Positive vorticity advection, or PVA, is the result of more cyclonic values of
vorticity
In continuum mechanics, vorticity is a pseudovector (or axial vector) field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point an ...
advecting into lower values of vorticity. It is more generally referred to as "Cyclonic Vorticity Advection" (CVA). In the Northern Hemisphere this is positive, whilst in the Southern Hemisphere it is negative.
Development
Vorticity in the
atmosphere
An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...
is created in three different ways, which are named in their resultant vorticity. These are;
Coriolis Coriolis may refer to:
* Gaspard-Gustave de Coriolis (1792–1843), French mathematician, mechanical engineer and scientist
* Coriolis force
In physics, the Coriolis force is a pseudo force that acts on objects in motion within a frame of ref ...
vorticity,
curvature
In mathematics, curvature is any of several strongly related concepts in geometry that intuitively measure the amount by which a curve deviates from being a straight line or by which a surface deviates from being a plane. If a curve or su ...
vorticity, and
shear vorticity. For example, at the base of a
trough
Trough may refer to:
In science
* Trough (geology), a long depression less steep than a trench
* Trough (meteorology), an elongated region of low atmospheric pressure
* Trough (physics), the lowest point on a wave
* Trough level (medicine), the l ...
, there is curvature and shear vorticity. Curvature vorticity is due to the increasing cyclonic turning as an air parcel enters the trough base. The maximum counter-clockwise spin (positive vorticity in the Northern Hemisphere) is at the trough base. Shear vorticity is caused by the difference in wind speed between air moving through the trough base (typically a jet or jet finger) and slower moving air on either poleward and equatorward side of the faster flow. Consider that slower air to the poleward side will be imparted counter-clockwise spin (picture faster moving air (jet) south and slower air to the north, spin is created). Thus, to the north (poleward) of the trough base an air parcel will experience positive vorticity. Likewise, to the south of the faster flow the air is spun in a clockwise direction (faster air (jet)to the north with slower air to the south, spin is created). Thus, to the south of the faster winds will be an area of negative vorticity.
When these areas of negative and positive vorticity are moved (advected) they produce areas of negative vorticity advection (NVA) and positive vorticity advection (PVA) respectively, downstream from the trough base. The positive vorticity advection area is typically associated with divergence and upward motion. The negative vorticity advection area will be associated with convergence and downward motion.
This produces
convergence
Convergence may refer to:
Arts and media Literature
*''Convergence'' (book series), edited by Ruth Nanda Anshen
*Convergence (comics), "Convergence" (comics), two separate story lines published by DC Comics:
**A four-part crossover storyline that ...
because of the way the air gains cyclonic vorticity while entering the base of the trough. The opposite happens when air is exiting the base of a trough. This air has more cyclonic vorticity than the air it is entering and therefore produces CVA. CVA produces
divergence
In vector calculus, divergence is a vector operator that operates on a vector field, producing a scalar field giving the rate that the vector field alters the volume in an infinitesimal neighborhood of each point. (In 2D this "volume" refers to ...
as a result of how there is a loss of cyclonic vorticity. Coriolis vorticity in this situation is ignored because it acts about the same on all the air flowing through the base of the trough.
Significance in forecasting
The divergence with CVA is significant because it creates forced
lift
Lift or LIFT may refer to:
Physical devices
* Elevator, or lift, a device used for raising and lowering people or goods
** Paternoster lift, a type of lift using a continuous chain of cars which do not stop
** Patient lift, or Hoyer lift, mobile ...
in the atmosphere. This forced lift, in the presence of conditions favorable for
atmospheric convection
Atmospheric convection is the vertical transport of heat and moisture in the atmosphere. It occurs when warmer, less dense air rises, while cooler, denser air sinks.
This process is driven by parcel-environment instability, meaning that a "par ...
, can cause
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles, suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s or
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwe ...
. AVA will do the opposite and lead to a stable atmosphere. In combination with a jet streak, CVA can lead to the amplification of a trough which is significant for forecasting many conditions of the atmosphere.
References
{{reflist
External links
Vorticity Advection
Atmospheric dynamics