WV Vortices in the northern hemisphere are cyclonically rotating and therefore associated with a trough or low in the upper levels of the troposphere. This can also be seen in PV fields, which show high values near the centre of the vortex. High values of PV are related to low tropopause height, which explain the Dark Stripe in the WV image. (See Additional Parameters for the diagnosis of cloudiness - Potential vortcity and Water Vapour Dark Stripe ). The Dark Stripe implies that relatively dry stratospheric air is penetrating down into the higher levels of the troposphere. Within the Dark Stripe, a local maximum of PV can often be seen. Because of the local maximum, the cyclonic circulation is enhanced. (See schematics below). Therefore, the Dark Stripe is being deformed and the moister air spirals around the dryer air. This process either leads to the formation of a WV Eddy or a WV Eye. A complete scientific reason for the different developments can't be given yet. In the example above the development of a vortex can be clearly seen. Note that the PV field is coinciding reasonably well with the Water Vapour Dark Stripe. However, the exact structure is not completely reproduced by the numerical model. Also note that the PV field indicates a narrow elongated zone of high values, with a local maximum within this zone. As expected, the vortex develops close to this local PV maximum.

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Discussion

Research of numerous cases has shown that, based on the WV imagery, a distinction between two prevailing structures can be made: The WV Eddy and the WV Eye. However, in the investigation so far, a simple scientific reason for the different structures can't yet be given. There are some ideas about the connection between the strength of the shear zone and the width and the angle of the PV-anomaly: To surround a dry area (eye) you need a relative broad PV-trough. However, the spatial resolution of the model fields used is not high enough to confirm this hypothesis.
The model fields show similar characteristics for the two different structures. Therefore, in the next chapters, no distinction will be made between the WV Eddy and the WV Eye.

WV Vortices often exist in front of frontal zones; consequently there are no distinct or intensive cloud structures. The reason that no other distinct cloud structures exist, is due to the fact that WV Vortices mainly develop in the upper levels of the troposphere. There is less cloud development in the lower levels because the lower parts of the troposphere often show a stable stratification, or insufficient moisture. This is one of the main differences with the conceptual model Upper Level Low. (See Upper Level Low and look at the table below). In the case of an Upper Level Low, the lower troposphere shows an unstable stratification and, as a consequence, clouds develop as convection occurs.

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As already mentioned, a smaller scale trough is often superimposed on a large scale trough resulting in a small scale vortex. Smaller troughs are mobile and therefore WV Vortices move with the general flow. The reason for the movement of small troughs can be explained relatively simply.
From the geopotential tendency equation it can be seen that advection of vorticity can lead to the change of the geopotential height pattern. This vorticity advection term consistst of two parts: Advection of relative vorticity, and advection of earth's vorticity.

The relative vorticity can be writen as:

where ζ_g is the relative (geostrophic) vorticity, f₀Coriolis parameter, ∇² Laplacian operator, φ geopotential height.

From this equation it can be seen that the relative vorticity is proportional to the Laplacian of the geopotential height field. If we suppose that the geopotential height field is a sinusoid it can be shown that:

where L is the wavelength of the sinusoid.

The relative vorticity is inverse proportional to the square of the wavelength of the sinusoid. Note that the relative vorticity is inversely proportional to the square of the wavelength of the sinusoide. The shorter the sinusoidal wave, the stronger the vorticity will be. It can also be shown that the Earth's vorticity is inverse proportional to the wavelength of the sinusoid, therefore, relative vorticity will dominate the behaviour of short Waves and the earth's vorticity will dominate the long Waves. As a consequence, a WV Vortex, which is related to a short Wave, tends to move quickly. However, an Upper Level Low which is associated with a large scale trough, is stationary or may even retrograde.

The differences and similarities between a WV Vortex and an Upper Level Low are listed in the tables below.

Differences:

	WV-vortex (Eye/Eddy)	Upper Level Low
Process	Small upper level cyclonic circulation, often embedded in a large scale upper level trough	Cut-off process from a large scale Rossby Wave
Cloudiness	Can only be seen in WV imagery. Sometimes small convective cloud cells develop which can also be seen in IR and VIS imagery	Vigorous convective activity develops especially in the mature stage which can be clearly seen in all channels
Scale	Small scale/ Mesoscale system	Synoptical scale system
Velocity	Moving system, associated with a mobile trough	Associated with a quasi-stationary trough

Similarities:

Timescale	Persistent, can last for several days
WV imagery	Both appear as a cyclonic circulation of dry and moist air streams.