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Cloud Structure In Satellite Images

Appearance in METEOSAT Second Generation imagery

IR10.8 imagery:

  • Two main cloud configurations are seen during the initial stages of a Rapid Cyclogenesis: a frontal cloud band usually oriented east to west and clouds with warmer tops that form an increasingly dense shield, a so-called cloud head, on the poleward side of the frontal cloud band and protruding from below it. The developing cloud head varies between grey and light grey in the IR channels, mostly with higher tops on the poleward side.
  • Dry sinking air originating from the lower levels of the stratosphere on the cyclonic side of a jet stream is advected downstream. This leads to a dark, cloud-free area between the cloud band of the cold front and the cloud head, thereby creating a V-pattern (often also called a dry slot).
  • During the advanced stages the cloud head grows to a cyclonically curved cloud spiral with a broad dark area between the spiral and the frontal cloud band; additionally, the rear part of the cold frontal cloudiness frequently dissolves as a consequence of sinking dry air.
  • Further development leading to the mature stage of the cyclone often includes a cloud spiral around the low center.

WV6.2 imagery:

  • WV images have an important role in identifying the possibility of rapid cyclogenesis initiation.
  • In the initial stage of the development a dark grey stripe can be seen along the northward edge of the white frontal cloud band. This stripe represents dry air sinking from the stratosphere along the cyclonic side of a jet stream.
  • Before the development starts, the dark grey stripe is more distinct upstream of the cloud head.
  • In the next stage the dark grey stripe approaches the cloud head while becoming darker and darker (indicating dry air moving downwards in the upper troposphere) and finally forming a typical V-pattern together with the cloud head.
  • During the advanced stages the cloud head grows into a cyclonically curved cloud spiral with a broad black area between the spiral and the frontal cloud band.
  • A further dark stripe often develops along the north-northeastern boundary of the cloud head in the cold air within the cold airmass, indicating sinking movement in connection with a second jet streak which often develops there.

VIS0.6 imagery:

  • During their initial stages both the west-to-east oriented frontal cloud band as well as the cloud head appear white in the VIS image, indicating fairly thick cloudiness, but with an irregular structure and sometimes fibrous edges on the poleward edge of the frontal cloud band.
  • As the top of the frontal cloud band is higher than that of the cloud head, the frontal clouds can cast a distinct shadow on the cloud head, which is visible in VIS images.

Airmass RGB imagery:

  • The Airmass-RGB (see Basics: Artificial and Combination Channels) is appropriate for differentiating between cold, warm and dry air masses.
  • The dry intrusion and jet streak axis take the distinctive forms of a reddish stripe along the frontal cloudiness and a reddish hole between the cloud head and frontal clouds. This makes RGB images a good tool for the identification of Rapid Cyclogenesis developments.

Dust RGB imagery:

  • The Dust RGB (see Basics: Artificial and Combination Channels) is appropriate for differentiating between clouds of different thickness, especially for identifying high cirrus cloud.
  • Its abilities help to identify the different elements of frontal and cloud head elements, as well as the cirrus fibers connected to the jet axis.

The following schematics show the typical features in Airmass RGB images at three important development stages of a Rapid Cyclogenesis:

Fig. 1: Initial stage Fig. 2: Development stage
Fig. 3: Advanced stage

The typical features of a Rapid Cyclogenesis seen in satellite images and image loops are:

  1. A (south)west to (north)east oriented cold front-warm front cloud band together with a cloud head on the poleward side
  2. An approaching dark grey stripe in the WV image. As soon as the leading edge of this dark grey strip becomes darker and wider close to the area between the cloud head and frontal cloud band, the development described in this chapter will take place in just a few hours.

The example used throughout this conceptual model took place over the eastern Atlantic in 7-9 January 2015 and displays structures and developments typical of rapid cyclogenesis.

The next images show four illustrative channels (IR 10.8 μm, WV 6.2 μm, Airmass RGB, Dust RGB) for the three stages of development in the schematic above: Initial stage, Development stage and Advanced stage.

Initial stage:

Main characteristics: Frontal cloud band, cloud head and dry tongue

Fig. 4a: 8 January 2015, 06 UTC; Meteosat 10, IR 10.8; Initial stage: frontal cloud band, cloud head. Fig. 4b: 8 January 2015, 06 UTC, Meteosat 10, WV 6.3; Initial stage: frontal cloud band, cloud head, dark grey stripes of dry air - V-pattern
Fig. 4c: 8 January 2015, 06 UTC; Meteosat 10, Airmass RGB; Intitial stage: frontal cloud band, cloud head, brownish stripes of dry air. Fig. 4d: 8 January 2015, 06 UTC, Meteosat 10, Dust RGB; Intitial stage: frontal cloud band, cloud head, black cirrus fibers.

All channels show the west-to-east oriented frontal cloud band as well as the separate cloud head area. The WV and Airmass images indicate with dark grey and brownish stripes the dry tongue at the rear side of the frontal cloud band, which has protruded further in the direction of the cloud head area. The IR and Dust images indicate the lower cloud tops with darker shades of grey and lighter shades of brown compared to the frontal cloud band.

Development stage:

Frontal cloud band, intensified dry intrusion, spiral structure of cloud head

Fig. 5a: 8 January 2015, 12 UTC; Meteosat 10, IR 10.8; Development stage: frontal cloud band, cloud head, intensified dry intrusion. Fig. 5b: 8 January 2015, 12 UTC, Meteosat 10, WV 6.3; Development stage: frontal cloud band, cloud head, intensified dry intrusion.
Fig. 5c: 8 January 2015, 12 UTC; Meteosat 10, Airmass RGB; Development stage: frontal cloud band, cloud head, brownish stripes of dry air, beginning dissolution of cold front cloudiness in dark blue. Fig. 5d: 8 January 2015, 12 UTC, Meteosat 10, Dust RGB; Development stage: frontal cloud band, cloud head, black cirrus fibers.

All channels show the west-to-east oriented frontal cloud band as well as a beginning spiral structure in the cloud head area accompanied by an intensified dark grey (WV) and brown (Airmass) area between the cloud head and cloud band. As a consequence of cloud evaporation, frontal cloudiness in this area has started to become thinner with a high cirrus stripe on the rearward side (blue in Airmass and black in Dust RGB).

The following image shows the VIS 0.6 μm and high resolution visible channels for 8 January 2015, 12 UTC:

Fig. 6a: 8 January 2015, 12 UTC; Meteosat 10, Vis 0.6. Development stage: Frontal cloud, cloud head, shadow of higher frontal cloud on the lower cloud head; low cold air cloudiness. Fig. 6b: 8 January 2015, 12 UTC; Meteosat 10, HRVis. Blue: high cloud; yellow: low cloud. Development stage: frontal cloud, cloud head, shadow of higher frontal cloud on the lower cloud head; low cold air cloudiness.

Convection in winter cases like this one does not reach as high as in summertime Rapid Cyclogenesis events.

The shadows of high and thick frontal clouds over the cloud head show as black and dark blue stripes in VIS images.

Advanced stage:

Fig. 7a: 8 January 2015, 18 UTC; Meteosat 10, IR 10.8; Advanced stage: frontal cloud band, dissolution of cold front clouds, cloud head changes to spiral form. Fig. 7b: 8 January 2015, 18 UTC, Meteosat 10, WV 6.3; Advanced stage: frontal cloud band, cloud head changes to spiral form, dry area in the center grows.
Fig. 7c: 8 January 2015, 18 UTC; Meteosat 10, Airmass RGB. Advanced stage: frontal cloud band, cloud head spiral, brownish stripes of dry air,dissolution of cold front cloudiness. Fig. 7d: 8 January 2015, 18 UTC; Meteosat 10, Dust RGB. Advanced stage: frontal cloud band, cloud head spiral, black cirrus fibres.

The most indicative structures for this advanced development stage of a Rapid Cyclogenesis are the more distinct cloud spiral with a large dry area in the spiral center, and the large cloud-free (dark) areas within the cold front cloud band. Both features are very well developed in this case. Convective cells develop in the center of the spiral; they show as white spots in the IR channel and Airmass RGBs and as brown spots in Dust RGBs.

The convectivity is often stronger in the warmer seasons.

Cloud dissolution shows in IR images and Airmass RGBs as a darkening of the clouds. In Dust RGBs it is signaled by bluish areas and black cirrus stripes.

IR channels as well as Airmass RGB images from 9 January at 00 and 06 UTC show the development from advanced to mature stage in the form of a cloud spiral that can circle the cyclone center several times.

Fig. 8a: 9 January 2015, 00 UTC; Meteosat 10, IR 10.8; Mature cloud spiral. Fig. 8b: 9 January 2015, 06 UTC; Meteosat 10, IR 10.8; Mature cloud spiral.
Fig. 8c: 9 January 2015, 00 UTC; Meteosat 10, Airmass RG; Mature cloud spiral. Fig. 8d: 9 January 2015, 06 UTC; Meteosat 10, Airmass RGB; Mature cloud spiral.

In the advanced and mature stages, when the cloud spiral is well developed and circling the cyclone's center, parallel cloud lines can appear close to the innermost part of the cloud spiral. These cloud bands are parallel rain bands. Although rain bands are by their nature more visible on radar than in satellite images, they are mentioned here because they can appear together with a very dangerous phenomenon, a sting jet. The sting jet will be described in more detail in the chapter on weather phenomena.

Fig. 9: 9 January 2015, 00 UTC - Meteosat 10, Airmass RGB. The black line indicates an area of banded cloud.

The following loops show a very rapid development of the depression over 12 hours from 990 hPa on 8 January 2015, 06 UTC to the extreme low value of 965 hPa on 8 Januar 2015, 18 UTC.

Fig. 10: 9 7 January 2015, 18 UTC - 9 January 2015, 06 UTC, 15 minute image loop; IR 10.8.

Appearance in AVHRR imagery

As usual, the advantage of AVHRR images is the high resolution which is most useful for observing the development of convection, especially in high latitudes.

Meteorological And Physical Background

A Rapid Cyclogenesis is accompanied by distinct and typical cloud configurations in satellite images. Other names have been used in the literature to describe the same conceptual model, such as "explosive cyclogenesis" or "bomb" as well as names which describe only parts of the whole conceptual model, such as "emerging cloud head", "cyclogenesis within the left exit region of a jet streak" or "comma cloud".

1. Polar front theory and Rapid Cyclogenesis

The biggest differences between the the developments of classical polar fronts and Rapid Cyclogenesis are in the first phase. Here that is taken to be the stage between the first sign of a V-pattern (dry slot) and the development of a cloud spiral.

While a classical cyclogenesis through wave development is often slow and the wave bulge dissolves after some time or produces a spiral only after some days, the development of a Rapid Cyclogenesis is faster. The dry slot (V-pattern) typically evolves into a cloud spiral within 12 hours. After the mature stage is reached there is no longer any notable difference between the two processes.

The fast development of Rapid Cyclogenesis in its initial stage cannot be explained within the framework of the classical polar front theory developed by Bjerknes and Solberg (see Meteorological Physical Background of "Occlusion warm conveyor belt" and "Occlusion cold conveyor belt"). Consequently we must consider other processes and conditions that lead to rapid and intensive development.

From the features of WV and Airmass RGB images it is evident that sinking dry air has a significant impact on the cloud configuration.

2. Conveyor Belt Model

Features like a protruding cloud head suggest the theory of relative streams (see Basics, relative streams) as a likely method of investigation for a deeper understanding of cloud configurations.

As shown in the conceptual models "Occlusion: Cold Conveyor Belt Type" and "Occlusion: Warm Conveyor Belt Type" there are several conveyor belts involved in the development of a cyclogenesis, resulting in different cloud systems. This can be seen in the schematics below. The cloudiness of the main frontal zone is produced by a typical warm conveyor belt together with a humid relative stream from behind, the "upper relative stream" while the dry stream from behind, the dry intrusion, appears behind the poleward edge of the frontal cloud band. This stream often contains stratospheric air. As the schematics of the initial stages show, the warm conveyor belt and dry intrusion are, according to the polar front cyclogenesis model, involved in the developments of warm conveyor belt types, cold conveyor belt types, and also rapid cyclogenesis types. But there are also differences; the cloud head of a Rapid Cyclogenesis is formed within the lower and mid-levels of the troposphere by a rapidly ascending relative stream (conveyor belt) from the south, which advects warm and moist air from lower latitudes beneath the warm conveyor belt of the frontal zone. This relative stream of a Rapid Cyclogenesis has several similarities with the cold conveyor belt in the polar front theory of cyclogenesis, which also has an ascending relative stream below the warm conveyor belt, which forms the cloudiness of the occlusion spiral. But such a cold conveyor belt usually originates from the east and is usually less warm and humid.

After crossing the cold front zone the ascending conveyor belt often splits into two branches that flow east and west. This splitting of the relative flow leads to the convex-shaped cloud edge at the poleward side of the cloud head.

As the advected air of the dry intrusion originates from the upper troposphere and the lower stratosphere, this flow is characterized by high values of potential vorticity. A potentially unstable stratification of the troposphere develops in the area where the dry intrusion flow over the ascending conveyor belt from the south. This is one of the factors that contribute to the development of convective storms in the same area (see the chapters 'Weather events' and 'Cloud structure in satellite images'). This is indicated in the schematic of the Advanced Stage below.

Fig. 11a: Development stages

Fig. 11b: Advanced stage

3. Jet, Jet streaks and 4-Quadrant Model by Uccellini

As mentioned above, the speed of a rapid cyclogenesis's development cannot be explained with the polar front theory alone; a different process is needed for a more detailed explanation. Rapid developments take place in the left exit region of jets and jet streaks. The structure of dark stripes along the rear edge of the frontal cloud band, which is best seen in the WV image WV images and Airmass RGB RGBs, also indicate that jets play an important role in the process of a Rapid Cyclogenesis.

Therefore, in addition to the development models of polar fronts and conveyor belts, upper air phenomena like the 4-quadrant jet streak model from Uccellini are taken into consideration and - as will be shown - play key roles.

Jet streaks can have straight or curved axes; during the initial stages of a Rapid Cyclogenesis the axes are mostly straight.

Fig. 12: Jet streak model

Jet streaks are the areas of maximum wind speed within the jet streams. The most interesting regions from the viewpoint of cyclonic developments are the entrance region where air particles accelerate, and the exit region where the air particles decelerate. As a consequence of these accelerations ageostrophic wind components develop in these regions, which leads to vertical movements.

Fig. 13: Vertical cross section of a jet

Both entrance and exit regions are favorable for frontogenesis, although in different ways.

In the entrance region the frontogenetic process occurs mostly in the lower troposphere, which causes the process to last a longer time, typically more than 24 hours.

In the exit region the frontogenetic area lies in the upper troposphere, where the PVA maximum connects to the left exit region. These circumstances favor a rapid (under 12 hours) cyclogenesis.

The jet stream axis lies parallel to the frontal cloud band with dry stratospheric air approaching the cloud head. This is the black stripe typically seen in WV imagery. Also, the PV values there are higher than 1 PVU. The emerging cloud head in the initial and development stage of the Rapid Cyclogenesis usually appears in the left exit region of a jet streak. This can be seen in the following WV 6.3 μm image from 8 January 2015 at 06 UTC with isotachs at 300 hPa, height of PVU > 2 and the PVA maximum at 300 hPa superimposed.

Fig. 14a: 8 January 2015, 06:00 UTC - Meteosat 10, WV 6.2 image. Yellow: isotachs 300 hPa. Fig. 14b: 8 January 2015, 06:00 UTC - Meteosat 10, Magenta: height of PV >= 1 PVU.
Fig. 14c: 8 January 2015, 06:00 UTC - Meteosat 10, IR 10.8 image. Yellow: isotachs 300 hPa; red: positive vorticity advection 300 hPa.

While there is ascending motion below the left exit region of the jet streak, descent takes place below the right exit region.

Quite often, and especially in cases with Rapid Cyclogenesis, this sinking motion leads to the dissolution of the cold front cloudiness. This can be seen clearly in the following image from 8 January 2015. The green star indicates the right exit region of the jet streak, where the cold front cloudiness has dissolved.

Fig. 15: 8 January 2015, 18 UTC - Meteosat 10; IR 10.8 image; yellow: isotachs 300 hPa; red: positive vorticity advection 300 hPa.

In summary, the part of the cloud head where the rapid development happens is an area of upward motion and cyclogenesis in the left exit region of a jet streak, and at same time an area where stratospheric air has protruded far downward, a so-called PV anomaly. This last fact in particular shows a difference between a classical polar frontal cyclogenesis and a Rapid Cyclogenesis; while in the case of classical wave development stratospheric air (if present at all) reaches the layer between 300 and 500 hPa, in the case of Rapid Cyclogenesis stratospheric air is a key feature and protrudes much further downward, with 500 hPa reached in the beginning of the process. This development process in the left exit region goes hand in hand with the dissolution of cold front cloudiness in the right entrance region.

4. Potential Vorticity

As demonstrated before, the development conditions in the left exit region of a jet streak can explain the rapid developments. On the other hand, there are also cases - especially in the cold conveyor belt (CCB) occlusion conceptual model - where the occlusion cloud band lies in the left exit region and thus under its physical influence. Therefore, there must still be yet another process to explain why some polar front developments are rapid and some are not.

The development of Rapid Cyclogenesis can be explained by the theory described by Hoskins et al. involving potential vorticity anomalies and their interaction with low-level baroclinic zones. (Compare: Basics/Numerical Parameters for Synoptic- to Mesoscale Cloud Systems /Potential Vorticity)

Fig. 16: Potential vorticity equation

When stratospheric air protrudes downward into the troposphere an upper level PV anomaly develops. As a consequence of the PV equation, positive vorticity will become more cyclonic because of the influence of the less stable environment of the troposphere.

When a positive upper level PV anomaly is being advected over a zone of strong, low-level temperature gradient a rotation is induced in the lower levels of the baroclinic zone, which causes warm advection at these levels and intensifies the cyclonic vortex there.

What takes place is a mutual intensification of cyclonic rotation from high to low and from low to high levels. The downward protrusion of high PV values can be seen in vertical cross sections. This does not mean that stratospheric air reaches the surface layers, but that the protrusion intensifies rotation in the lower layers.

In the schematic below the thick solid arrow around the PV maximum indicates the cyclonic rotation. This rotation is induced at lower levels of the baroclinic zone as shown by the thin solid circulation arrow. This low level circulation causes warm advection ahead leading to a low level positive temperature anomaly indicated by the open + sign in the right figure (b). This temperature anomaly is associated with a cyclonic vortex which is marked by the open arrow at low levels. In turn, this circulation has a positive feedback to the upper troposphere, shown by an open circulation arrow at higher levels.

Fig. 17a: Positive PV anomaly. Fig. 17b: Positive PV anomaly causing positive temperature advection.

At the same time another process is taking place: The induced low level vortex results in a strong equatorward wind component under the upper level PV anomaly. This southward component also influences the higher levels and leads to an equatorward advection of the upper level PV-anomaly which in turn intensifies the upper level wave.

Within this increased flow higher PV values to the west of the PV-anomaly are advected southward and lower PV values to the east of the PV-anomaly are advected northward. As a consequence of the latter process, the eastward movement of the PV-anomaly is decreased. Hence, the interaction between low and upper level circulations and the already ongoing cyclogenesis process will strengthen.

The next three images show the development of a Rapid Cyclogenesis. A PV anomaly (red arrow) protrudes downward and approaches the location of the surface low center (blue arrow) and simultaneously the cloud head turns into a cloud spiral.

At 06 UTC an intensive PV anomaly is located behind the cloud head, parallel to the frontal cloud band. The (dynamical) tropopause, here indicated with the height of 1 PVU, has lowered to a minimum level of 650 hPa. Six hours later a V pattern has developed. At 18 UTC the PV anomaly has reached the location of the surface low and lowered to 950 hPa.

Fig. 18a: 8 January 2015, 06:00 UTC - Meteosat 10; WV 6.2 image. Magenta: height of PV>2 PVU; red arrow: PV>2PVU height maximum; blue arrow: low at 1000 hPa. Fig. 18b: 8 January 2015, 12:00 UTC - Meteosat 10; WV 6.2 image. Magenta: height of PV>2 PVU; red arrow: PV>2PVU height maximum; blue arrow: low at 1000 hPa.
Fig. 18c: 8 January 2015, 18:00 UTC - Meteosat 10; WV 6.2 image. Magenta: height of PV>2 PVU; red arrow: PV>2PVU height maximum; blue arrow: low at 1000 hPa.

In the following vertical cross sections the downward protrusion of the PV anomaly is seen even more clearly: the minimum of 950 hPa was already reached by 12 UTC.

Fig. 19a: 8 January 2015, 06:00 UTC - vertical cross section. Black: isentrops; red: potential vorticity (PV); heavy > 2 PVU (stratospheric values); light: < 2 PV units (tropopsheric values). Fig. 19b: 8 January 2015, 12:00 UTC - vertical cross section. Black: isentrops; red: potential vorticity (PV); heavy > 2 PVU (stratospheric values); light: < 2 PV units (tropopsheric values).
Fig. 19c: 8 January 2015, 18:00 UTC - vertical cross section. Black: isentrops; red: potential vorticity (PV); heavy > 2 PVU (stratospheric values); light: < 2 PV units (tropopsheric values)

Another sign of the interaction between high and low layers is the movement and intensification of the surface low. In the beginning it is rather weak and situated on the anticyclonic side of the jet streak. During the development it moves to the cyclonic side and intensifies strongly when it encounters the left exit region. In the case of 8 January 2015 the pressure at 1000 hPa drops by 8 to 28 hPa within 12 hours.

Fig. 20a: 8 January 2015, 06 UTC - Meteosat 10, IR 10.8 image. Magenta: height contours 1000 hPa; black/blue: shear vorticity 300 hPa; black: jet axis. Fig. 20b: 8 January 2015, 12 UTC - Meteosat 10, IR 10.8 image. Magenta: height contours 1000 hPa; black/blue: shear vorticity 300 hPa; black: jet axis.
Fig. 20c: IR 10.8 image. Magenta: height contours 1000 hPa; black/blue: shear vorticity 300 hPa black: jet axis

5. Under discussion: Another Rapid Cyclogenesis development model

Another theory of cyclogenesis that also deviates from classical polar front theory has been discussed in the literature more and more often in the last decades. It is the Shapiro-Keyser cyclone model and it is mentioned here because some authors suggest that a Rapid Cyclogenesis is the result of this cyclogenetic process.

Although most of the presented case studies do not contain and take into account satellite images and the typical cloud configurations, the theory will be briefly summarized here.

According to the Shapiro-Keyser model, cyclogenesis deviates from Bjerknes and Solberg's polar front theory from a phase immediately after the wave development to the end stage of the spiral structure. The cold front is weak and soon dissipates from the warm front. In contrast, the warm front is very strong and starts to rotate around the low center, creating a warm seclusion. This development often occurs in west-east oriented fronts with air masses from warmer latitudes, and in the area of confluent flows in jet streak entrance regions.

Fig. 21: Shapiro-Keyser cyclogenesis model

The polar front theory of Bjerknes and Solberg is an occlusion process in which a cold front catches up to a warm front and lifts warm air upward. The Shapiro-Keyser model is based on the idea of wrapping the different air masses around the center, resulting in the warm seclusion.

There are some similarities between Rapid Cyclogenesis events and the Shapiro-Keyser model, such as the west-east orientation of the frontal system, the very intensive warm advection in the warm front and cloud head cloudiness (especially in the beginning of the process), the dissolution of cold front cloudiness in a more advanced stage, and, in nearly all cases, a recognizable cloud spiral around the low center in the final stage.

However, Rapid Cyclogenesis also deviates from the Shapiro-Keyser model in important ways. In the case of Rapid Cyclogenesis, the main development takes place in the left exit region of a jet streak, which is to say: in the diffluence and not the confluence area of a jet streak. Also, if one looks into the development of cloud configurations in satellite images, frontal (especially warm front) cloudiness and cloud heads are clearly different configurations which are best explained with conveyor belts.

Further investigation is recommended to clarify the open questions.

Key Parameters

  • Height contours at 1000 hPa and 500 hPa:
  • The height contours at 1000 hPa start with a weak low or trough but undergo rapid deepening during the initial stages of development. The upper level height contours at 500 or 300 hPa show only a very weak trough in the beginning to the west of the surface low. The trough deepens during the process and finally catches up with the position of the surface low. The height contours at 500 (or 300) hPa are perpendicular to the height contours at 1000 hPa in the initial phases, indicating the cyclogenesis is in the developing stage.

    The mature stage is characterized by a low center with very low values at 1000 hPa as well as at 500/300 hPa. At this point the trough axis is vertical and situated in the center of the cloud spiral.

  • Temperature advection (TA):
  • The field of temperature advection shows a very typical situation for a wave: a maximum of warm advection (WA) within the cloud head and the warm front (WF) shield and cold advection (CA) behind. This pronounced WA/CA dipole is a sign of the ongoing cyclogenesis. However, one characteristic of Rapid Cyclogenesis events is that compared to more classical developments the WA is very strong, with values sometimes even higher than those of the CA.

  • Isotachs at 300 hPa:
  • The isotachs at 300 hPa show a pronounced jet streak along the rear edge of the frontal cloud band. The cloud head can be found within the left exit region of this jet streak. In the mature stage a second jet streak often forms at the east-northeastern (polar) side of the cloud head.

  • Vorticity advection at 500 and 300 hPa (positive vorticity advection or PVA, and negative vorticity advection or NVA):
  • The field of vorticity advection shows a pronounced PVA maximum at 500 and 300 hPa, situated mostly within the area of the cloud head and therefore also in the area of the left exit of the main jet streak. If the second jet streak, downstream from the low, has already developed, the maximum is also located near the right entrance of the second jet streak. As a consequence both PVA maxima work together and have an enhanced impact.

    During the phase of strongest development NVA in the right exit region intensifies, which goes hand in hand with the dissipation of clouds within the cold front of the cold front cloud band.

  • Potential vorticity (PV):
  • The potential vorticity shows a PV anomaly (values bigger than 1 PVU) protrude downward in the upper troposphere along the rear cloud edge of the cold front.

    The anomaly, which separates tropospheric and stratospheric air, goes lower than in classical cyclogenesis. In the initial stage it is approximately at 400-500 hPa and goes down to 700 hPa or even lower. In the beginning of the process the anomaly is upstream of the low center, and approaches it while the Rapid Cyclogenesis proceeds.

    In the phase where the deepening of the surface low is strongest, the PV anomaly coincides with the location of the low and the intensified PV values of the troposphere.

    This does not automatically mean that stratospheric air reaches the surface. The increase of PV in the low layers can also have other reasons, such as the mutual increase of rotation between high and low levels as described by Hoskins (see Meteorological Physical Background) and the release of latent heat, especially in convective situations.

The images from 8 January 2015 show three time steps in the life cycle of a Rapid Cyclogenesis: 06:00, 12:00 and 18:00 UTC from initial to advanced stage. The schematics summarize the most typical features which can appear in some or all phases of development.

Height contours at 1000 hPa and 500 (300 hPa)

Fig. 22a: Schematic of height contours at 1000 hPa and 500 hPa. Fig. 22b: 8 January 2015, 06:00 UTC - Meteosat 10, IR 10.8 image. Magenta: height contours 1000 hPa, cyan: height contours 500 hPa.
Fig. 22c: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Magenta: height contours 1000 hPa, cyan: height contours 500 hPa. Fig. 22d: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Magenta: height contours 1000 hPa, cyan: height contours 500 hPa.

Temperature advection (TA) at 700 hPa

Fig. 23a: Schematics of temperature advection. Fig. 23a: 8 January 2015, 06:00 UTC - Meteosat 10, IR 10.8 image. Temperature advection 700 hPa; red: warm advection; blue: cold advection.
Fig. 23b: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Temperature advection 700 hPa; red: warm advection; blue: cold advection. Fig. 23c: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Temperature advection 700 hPa; red: warm advection; blue: cold advection.

Isotachs at 300 hPa

Fig. 24a: Schematics of isotachs at 300 hPa. Fig. 24b: 8 January 2015, 06:00 UTC - Meteosat 10, IR 10.8 image. Yellow: isotachs 300 hPa.
Fig. 24c: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Yellow: isotachs 300 hPa. Fig. 24d: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Yellow: isotachs 300 hPa.

Vorticity advection at 300 hPa (PVA/NVA)

The field of vorticity advection is rather noisy and it is important to filter out the meteorologically irrelevant areas from the important ones. To make this easier the relevant PVA fields are marked by a black star and the NVA areas are encircled by a black line.

Fig. 25a: Schematics of vorticity advection at 300 hPa. Fig. 25b: 8 January 2015, 06:00 - Meteosat 10, IR 10.8. image. Vorticity advection 300 hPa; red: PVA; cyan: NVA; blue line: TFP CF; yellow line: jet axis 300 hPa; black line: CF cloud dissolution.
Fig. 25c: 8 January 2015, 12:00 - Meteosat 10, IR 10.8. image. Vorticity advection 300 hPa; red: PVA; cyan: NVA; blue line: TFP CF; yellow line: jet axis 300 hPa; black line: CF cloud dissolution. Fig. 25d: 8 January 2015, 18:00 - Meteosat 10, IR 10.8 image. Vorticity advection 300 hPa; red: PVA cyan: NVA; blue line: TFP CF; yellow line: jet axis 300 hPa; black line: CF cloud dissolution.

Potential Vorticity (PV)

Fig. 26a: Schematics of potential vorticity Fig. 26b: 8 January 2015, 06:00 UTC - Meteosat 10, IR 10.8 image. Magenta: height of PV >1 unit.
Fig. 26c: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Magenta: height of PV >1 unit. Fig. 26d: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Magenta: height of PV >1 unit.

Typical Appearance in Vertical Cross Sections

  • Moist isentropes
  • The isolines of moist isentropes form a downward-inclined zone with a high gradient typical for cold fronts. In the case of a Rapid Cyclogenesis the zone is broad and contains isolines of both the cold front and the cloud head, which are close to each other, or even merge together.

    In the initial stages, where the cold front and cloud head are clearly separated, there is often a double structure within this zone as well. This double structure vanishes as the distinct cloud spiral develops.

  • Relative humidity:
  • The relative humidity is high within the frontal cloud band and cloud head. In between them, within the dry tongue, can be found low values of relative humidity which reach from the tropopause down to relatively low levels in the atmosphere.

  • Potential vorticity (PV):
  • Potential vorticity shows an anomaly protruding downward deep into the troposphere below the zone with the highest moist isentrope gradient in the cold front. This lowering of the so-called dynamical tropopause (1 - 2 PV Units) is already clearly seen in the initial stage, where the tropopause height sinks below about 400 hPa. In the mature stage the tropopause is usually below 500 hPa and in extreme cases below 700 hPa. At the same time there is an intensification of PV in the lowest tropospheric layer from the typical values of PV < 1 unit to PV > 1 unit and more. In the final phase a column of PV values > 1 unit develops and stretches from the stratosphere down almost to the surface.

  • Temperature advection (TA):
  • The temperature advection shows a very distinct situation similar to but more pronounced than a classical wave; there is a maximum of warm advection within and ahead of the cloud head, and cold advection directly behind it.

  • Vorticity advection (PVA/NVA):
  • The vorticity advection shows a pronounced PVA maximum in the upper levels, mostly situated within the cloud head, and an NVA maximum on the other side of the dry tongue in the gradient zone of the cold front. Both correlate well with the left and right exit regions of the jet streak.

  • Isotachs:
  • The isotachs show a pronounced jet stream along the rear edge of the frontal gradient of the cold front. The cloud head can often be found within the left exit region of a jet streak situated upstream of the low. In later stages of development, another maximum of the isotachs is often present at the poleward side of the cloud head.

    A sting jet can develop in the advanced development stage and mature stage. It can be seen as an isotach maximum below the frontal zone, reaching from the surface up to 700 hPa.

The images from 8 January 2015 show three time steps in the life cycle of a Rapid Cyclogenesis at 06, 12 and 18 UTC.

The cross sections are taken across the cloud head and the cold front from north-northwest to south-southwest through the deepening surface low and the developing dry intrusion. For the different stages of development slightly different cross sections were made to show the most indicative features of each stage.

Fig. 27a: 8 January 2015, 06:00 UTC - Meteosat 10, IR 10.8 image. Orange: location of vertical cross section A. Fig. 27b: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Orange: locations of vertical cross sections A, B and C.
Fig. 27c: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Orange: locations of vertical cross sections A and B.

Relative Humidity

Fig. 28a: Schematics for moist isentropes and relative humidity: Initial and development stage. Fig. 28b: 8 January 2015, 06:00 UTC - vertical cross section A. Black: moist isentropes (ThetaE), green/brown: relative humidity.

Potential Vorticity

Fig. 29a: Schematics for moist isentropes and potential vorticity (PV): all development stages. Fig. 29b: 8 January 2015, 18:00 UTC - vertical cross section A. Black: moist isentropes (ThetaE), red: potential vorticity: heavy: > 2 PVU, light: < 2 PVU.

Temperature Advection

Fig. 30a: Schematics for moist isentropes and temperature advection: initial to advanced stages. Fig. 30b: 8 January 2015, 12:00 UTC - vertical cross section A. Black: moist isentropes (ThetaE), temperature advection: red: warm advection; blue: cold advection.

Vorticity Advection and Isotachs

Fig. 31a: Schematics for moist isentropes, isotachs and vorticity advection: initial to advanced stages.
Fig. 31b: Schematics for moist isentropes and temperature advection: initial to advanced stages. Fig. 31c: 8 January 2015, 12:00 UTC - vertical cross section A. Black: moist isentropes (ThetaE), temperature advection: red: warm advection; blue: cold advection.

Weather Events

Parameter Description
Precipitation
  • Intense precipitation associated with the warm conveyor belt.
  • Thunderstorms within the inner edge of the cloud head on the western side of the surface low. They can also develop within the dry slot.
Temperature
  • Substantial rise in surface temperature within the area of warm conveyor belt.
Wind (incl. gusts)
  • Strong winds within the area of the cloud head. In extreme cases winds can reach hurricane force.
  • Very strong gusts in the transition zone between dark and white as can be seen in the WV channels near the cold front and the cloud head.
  • Possibility of the development of a sting jet in the advanced and mature stages in the southern semicircle of the low and south of the decaying cloud spiral. (See below)
  • Sting jet cases may involve hurricane force winds with gusts up to 200 km/h.
Other relevant information
  • Very strong falls and rises in pressure. In the initial stage, falls ahead exceed the rises behind.
Fig. 32a: Weather events at the ground level Fig. 32b: Anatomy of an RC cyclone

The case of 8 - 9 January 2015 developed over the ocean, which means synoptic measurements are only available for the last phases and only from the British Isles and the North Sea. Although the cloud spiral is not directly over land, heavy showers and high wind speed can be seen in synoptic measurements over the Hebrides (NW Scotland).

Fig. 33: 9 January 2015, 00 UTC, Meteosat; IR 10.8. Synoptic observations.

10 meter wind speed maximum and its location, typical for a sting jet - discussed in the "Meteorological Physical Background" chapter - can be seen in the data from 9 January 2015, 00:00 UTC.

Fig. 34: 9 January 2015/00.00 UTC; Meteosat 10; IR 10.8 image, ECMWF 10 m wind

As already mentioned in the table above, sting jets may appear in the innermost part of the cloud spiral in the advanced and mature stages of a Rapid Cyclogenesis.

A sting jet is a mesoscale (no wider than about 100 km) zone of fast moving air descending from a height of 3-4 km to the surface south of the low center. The high winds last only for a few hours, but they can cause damage on the surface.

Sting jets may occur in connection with any intensive cyclogenesis. Their structure and development is not yet fully understood; both dynamical and thermodynamical approaches have been applied.

The next two images show the moist isentropes and isotachs in vertical cross sections from 8 January 2015 at 12:00 and 18:00 UTC. Both cross sections show the maximum of the upper level jet with the core between about 250 and 300 hPa. But during these 6 hours a second wind maximum has developed in the low layers with a core between 950 and 800 hPa.

Although it is not so easy to decide if this is a sting jet or some other wind speed maximum, these two figures do demonstrate the applicability of VCSs for a more detailed investigation of strong winds and the possibility of their damaging impact.

Fig. 35: 8 January 2015, 12:00 UTC - Meteosat 10, IR 10.8 image. Black: moist isentropes; brown: isotachs; yellow: orientation of VCS line; yellow arrow: position of upper level jet core; blue line: area under the front from the surface up to 700 hPa.

Fig. 36: 8 January 2015, 18:00 UTC - Meteosat 10, IR 10.8 image. Black: moist isentropes; brown: isotachs; yellow: orientation of VCS line; yellow arrow: position of upper level jet core; blue arrow: position of a low level jet (possibly a sting jet); blue line: area under the front from the surface up to 700 hPa.

Due to the strong winds that quite often occur during rapid cyclogenesis events, wind warnings for the relevant areas are issued by MeteoAlarm. The example below is from 31 January 2008 and shows a similar situation and cloud spiral as in the 8 January 2015 case. Also shown is a wind warning for Norway.

Fig. 37: 31 January 2008, 18:00 UTC - Meteosat 9 IR10.8 10.8 image. Weather events (green: rain and showers, blue: drizzle, cyan: snow, red: thunderstorm with precipitation, purple: freezing rain, orange: hail, black: no actual precipitation or thunderstorm with precipitation).

Fig. 38: 31 January 2008, 03:59 UTC - MeteoAlarm warning issued by the national meteorological services.