Positive vorticity advection (PVA) is a process that contributes to a lifting of the air. This is important to a weather forecast since rising air contributes to clouds and precipitation. This mini lecture will address the analysis of the positive 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). Positive vorticity advection occurs within troughs (both shortwave and long wave troughs). Positive vorticity advection is the replacement of lower values of vorticity by higher values of vorticity. Thus, the term “positive” is similar to “replacement by higher values”. The term advection means the wind flow is allowing a movement of higher values of vorticity 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 in the last paragraph. The right side of a trough is generally where the vorticity advection will be positive. This goes right in line with the right side of the trough being the region that generally has the greatest amount of lifting, clouds and precipitation.  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 right of the vort max, the wind flow is advecting in higher values of vorticity. This region (downstream region of vort max) has lifting due to positive 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.  |