Cyclogenesis is a thermally direct circulation with warm air rising as it flows poleward, and cold air sinking as it flows Equatorward.

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From: Encyclopedia of Atmospheric Sciences (Second Edition), 2015

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Cyclogenesis and Atmospheric Fronts 97

3.5.1

Extratropical and Tropical Cyclones: Energy Source and Main Thermodynamic Characteristics 97

3.5.2

Cyclogenesis Within Baroclinic Troughs: Leaf and Baroclinic Leaf Features in the Water Vapor Imagery 97

3.5.3

Cyclogenesis With Upper-Level Precursors 1033.5.3.1Cyclone Development in the Western North Atlantic 104

3.5.3.2

Explosive Cyclogenesis in the Southern West Pacific 110

3.5.3.3

Water Vapor Imagery Dry Slot as a Precursor of Cyclone Deepening 113

3.5.4

Usefulness of Water Vapor Imagery to Identify “Sting Jet” and Related Surface Wind Gusts 116

3.5.5

Split Cold Front Seen in Water Vapor Imagery 122


S.A. Hsu, in Encyclopedia of Physical Science and Technology (Third Edition), 2003

Cyclogenesis

Cyclogenesis is defined as any development or strengthening of cyclonic circulation in the atmosphere. In certain coastal regions, cyclogenesis is a very important phenomenon, for example, along the mid-Atlantic coast of the United States and in the northwestern Gulf of Mexico. The cyclones that develop over the Yellow Sea and East China Sea often cause strong gales, and the cold air west of the cyclones spreads southward as an outbreak of the winter monsoon. Main features and processes contributing to coastal cyclogenesis along the U.S. East Coast are significant sensible heat transport over the ocean and latent heat release along the East Coast, coastal frontogenesis, and a polar jet streak propagating eastward. In a case of East Asian coastal cyclogenesis, a numerical experiment that included all physical processes simulated the development of a cyclone that developed rapidly in a way similar to that observed. In an experiment without latent heat feedback, only a shallow low appeared when the upper short-wave trough approached the inverted surface trough situated on the coast, but no further development took place. This suggests that the baroclinic forcing was enhanced by the feedback of physical processes. The latent heating had a profound impact on the amplifying jet streak circulation and the vertical coupling within the system, which appeared to prime the rapid cyclogenesis along the coast. Sensible heating contributed nearly 18 % to the surface development. It helped to build a potential temperature contrast along the coast below 900 mb. Without sensible heating, the model-latent heat release was reduced. Thus, the impact of sensible heating was partly through the moist processes rather than direct heating.


Cyclogenesis in the western Gulf of Mexico is contributed in most cases by a mountain-induced standing wave developed on a climatological surface baroclinic zone over the Gulf. The effect of surface-layer baroclinicity on cyclogenesis is shown in Fig. 1. A close relationship is found between the frequency of occurrence of frontal overrunning over New Orleans, LA, and the air temperature difference between the shelf (shallow) water and deeper (warm) ocean water. Because the surface-based vorticity is directly and linearly proportional to the temperature difference across the cold shelf water and warmer Gulf water, Fig. 1 indicates that there is definitely a correlation between the surface-based, vorticity-rich air and the temperature difference or baroclinic zone occurring between the colder land/shelf water and the warmer deep-ocean water. A larger scale baroclinic or solenoidal field from Key West, FL, to Del Rio, TX, via Victoria, TX, and the Gulf of Mexico is shown in Fig. 2. The cold pool or cold-air damming, as discussed previously, over the cold shelf water off the coast of southern Texas and the Victoria region is clearly delineated. On the other hand, a warmer region over the Loop Current west of Key West is also illustrated.


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FIGURE 1. Correlation between the frequency of frontal overrunning over New Orleans, LA, and the difference in air temperature over warmer Gulf of Mexico and colder shelf waters.


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FIGURE 2. An example of a baroclinic (or solenoidal) field from Key West, FL, to Del Rio, TX, via the Gulf of Mexico and Victoria, TX, on February 22, 1986, during a special experiment. Based on radiosonde ascents from weather stations and radiosonde drops from airplanes.


An example of the cyclogenesis over the Western Gulf of Mexico including its effect and classification is provided in Figs. 3-6456.


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FIGURE 3. An example of cyclogenesis which took place over the Gulf of Mexico on February 16, 1983. This shot was taken from the GOES satellite. Notice the comma-shaped whirlpool cloud pattern and also the fact that this system was not linked to other larger scale systems. This was one of the top five cyclones generated over the Gulf of Mexico during the 1982–1983 El Niño period. Not only do surface conditions, such as sea surface temperatures, play an important part in the development and intensification of these storms, but the upper atmospheric conditions are critical as well.

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FIGURE 5. The time series analysis for this storm (Fig. 4) was made from a data buoy for atmospheric pressure, wind speed, and significant wave height during the period of cyclogenesis. Note the relationship between pressure and winds. The maximum wind speed does not usually occur at the time of the lowest pressure, but in general the lower the pressure the stronger the wind will be. This particular time series very much resembles a typical tropical cyclone plot as the wind speed would be expected to drop off dramatically in the eye or center of lowest pressure.


FIGURE 6. The cyclogensis classification (top panel) is based on the minimum pressure of winter storms in the Gulf of Mexico. The bottom panel shows the number of storms studied, the relationship between the pressure gradient parameter and the reported maximum winds, while the vertical bars are the standard deviations. Mariners Weather Log 37 (2), 4.>