METEOROLOGIST JEFF HABY
DPVA stands for Differential
Positive Vorticity Advection. One way
to understand what this is can be done by
breaking down and defining each of the four terms that make up DPVA. The word positive is in reference to
"higher values". Higher values of VA are moving toward the forecast region. Differential refers to "a change
in the vertical". VA is changing from the low levels of the troposphere to the upper levels of the troposphere.
RULE OF THUMB: vorticity will generally increase from the surface to 500 millibars. Vorticity advection is
at 500 millibars than in the low levels of the troposphere because the wind speeds tend to increase with height
(500 millibar winds near a
trough will often be stronger
than low-level winds). Because of this general rule,
the "D" is often left out. Then what is left is PVA (positive vorticity advection). Vorticity is any twisting motion
in the troposphere.
Positive vorticity can be broken down into three components, which are
positive shear vorticity, positive curvature vorticity and earth vorticity. A
counterclockwise spin in the Northern Hemisphere will produce
positive vorticity. Since the earth spins counterclockwise, earth vorticity is always positive. Motion within a
trough produces positive curvature vorticity (because air is rotating counterclockwise within a trough). Wind speed
increasing away from a point source will produce positive shear vorticity. Positive shear vorticity is also common
in a trough since the highest winds are often located away from the center of the trough.
Vorticity maximums are located in
regions where there is high positive shear vorticity co-located with positive curvature vorticity. Again, this is
most common just to the south of the center of a trough. The speed of the winds and the amplification of the trough
will determine the positive vorticity it will generate. The last term "A" means the DPV is moving from one place
to another over time. Wind speed and the orientation of the lines of constant vorticity to the
determines the rate at which vorticity advection will take place.
Putting this all together, DPVA means: positive
vorticity increasing from the surface to the upper levels is being advected. DPVA causes the troposphere to become
more dynamically unstable. DPVA occurs in
shortwaves and within just about any trough.
DPVA contributes to
rising air. The spin up of vorticity causes the troposphere to cool since rising air cools. This cooling will
lower heights aloft.
Cooling the middle and upper levels of the troposphere causes the troposphere to become more unstable. DPVA is
associated with upper level divergence and rising air. DPVA along with
low level warm air advection is a favorable
environment for precipitation and storms.