Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area).[9] Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. It can occur at various scales, from the microscale to the synoptic scale. Extratropical cyclones form as waves along weather fronts before occluding later in their life cycle as cold core cyclones. Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core.[10] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.[11] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.[12] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high pressure areas—Anticyclogenesis.[16]
The surface low has a variety of ways of forming. Topography can force a surface low when dense low-level high pressure system ridges in east of a north-south mountain barrier.[17] Mesoscale convective systems can spawn surface lows which are initially warm core.[18] The disturbance can grow into a wave-like formation along the front and the low will be positioned at the crest. Around the low, flow will become cyclonic, by definition. This rotational flow will push polar air equatorward west of the low via its trailing cold front, and warmer air with push poleward low via the warm front. Usually the cold front will move at a quicker pace than the warm front and “catch up” with it due to the slow erosion of higher density airmass located out ahead of the cyclone and the higher density airmass sweeping in behind the cyclone, usually resulting in a narrowing warm sector.[19] At this point an occluded front forms where the warm air mass is pushed upwards into a trough of warm air aloft, which is also known as a trowal.[20]
Tropical cyclogenesis is the technical term describing the development and strengthening of a tropical cyclone in the atmosphere.[22] The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which mid-latitude cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment. There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a preexisting low level focus or disturbance, and low vertical wind shear.[23] An average of 86 tropical cyclones of tropical storm intensity form annually worldwide, with 47 reaching hurricane/typhoon strength, and 20 becoming intense tropical cyclones (at least Category 3 intensity on the Saffir-Simpson Hurricane Scale).[24]
Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column. Mesocyclones are normally relatively localized: they lie between the synoptic scale (hundreds of kilometers) and microscale (hundreds of meters). Radar imagery is used to identify these features.[25]
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