The cyclone tracking algorithm outlined in Sect. 2.2 on the MERRA data provides a set of 53 cyclones. Originating from latitudes equatorward of 37.5\(^\circ\)N, these cyclones encounter different physical circumstances when crossing the North Atlantic and undergo various kinds of structure evolution before reaching Europe. Using the phase-space analysis of Hart (2003), four physically distinct life cycle classes have been identified. These life cycle classes describe the different possible transitions a cyclone may undergo when entering the midlatitudes. In the classification we have been guided by the known structure evolutions that have been described in the literature (Hart 2003; Hart and Evans 2001; Jones et al. 2003; Maue 2010; Hart et al. 2006). The four classes are described below and summarized in Table 1. Do note that, due to the limited set of cyclones, some of the differences among the stated variables are statistically not significant.
Table 1 Characteristics of the four different life cycle classes, showing number of storms, average pressure minimum and average wind maximum (shown with standard deviations). The last column shows the latitude where they enter Europe (i.e. entering the rectangle with lower left corner 15\(^\circ\)W by 37.5\(^\circ\)N) and subsequent standard deviation. The warm seclusion life cycle has been subdivided into two classes (see Sect. 4.2)
Symmetric warm-core development: tropical cyclone life cycle
Cyclones with tropical characteristics can be characterized by a thermally symmetric or non-frontal structure (B <10 m) and a warm core (\(\mathrm {T}_L\) >0, \(\mathrm {T}_U\) >0). Cyclones of the tropical life cycle start with these characteristics. The lower-troposphere warm core increases upwards due to sustained convection (Hart 2003). Combined with subsidence within the eye, a warm core is established, visible at the start of Fig. 1a by a high \(\mathrm {T}_L\) and a slightly positive \(\mathrm {T}_U\) . They generally lose this warm-core structure when they encounter colder SSTs on their way to Europe. The fact that these cyclones do not develop thermal asymmetry (B remains <10 m, Fig. 2a) can be explained by the fact that they are not subject to strong vertical shear, trough interaction or baroclinicity. In the early phase of this life cycle, the cyclone’s spatial extent in wind field is usually small (200–400 km), and steadily grows in time. The pressure minimum is in some cases found early in the life cycle during tropical intensification, but in other cases it is found near the point where the cyclone, after attaining a deep cold-core structure that has been developed over cold SSTs, re-establishes a shallow warm core. The cold core development in this life cycle (Fig. 2a) is different from other literature (e.g. Hart 2003). We speculate that this caused by the fact that we only investigate cyclones with this life cycle that cross the North Atlantic and reach Europe, whereas in previous literature this category contained mostly cyclones confined to the tropics.
We find that about 15\(\%\) of the observed cyclones follow this life cycle. These cyclones tend to enter Europe in France and Great Britain and never attain latitudes higher than 60\(^\circ\)N. The average wind speed maximum and average pressure minimum of this class are 21.9 ± 1.7 m s\(^{-1}\) and 977 ± 9 hPa, respectively, which makes this the weakest class of low-latitude originating storms entering Europe (see Table 1).
Asymmetric cold-core development: extratropical cyclone life cycle
The cyclones pertaining to the extra-tropical life cycle originate with a cold-core structure (\(\mathrm {T}_L\) < 0, \(\mathrm {T}_U\) < 0) (Fig. 1b) and retain it throughout their development in combination with a strong thermal asymmetry (B > 10) (Fig 2b). Although coming from below 37.5 N, these are the characteristics of an extratropical cyclone. Often visible in these cyclone life cycles is an increase of the cold-core structure in an early phase, which is reflected by \(\mathrm {T}_L\) becoming increasingly negative. The middle- and upper-tropospheric height gradients above the surface cyclone then intensify (isobaric heights decrease) more rapidly than near the surface, leading to an increasing cold-core cyclone signature. With B increasing, thermally direct circulation dictates that cold air is advected in the rear of the cyclone and warm air is advected further north, sustaining the strongly negative \(\mathrm {T}_L.\) This is often followed by an increase of \(\mathrm {T}_L\) as can be seen in Figs. 1b and 2b, where also an increase in pressure is clearly visible. For detailed analyses of this life cycle, see Hart (2003). Occlusion (following the Norwegian cyclone model) of the cyclone fronts can be recognized by re-establishing thermal symmetry in the latter phase of the cyclone life cycle. If the cyclone does not interact with another trough or is not subject to increased surface fluxes, further intensification stops and the cyclone decays. The radius of the system (mean gale force wind radius) is relatively large throughout the extratropical cyclone life cycle (Fig. 1b).
About 13\(\%\) of analyzed cyclones follow an extratropical life cycle. These cyclones tend to penetrate further north than cyclones with a tropical life cycle, reaching Europe at the northernmost point of Great Britain or even near Iceland. The average wind speed maximum and average pressure minimum of this class are 21.5 ± 2.3 m s\(^{-1}\) and 970 ± 13 hPa, respectively, which makes these storms of moderate strength among the low-latitude originating storms entering Europe (see Table 1).
Extratropical transition: classic ETT cyclone life cycle
The first two classes mentioned above (tropical and extratropical cyclone life cycles) are called conventional or single phase life cycles as they do not undergo major phase transitions in thermal symmetry and thermal wind (Hart 2003). The remaining two classes do undergo major phase transitions and differ in the experienced forcing mechanisms throughout their life cycle.
One of these major phase transitions is extratropical transition (ETT), changing the warm-core symmetric structure of a tropical cyclone into the cold-core asymmetric structure of an extratropical cyclone (Jones et al. 2003). The tropical origin is visible in Fig. 2c, with near-zero or even negative values of B and positive values of \(\mathrm {T}_L.\) This is called the tropical stage. One would expect to find positive values of \(\mathrm {T}_U\), too, which is on average not the case (Fig. 1c). This depends on the maturity of the tropical cyclone. Some do start with a positive \(\mathrm {T}_U\), while others do not as can be seen from the individual life cycles in Fig. 1c. In this tropical stage, the cyclone radius is relatively small but slowly increasing, similar to the early phase of the tropical (conventional) life cycle. We adopt the criterion Hart (2003) proposed for the onset time of the ETT, when B exceeds 10 m, which corresponds to the cyclone entering a baroclinic environment (Hart 2003; Klein et al. 2000).
After the ETT, in the second transformation or hybrid stage, the cyclone interacts with its new environment, which usually consists of merging with an extratropical system or upper-level trough. During this stage, the tropical cyclone generally develops an increased translation speed. In the early stages of ETT, the cyclone tends to weaken first (Hart and Evans 2001), which could be attributed to the interaction between the cyclone and an upper-level trough, as this is associated with high vertical wind shear. The decrease in intensity of the cyclone also depends on the inner-core convection evolution by the environmental changes (Jones et al. 2003). Important changes after an ETT are the loss of organized convection in the inner core, the increase in translation speed, the loss of upper-level outflow circulation, increased frontogenesis, cyclone vertical tilt, asymmetry in the precipitation, moisture and temperature fields and the expansion of the gale force winds area (Hart and Evans 2001). Throughout the ETT, some cyclones directly attain a lower cold core and a thermally asymmetric structure (B >10 m; Fig 2c). However, ETT as it is described in Hart (2003), implies first a transition to the upper-right quadrant in Fig. 2c. The individual cyclone life cycles depicted in grey reveal that indeed many of them undergo this transition. This quadrant refers to the so-called hybrid stage, where a lower warm core (\(\mathrm {T}_L\) > 0) pertains in a baroclinic environment (B >10 m). The latent heat release possible in the warm-core structure of the cyclone during this phase, may contribute to a deepening. The end time of the hybrid stage (\(\mathrm {T}_L\) becomes negative) is sometimes referred to as the end of the ETT (Hart and Evans 2001). The duration of the hybrid stage differs among cyclones. The extratropical stage contains the decay and sometimes a reintensification of the cyclone. Both weak and strong cyclones can reintensify, but weak cyclones need to have a smaller duration of the hybrid stage for them to survive longer. The pressure evolution is therefore determined by weakening in an early phase of the ETT, the intensification during the hybrid stage (of cyclones that have one), and possible reintensification after the ETT (see Fig. 2c). Cyclones of this life cycle fade as cold-core, thermally asymmetric cyclones.
About 19\(\%\) of the observed cyclones follow this life cycle. The tracks of these cyclones start relatively deep in the tropics. They usually arrive in Europe around Great Britain. The average wind speed maximum and average pressure minimum of cyclones of this class are 22.5 ± 1.7 m s\(^{-1}\) and 972 ± 7 hPa, respectively, which is relatively weak (Table 1).
Transition to warm seclusion: warm seclusion life cycle
Finally, more than 50\(\%\) of the storms undergo another major transition and become what is known as a warm seclusion storm. A warm seclusion occurs, when during the bending of the cold and warm front, part of the cold front gets separated from the cyclone center and propagates eastwards, perpendicular to the warm front (Shapiro and Keyser 1990). This process is called a frontal T-bone fracture. This is associated with trapping of warm air (the so-called bent-back warm front) in the core of the cyclone. A large part of the analyzed cyclones that attain a warm seclusion structure start with a warm \(\mathrm {T}_L\) and a symmetric structure as seen in Figs. 1d and 2d. In their path towards Europe they lose their warm core structure and become asymmetric. During this transition they weaken. What distinguishes this life cycle from the classic ETT life cycle is that after this transition, due to the seclusion, a warm core is re-established again. This is caused by the release of latent heat and advection within the warm conveyor belt. The warm seclusion is often accompanied by a rapid intensification because both baroclinic processes and latent heat release contribute to the development of the storm. The average deepening is about 30 hPa in 2 days. The minimum pressure is attained when the warm seclusion is completed. Thereafter the cyclone tilt disappears and they lose their warm core before they fade away. This life cycle is in agreement with the analysis of Maue (2010). It is interesting to note that sting jets are mainly found in this type of cyclones (Browning, 2004). This structure development is dominant for the storms that reach Europe. Moreover, the warm seclusion life cycle class contains the strongest storms that originate in the tropics. Eight out of the ten strongest storms are all warm seclusion storms. Also, on average they are the strongest storms with an average wind speed maximum of 22.5 ± 2.4 m s\(^{-1}\) and average pressure minimum of 963 ± 14 hPa.
Summary
We have divided the cyclones that reach Europe and originate from lower latitudes in four different classes based on the characteristic pathways in the Hart diagrams. Each of these classes describes a physically different life cycle. The life cycles of the individual cyclones shown in Figs. 1 and 2, support this classification. The four possible life cycles are schematically represented in Fig. 3, and correspond to the physical processes and transitions that cyclones may experience when they enter the midlatitudes with colder SSTs. This schematic picture aligns with previous studies (Hart 2003; Hart et al. 2006).
The minority of the cyclones experience small structure transitions (tropical and extratropical life cycle, 28\(\%\) in total), but the majority (72\(\%\)) undergoes major transitions in cyclone structure like extratropical transition and warm seclusion. Intensification of these cyclones mainly appears in the form of a reintensification at midlatitudes while warm seclusion intensification generates the lowest pressure values.