|HABYTIME MINI LECTURE 10:|
NEGATIVE VORTICITY ADVECTION
METEOROLOGIST JEFF HABY
Negative vorticity advection (NVA) is a process that contributes to a sinking of the air. This is important to a weather forecast since
sinking air contributes to fewer clouds and less precipitation chances. This mini lecture will address the analysis of the negative
vorticity advection process. Vorticity advection is analyzed at 500 mb since this pressure level is about half way through the
vertical depth of the mass of the atmosphere. This pressure level gives a more pure analysis of the vorticity advection process
as compared to using pressure levels that are above or below 500 mb (i.e. 700 mb and 300 mb). Negative vorticity advection occurs
within troughs (both shortwave and long wave troughs). Negative vorticity advection is the replacement of higher values of
vorticity by lower values of vorticity. Thus, the term “negative” is similar to “replacement by lower values”. The term
advection means the wind flow is allowing a movement of vorticity isopleths into a fixed point.
The location of a region of vorticity with the highest value is termed the “vort max”. Vorticity is increased by an increasing
counterclockwise curvature of a wind flow (Northern Hemisphere). Thus, the location that has the greatest curvature though the
combination of counterclockwise speed shear, counterclockwise curvature shear and Coriolis will be the location of the vort max. This
will be further explained later in this writing. The left side of a trough is generally where the vorticity advection will be negative. This
goes right in line with the left side of the trough being the region that generally has the decreasing
amounts of lifting, clouds and precipitation.
Note that PVA and NVA regions are adjacent to each other. Thus, where strong PVA is found there will also be NVA adjacent to it. Both
PVA and NVA will occur near the vort max, with the occurrence of NVA or PVA depending on which side of the vort max a location is at
relative to the wind flow. The diagram below shows a trough with the vorticity values (with vort max) within the trough. The wind
is fairly parallel to the height contours. In the region to the left of the vort max, the wind flow is advecting in lower values
of vorticity. This region (upstream region of vort max) has sinking air due to negative vorticity advection.
The vort max tends to be located within the center of the trough. In this region, the counterclockwise curvature tends to be greater
and the wind flow tends to be greater (tighter height contours). The enhanced curvature leads to higher curvature vorticity while
the closer height contours leads to higher shear vorticity. Shear vorticity is generated by a change in wind speed over distance. For
example, when wind to the south of a location is strong while wind to the north of a location is weak, this will lead to a
counterclockwise twisting over time. Vorticity will be weakened when the wind flow is undergoing clockwise curvature or when
the wind shear produces a clockwise turning over time (anticyclonic shear). See diagram below. Note that NVA is not a process
that is causing vorticity reduction. The best NVA will be found next to a high magnitude vort max and PVA. Vorticity reduction
is mentioned here since a reduction in vorticity will cause a weakening in the magnitude of a vort max and thus the weakening
of both PVA and NVA.