Greenland blocking.
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| 3. |
- Harnik, Nili, et al.
(författare)
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On the equilibration of asymmetric barotropic instability
- 2014
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 140:685, s. 2444-2464
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Tidskriftsartikel (refereegranskat)abstract
- The conjunction of turbulence, waves and zonal jets in geophysical flows gives rise to the formation of potential vorticity staircases and to the sharpening of jets by eddies. The effect of eddies on jet structure, however, is fundamentally different if the eddies arise from barotropic rather than from baroclinic instability. As is well known, barotropic instability may occur on zonal jets when there is a reversal of potential vorticity gradients at the jet flanks. In this article we focus on the nonlinear stages of this instability and its eventual saturation. We consider an idealized initial state consisting of an anticyclonic potential vorticity strip sitting in the flanks of an eastward jet. This asymmetric configuration, a generalization of the Rayleigh problem, is one of the simplest barotropic jet configurations which incorporates many fundamental aspects of real flows, including linear instability and its equilibration, nonlinear interactions, scale cascades, vortex dynamics, and jet sharpening. We make use of the simplicity of the problem to conduct an extensive parameter sweep, and develop a theory relating the properties of the equilibrated flow to the initial flow state by considering the marginal stability limit, together with conservation of circulation and wave activity.
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| 4. |
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| 5. |
- Kral, ST, et al.
(författare)
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Observations of summer turbulent surface fluxes in a High Arctic fjord
- 2014
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 140, s. 666-675
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Tidskriftsartikel (refereegranskat)abstract
- The summer atmospheric boundary layer over a fjord in the High Arctic has beeninvestigated during three consecutive years (2008–2010). Measurements of turbulentsurface layer fluxes of momentum and sensible heat using a sonic anemometer and slowresponseinstruments were taken from a tower on the coast of Isfjorden, Svalbard, andanalysed for seasonal variability and differing fetch conditions. The results resembled theatmospheric boundary layer characteristics previously found for ice-free winter conditions.The momentum flux was usually directed downwards, but for low wind speeds there was apossible contribution of swell, giving an upward directed momentum flux. The cross-windcomponent of the momentum flux sometimes contributed significantly to the total flux. Thesensible heat flux was very dependent on the origin of air, whether it had a long over-waterfetch, or originated from land areas with or without glaciers. In addition to non-stationarityof the flow due to the influence of the fjord’s shape and its surrounding topography, lowwind speeds questioned the validity of the Monin–Obukhov similarity theory, whereas theover-water fetch and off-wind angle were of minor importance. Moreover, some results,especially the off-wind angle, were very sensitive to how the raw data were treated, inparticular which coordinate rotation method was employed, the double rotation or theplanar fit method.
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| 6. |
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| 7. |
- Nilsson, Erik, 1983-, et al.
(författare)
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Convective boundary-layer structure in the presence of wind-following swell
- 2012
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 138:667, s. 1476-1489
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Tidskriftsartikel (refereegranskat)abstract
- The marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area, where they were initiated by momentum transferred to the ocean wave field during storms. The atmospheric boundary layer during wind-following swell and various stability states has been investigated using large-eddy simulation (LES) data. The dominant energy-containing motions in the near-neutral atmospheric boundary layer over flat terrain are known to be dominated by near-ground shear-induced regions of high- and low-speed flow. Windfields and momentum fluxes from LES for swell-dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves. Statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary-layer winds and fluxes. A mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of LES data. Coherent detached eddies from the directly wave-induced motions near the surface are found to maintain an upward momentum transfer. This mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions. In this way, it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell. This has implication for the turbulence length-scales during wind-following swell.
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| 8. |
- Nilsson, Erik, 1983-, et al.
(författare)
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Turbulent momentum flux characterisation using extended multiresolution analysis
- 2014
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 140, s. 1715-1728
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Tidskriftsartikel (refereegranskat)abstract
- The variability of turbulent momentum flux in neutral and unstable atmospheric boundary layers is characterised by analysing surface-layer measurements and data from large-eddy simulations (LES). The method involves multiresolution (MR) decomposition of vertical wind and advected variables into eddy fluctuations on different scales. It provides a measure of the amount of flux variability that stems from same-scale correlations and from combinations of different scale eddy fluctuations. Combining two analysis methods enabled MR component cospectra to be introduced in order to study the contribution of downward and upward flux on different scales. These component cospectra were used to investigate at which scales most of the upward and downward momentum flux occurs. By using MR spectra, cospectra, and flow visualisation this investigation provides insights into turbulence structure and fluxes in neutral and unstable stratification. It is shown that most of the flux variability in the lower part of the boundary layer can be characterised as a combination of larger scale streamwise elongated horizontal wind streaks and smaller scale vertical wind fluctuations. These streaks are found to account for a large part of downward momentum flux at relatively large, energy-containing scales. Most of the upward momentum flux is found to occur at smaller scales. This can be interpreted as showing that upward momentum flux in these conditions is caused by the generation of smaller scale secondary motions when larger scale turbulence elements break down and dissipate. Differences in the height dependence of turbulence structure and momentum flux for neutral and unstably stratified conditions are also investigated and related to the existence of wind streaks and horizontal rolls in these different conditions.
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| 9. |
- Raisanen, P., et al.
(författare)
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Impact of dust particle non-sphericity on climate simulations
- 2013
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 139:677, s. 2222-2232
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Tidskriftsartikel (refereegranskat)abstract
- Although mineral aerosol (dust) particles are irregular in shape, they are treated as homogeneous spheres in climate model radiative transfer calculations. Here, we test the effect of dust particle non-sphericity in the ECHAM5.5-HAM2 global aerosol-climate model. The short-wave optical properties of the two insoluble dust modes in HAM2 are modelled using an ensemble of spheroids that has been optimized to reproduce the optical properties of dust-like aerosols, thereby providing a significant improvement over spheres. First, the direct radiative effects (DRE) of dust non-sphericity were evaluated diagnostically, by comparing spheroids with both volume-equivalent and volume-to-area (V/A) equivalent spheres. In the volume-equivalent case, the short-wave DRE of insoluble dust at the surface and at the top of the atmosphere (TOA) was slightly smaller (typically by 3-4%) for spheroidal than for spherical dust particles. This rather small difference stems from compensating non-sphericity effects on the dust optical thickness and asymmetry parameter. In the V/A-equivalent case, the difference in optical thickness was virtually eliminated and the DRE at the TOA (surface) was approximate to 20% (approximate to 13%) smaller for spheroids than for spheres, due to a larger asymmetry parameter. Even then, however, the global-mean DRE of non-sphericity was only 0.055 W m(-2) at the TOA and 0.070 W m(-2) at the surface. Subsequently, the effects of dust non-sphericity were tested interactively in simulations in which ECHAM5.5-HAM2 was coupled to a mixed-layer ocean model. Consistent with the rather small radiative effects noted above, the climatic differences from simulations with spherical dust optics were generally negligible.
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| 10. |
- Linders, Torsten, 1971, et al.
(författare)
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Limited polar low sensitivity to SST
- 2011
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - 1477-870X. ; 137:654, s. 58-69
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Tidskriftsartikel (refereegranskat)abstract
- The sensitivity of the intensity of polar lows to changes in the sea-surface temperature (SST) is investigated. This is done by using an axisymmetric non-hydrostatic numerical model. It is found that the intensity of the simulated polar lows responds linearly to SST perturbations, with positive perturbations leading to deeper sea-level pressure (SLP) depressions and higher azimuthal winds. The numerical simulations give an SLP depression sensitivity of only about −0.6 hPa K−1 and a sensitivity of the maximum azimuthal wind of only about +0.6 m s−1 K−1. This is about one order of magnitude less than the sensitivity of the theoretical maximum intensity. It is suggested that the very limited sensitivity can be explained within the wind-induced surface heat exchange (WISHE) theory. The limited sensitivity can thus be attributed, in roughly equal parts, to the limited efficiency with which the polar low extracts heat from the ocean and the limited mean height of the convection. Copyright © 2011 Royal Meteorological Society
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