More Information on Left Exit Developments

The early stage of left-exit cyclogenesis is characterised by the formation of a baroclinic leaf poleward of a pre-existing polar-front cloud band.

Illustration of a 'left exit' low . The dotted area is the middle/upper-level cloud edge, the thin arrow is the 300 hPa jet-stream axis and the striped arrow is the 300 hPa jet-streak. (after Evans et al.1994)

During this stage, there may be a surface low underneath either, or both, of the cloud features. The longitudinal axis of the cloud band and the baroclinic leaf are aligned roughly parallel, and a zone of cloud-free air separates the two features. Frequently, during this stage, the baroclinic leaf will exhibit a warmer cloud-top temperature than will the polar-front cloud band. As development continues, the baroclinic leaf appears to rotate cyclonically as it merges with the polar-front cloud band.

During this process a dominant, surface-low center usually becomes established underneath the southwestern edge of the leaf, near its intersection with the cloud band. As the baroclinic leaf rotates, it begins to assume an increasingly "comma-head" shape. Following the merging of these features, the two clouds become progressively less distinguishable from infrared satellite imagery, as cloud-top temperatures associated with the baroclinic leaf decrease to values similar to those characterising the polar-front cloud band (Evans et al. 1994).

Left-exit cyclogenesis typically occurs underneath the left-exit region of a jet streak, in association with diffluent flow downstream of the axis of a large-amplitude, upper-level trough. The jet streak associated with this type of development is often located within a jet stream constituting the main zone of westerlies. Typically, an extensive cloud band is coincident with the polar front. The poleward edge of this cloud band is marked by the axis of the jet stream, and the baroclinic leaf forms poleward of the jet stream, underneath the left-exit region of the aforementioned jet streak. Rotation of the baroclinic leaf and the merging of the clouds occur as the jet streak and its associated short-wave disturbance move around the base of the upper-level trough, resulting in an increasingly negative meridional tilt of the trough axis. As rapid deepening begins, the dominant low-center becomes established under the southwestern edge of the baroclinic leaf.

Problems in analysing left exit developments.

According to the Norwegian polar-front theory, the fronts of a young deepening depression are located on the warm side of the polar jet and the surface low pressure center forms where the frontal wave develops (classical development). The occlusion process begins as the low pressure centre moves to the cold side of the polar jet. However, if the cyclogenesis takes place from the very beginning on the cold side of the jet stream, some problems with analyzing the case may arise. The analysing of left exit cyclogenesis varies slightly from case to case depending, for example, on whether a surface cyclone, or just a surface trough, has developed on the cold side. A proposal for analysing left exit cyclogenesis in different situations has been made as a part of a study for a Master's thesis in Finland (see below).

A proposal for analysing left exit cyclogenesis in different situations.

In all the figures on this page, black dashed lines are the 850 hPa isotherms and the green arrow represents the jet axis. Fronts are shown in the conventional way .

  A surface trough that may form under the left exit of the jet streak, seen in the satellite imagery as a comma cloud, can be analysed, as shown in Fig 1, if cold advection does not exist or is a minor feature. The use of a surface trough symbol is reasonable even though the comma cloud seems to merge with the frontal cloud band (on the warm side of the jet) so that the trough seems to be connected with the frontal wave as in Fig 2 (below left). As the development proceeds, a surface depression may form near the intersection point and this will quickly deepen. Later, when the surface cyclone moves some distance away from the jet streak (still on the cold side of jet), the warm sector begins to narrow. The temperature structure in the area of the surface trough is then similar to a classical occlusion, even though near the centre of the low the surface trough is still where the coldest airmass lies. A suitable and simple symbol for this feature though is an occlusion (Fig 3, below right).

  When the cold advection is strong under the left exit region at the beginning of development, or if such conditions develop later on in the surface trough seen in figure 1, a separate cold front symbol is more justified (see figure 4, left). In this case, when the surface trough associated with the comma cloud is connected to the frontal wave, the temperature structure of the deepening cyclone resembles the structure of a cold occlusion (figure 5, below), but continuing the use of cold front symbol instead of occlusion gives the correct picture of the situation: the deepening still continues. When the cyclone is maturing and no formation of new warm front on the cold side is observed, the structure resembles the situation depicted in figure 3 above.

  When the cyclogenesis under the left exit quadrant is strong enough, there is also often frontogenesis on the side of the warm advection. A structure as in figure 6 (left), often with two long-lasting warm sectors within each other, develops or a strong cold front with two warm frontal zones like W1 and W2 in figure 7 below. In the case of a particularly diffluent exit region branching to the south and north, both frontal waves can later disconnect from each other and occlude separately (figure 8). More often however, one of the warm fronts (W1 or W2) weaken leading to the situation depicted in figure 3: either fast (W2 weakens and dissappears) or slow (W1 disappears and later W2 occludes). Figure 2c of Evans et al. 1994 represents the latter, as the jet stream crosses the warm sector.