Cyclogenesis - WikiHQ

thumb|upright=1.35|This collage of [[GOES 13 satellite images shows the development of a nor'easter over two days.]]

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low-pressure area). Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale.

Tropical cyclones form due to latent heat driven by significant thunderstorm activity, developing a warm core.
Extratropical cyclones form as waves along weather fronts before occluding later in their life cycle as cold core cyclones.
Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.

The process in which an extratropical cyclone undergoes a rapid drop in atmospheric pressure (24 millibars or more) in a 24-hour period is referred to as explosive cyclogenesis, and is usually present during the formation of a nor'easter. Similarly, a tropical cyclone can undergo rapid intensification.

The anticyclonic equivalent, the process of formation of high-pressure areas, is anticyclogenesis. The opposite of cyclogenesis is cyclolysis.

Meteorological scales

There are four main scales, or sizes of systems, dealt with in meteorology: the macroscale, the synoptic scale, the mesoscale, and the microscale. The macroscale deals with systems with global size, such as the Madden–Julian oscillation. Synoptic scale systems cover a portion of a continent, such as extratropical cyclones, with dimensions of across. The mesoscale is the next smaller scale, and often is divided into two ranges: meso-alpha phenomena range from across (the realm of the tropical cyclone), while meso-beta phenomena range from across (the scale of the mesocyclone). The microscale is the smallest of the meteorological scales, with a size under (the scale of tornadoes and waterspouts). These horizontal dimensions are not rigid divisions but instead reflect typical sizes of phenomena having certain dynamic characteristics. For example, a system does not necessarily transition from meso-alpha to synoptic scale when its horizontal extent grows from .

Extratropical cyclones

thumb|The initial frontal wave (or low-pressure area) forms at the location of the red dot on the image. It is usually perpendicular (at a right angle) to the leaf-like cloud formation (baroclinic leaf) seen on satellite during the early stage of cyclogenesis. The location of the axis of the upper level [[jet stream is in light blue.]]

Norwegian cyclone model

thumb|An upper-level jet streak. DIV areas are regions of divergence aloft, which will lead to surface convergence and aid cyclogenesis.

The Norwegian cyclone model is an idealized formation model of cold-core cyclonic storms developed by Norwegian meteorologists during the First World War. The main concept behind this model, relating to cyclogenesis, is that cyclones progress through a predictable evolution as they move up a frontal boundary, with the most mature cyclone near the northeast end of the front and the least mature near the tail end of the front.

Precursors for development

A preexisting frontal boundary, as defined in surface weather analysis, is required for the development of a mid-latitude cyclone. The cyclonic flow begins around a disturbed section of the stationary front due to an upper level disturbance, such as a short wave or an upper-level trough, near a favorable quadrant of the upper-level jet. However, enhanced along-frontal stretching rates in the lower troposphere can suppress the growth of extratropical cyclones.

Vertical motion affecting development

Cyclogenesis can only occur when temperature decreases polewards (to the north, in the northern hemisphere), and pressure perturbation lines tilt westward with height. Cyclogenesis is most likely to occur in regions of cyclonic vorticity advection, downstream of a strong westerly jet. The combination of vorticity advection and thermal advection created by the temperature gradient and a low pressure center cause upward motion around the low.