General characteristics of the dynamical and thermal structure and evolution of strong explosive cyclones in the northwestern Atlantic near North America (18 cases) and the extreme northeastern Atlantic near Iceland (19 cases) are compared and contrasted through a composite study. Twice-daily gridded analyses from the European Centre for Medium-Range Weather Forecasts at 2.58 resolution from January 1985 to March 1996 are used. In the process of case selection, it is found that the frequency of rapid cyclogenesis in the Greenland‐Iceland region is higher than previously thought, and some of the events can be extremely violent. Many dynamically consistent differences are found when composite cyclones in the two sectors of the North Atlantic are compared. The upper-level forcing that triggers the development in the northeast Atlantic (NEA) is no less intense at the onset of rapid deepening. The NEA cyclones are also associated with lower static stability and locally concentrated but shallower thermal gradient, with less overall environmental baroclinicity. These factors lead to rapid depletion of available potential energy and result in a faster evolution and a shorter life cycle. Therefore, low-level thermal gradient and upper-level forcing components all weaken immediately after rapid deepening. The low-level incipient low in the NEA composite is also stronger, with a distinct potential vorticity (PV) anomaly visible at least 24 h prior to most rapid deepening, and the development produces a more pronounced warm core seclusion. Explosive cyclones in the northwest Atlantic, on the other hand, tend to have a higher stability and a greater amount of environmental baroclinicity, with temperature gradients in a broader area and deeper layers. These factors correspond to slower evolution and a longer life cycle. For cases in the NEA near Iceland, it appears that both upper-level forcing and initial system strengths affect the maximum deepening rate. The close proximity of this region to the high PV reservoir in the lower stratosphere is helpful in the generation of very strong forcing and a violent development under favorable synoptic conditions, when a ‘‘parent cyclone’’ with appreciable strength exists to the north/northeast of the incipient system.

/pdf/a-composite-study-of-explosive-cyclogenesis-in-different-3ibnv1ao9k.pdf

A composite study of explosive cyclogenesis in different sectors of the North atlantic. Part I: Cyclone structure and evolution

In this paper, the ability of the MM5 mesoscale forecast model to simulate the air‐sea interaction, boundary layer development, and mesoscale structure associated with a cold-air outbreak over the Labrador Sea is investigated. The case chosen was one for which research aircraft data and satellite imagery are available for validation. The default surface-layer parameterization included in the model is shown to grossly overestimate the magnitude of the air‐sea interaction resulting in forecasts of boundary layer growth and mesoscale development that differ substantially from observations. It is also shown that a representation of the inhomogeneities in sea-ice cover results in a significant improvement in simulations of the air‐sea interaction, boundary layer development, and mesoscale structure both within the marginal ice zone and downstream over the open ocean. Finally, the mesoscale cyclones or polar lows observed in the wake of the cold-air outbreak are shown to be coupled to the evolution of an upper-tropospheric potential vorticity anomaly that was advected over the region. The model simulations suggest that MM5, most probably due to inaccuracies in and the limited resolution of the analyzed fields that supply the initial and boundary conditions to the model, was unable to correctly simulate the development and track of this anomaly and this ultimately led to an incorrect forecast of the polar lows’ finite-amplitude behavior.

/pdf/a-numerical-study-of-an-extreme-cold-air-outbreak-over-the-3i2eckvxye.pdf

A Numerical Study of an Extreme Cold-Air Outbreak over the Labrador Sea: Sea Ice, Air–Sea Interaction, and Development of Polar Lows

The two-way relationship between Rossby wave-breaking (RWB) and intensification of extratropical cyclones is analysed over the Euro-Atlantic sector. In particular, the timing, intensity and location of cyclone development are related to RWB occurrences. For this purpose, two indices based on potential temperature are used to detect and classify anticyclonic and cyclonic RWB episodes from ERA-40 reanalysis data. Results show that explosive cyclogenesis over the North Atlantic (NA) is fostered by enhanced occurrence of RWB on days prior to the cyclone's maximum intensification. Under such conditions, the eddy-driven jet stream is accelerated over the NA, thus enhancing conditions for cyclogenesis. For explosive cyclogenesis over the eastern NA, enhanced cyclonic RWB over eastern Greenland and anticyclonic RWB over the subtropical NA are observed. Typically only one of these is present in any given case, with the RWB over eastern Greenland being more frequent than its southern counterpart. This leads to an intensification of the jet over the eastern NA and enhanced probability of windstorms reaching western Europe. Explosive cyclones evolving under simultaneous RWB on both sides of the jet feature a higher mean intensity and deepening rates than cyclones preceded by a single RWB event. Explosive developments over the western NA are typically linked to a single area of enhanced cyclonic RWB over western Greenland. Here, the eddy-driven jet is accelerated over the western NA. Enhanced occurrence of cyclonic RWB over southern Greenland and anticyclonic RWB over Europe is also observed after explosive cyclogenesis, potentially leading to the onset of Scandinavian blocking. However, only very intense developments have a considerable influence on the large-scale atmospheric flow. Non-explosive cyclones depict no sign of enhanced RWB over the whole NA area. We conclude that the links between RWB and cyclogenesis over the Euro-Atlantic sector are sensitive to the cyclone's maximum intensity, deepening rate and location.

Rossby wave-breaking analysis of explosive cyclones in the Euro-Atlantic sector

Explosive cyclones are intense extra-tropical low pressure systems featuring large deepening rates. In the Euro-Atlantic sector, they are a major source of life-threatening weather impacts due to their associated strong wind gusts, heavy precipitation and storm surges. The wintertime variability of the North Atlantic cyclonic activity is primarily modulated by the North Atlantic Oscillation (NAO). In this study, we investigate the interannual and multi-decadal variability of explosive North Atlantic cyclones using track density data from two reanalysis datasets (NCEP and ERA-40) and a control simulation of an atmosphere/ocean coupled General Circulation Model (GCM—ECHAM5/MPIOM1). The leading interannual and multi-decadal modes of variability of explosive cyclone track density are characterized by a strengthening/weakening pattern between Newfoundland and Iceland, which is mainly modulated by the NAO at both timescales. However, the NAO control of interannual cyclone variability is not stationary in time and abruptly fluctuates during periods of 20–25 years long both in NCEP and ECHAM5/MPIOM1. These transitions are accompanied by structural changes in the leading mode of explosive cyclone variability, and by decreased/enhanced baroclinicity over the sub-polar/sub-tropical North Atlantic. The influence of the ocean is apparently important for both the occurrence and persistence of such anomalous periods. In the GCM, the Atlantic Meridional Overturning Circulation appears to influence the large-scale baroclinicity and explosive cyclone development over the North Atlantic. These results permit a better understanding of explosive cyclogenesis variability at different climatic timescales and might help to improve predictions of these hazardous events.

/pdf/abrupt-transitions-in-the-nao-control-of-explosive-north-11kqwe3m4b.pdf

Abrupt transitions in the NAO control of explosive North Atlantic cyclone development

There is considerable evidence that many extra-tropical cyclones are generated by the inter-action between an upper-level potential vorticity anomaly and a low-level baroclinic zone. A variety of satellite data and the ECMWF high-resolution objectively-analyzed fields are used to investigate the origin of these tropopause features and their role in cyclogenesis for a remarkable rapidly developing cyclone that developed in the Labrador Sea on 1 and 2 March 1992. According to the ECMWF analysis, this storm deepened by 42 mb in 24 h, attaining a central pressure of less than 935 mb on 2 March The use of satellite column ozone data, from the TOMS and TOVS instruments is emphasized for the identification of mesoscale tropopause features implicated in cyclogenesis. Both data sets are able to resolve the intrusions of ozone-rich stratospheric air associated with this storm. DOI: 10.1034/j.1600-0870.1995.00124.x

Stratosphere‐troposphere interactions associated with a case of explosive cyclogenesis in the Labrador Sea

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Stratosphere‐troposphere interactions associated with a case of explosive cyclogenesis in the Labrador Sea