Introduction
Research on cyclogenesis has a long history, spanning more than 100 years. Many summaries of the relevant approaches and developments exist in the literature (see below).
Cyclogenesis research incorporates three components: "observations; diagnosis, including dynamical modelling; and theory". It leads to the construction of Conceptual Models (CM), and so, many CMs have been developed (Shapiro 1999) that describe cyclogenesis either as a whole process, or as only part of a more general process.
From about 1960, satellite images and data have been available with steadily improving quality and detail. While satellite data, in form of remote sensing data, have been included in the long history of cyclogenesis research, satellite images have not played an important role in the relevant literatures.
The intention of SatManu is to start the process of weather diagnosis and/or research by looking at the satellite images: recognising the typical and recurring cloud configurations and structures that accompany different meteorological and physical processes.
As cyclogenesis is a process (rather than an object), the focus in the relevant "Cyclogenesis" chapters must be on the phases of development.
Phases of cyclogenesis development and associated conceptual models in SatManu
This chapter presents an outline of the different of the different phases of the historical development of cyclogenesis research, and compares the CMs that have been developed with those treated in SatManu, with a focus on typical appearances in satellite images.
During the early stages of cyclogenesis research, two main theories developed in parallel:
- 1) Cyclogenesis takes place in frontal zones, at the polar front, and is described in the well-known Polar Front Theory developed within the Norwegian school from about 1922 onwards (Solberg, Bjerkness). This describes an occlusion process in which the cold front catches up the warm front, lifting the warm air, which forms the cloud band of the occlusion front.
- 2) Cyclogenesis develops in an area of baroclinic instability and fronts develop (if at all) as a consequence of and in connection with the developing cyclone (Charney 1947, Eady 1949).
In SatManu, fronts are treated as important CMs in the main chapters of Cold Fronts, Warm Fronts and Occlusions chapters. In these chapters, the types of fronts are differentiated mostly by their appearance in satellite images, as well as by their differing appearance in orographically different regions. The Polar Front Theory is presented briefly below, and in detail in extratropical cyclones "Polar front theory".
- 3) During the next period, the theory of cyclogenesis development at fronts was refined by including processes occurring at upper levels: CMs of tropopause folding, low tropopause levels above cold fronts, and jet streaks and their preferred areas for cyclogenesis in the right entrance and the left exit region (Scherhag 1934), were developed.
In SatManu many CMs refer to the interaction of frontal cloud bands and the jet streak, such as FIbyJet (Frontal intensification by jet streak), Split front and Front Decay.
- 4) The occurrence of cyclogenesis at already well-developed cold fronts is also called "secondary cyclogenesis" and starts with a wave stage at the front.
This phenomenon was observed back in the beginnings of cyclogenesis research (Bjerkness 1919), but a theoretical explanation was only given much later (1990 Schär and Davies; Joly and Thorpe).
However, not all waves develop into a cyclone: this is dependent on the interaction with the upper-level trough and horizontal wind shear and shear vorticity.
These developments and the associated CMs are described in SatManu in "Wave" and "Upper Wave"; whereby the latter is an example of a non-developing wave.
- 5) A later new approach (Hoskins 1985) is summarised by the title "PV Thinking": the conservation of stratospheric PV (potential vorticity) leads to a release and strengthening of cyclogenesis where PV anomalies interact with the low-level temperature waves.
In SatManu this is most often seen and presented in the CM of Rapid Cyclogenesis.
- 6) The polar front theory, although a big breakthrough in meteorology, has several deficiencies that later became clear in numerous case studies. Some of them show a development where, after the wave stage, the cold front weakens and finally tears off from the warm front (Shapiro, Keyser 1990). This cannot be seen in every case in satellite images but can be seen much better in the thermal parameters, which show a warm seclusion in the occlusion centre. This contrasts with the polar front theory, where there is cold air in the cyclone centre. The most important consequence of these CMs is a change in the understanding of occlusion development.
In SatManu, the Shapiro-Keyser CM is presented briefly below and in detail in the chapter of extratropical cyclones "Shapiro-Keyser cyclogenesis".
- 7) All the CMs discussed above refer to the observation of data at one (or, for developments, several) point in time, following the Eulerian perspective. In contrast to this, the observation of changes along a trajectory, following the Lagrangian perspective, led to the theory of the conveyor belts (Browning 1987, Carlson 1980).
In SatManu, cyclones are presented as conveyor belts in the chapter of extratropical cyclones "Conveyor Belt theory". Additionally, for all CMs in SatManu, the relevant involved conveyor belts are discussed in the meteorological and physical background chapter. Some recurring cloud configurations are optically so clearly related to a conveyor belt, that this is also included in the name of the conceptual models, such as Warm conveyor belt, Cold conveyor belt occlusion and Warm conveyor belt occlusion.
- 8) Motivated by cloud structures seen in satellite images, other cloud vortices were considered by many different researchers; these come from mesoscale forcing and from the cold part of well-developed cyclones, but can result in cloud configurations that look very similar to occlusion stages, without having undergone a cyclogenesis process.
In SatManu these CMs are Comma and the processes of Instant Occlusion and Cold Air Development (CAD).
References
David M. Schultz at al: Extratropical Cyclones: A Century of Research on Meteorology's Centerpiece; Meteorological Monographs 59, 1; Chapter 16;2019.
Promet 2020: Außertropische Zyklonen. Heft 103.