Vorticity and 500 mb

[H2Otown_WX]

Simple enough question..but I can't seem to think of why myself. According to my met. professor it is the level of quasi-non-divergence. Is that what makes it a could place to calculate vorticity?

[isohume]

Simple enough question..but I can't seem to think of why myself. According to my met. professor it is the level of quasi-non-divergence. Is that what makes it a could place to calculate vorticity?

That is certainly one of the reasons H5 is the level where vorticity is measured. In order for PVA and upward vertical motion theory to work you need the PVA to be above the LND. Above the LND air is rising and diverging. Below the LND...the atmos flow is assumed to be mainly converging. Also...the omega equation states that vertical motions are due to thermal advections and vorticity advections through a layer....or differential vorticity advection (dPVA). Since wind speeds generally get higher as you go up, you can assume the dPVA will be positive (upward vertical motion) and therefore you can use the magnitude of PVA at H5 as a decent proxy for dPVA for say the H7-H5 layer. Some software packages like AWIPs, PCGRIDDs, etc let you look at various layers of explicit dPVA, but if that's not available...the H5 proxy or even higher is good to use in most cases depending on the vertical wind shear.

[H2Otown_WX]

That is certainly one of the reasons H5 is the level where vorticity is measured. In order for PVA and upward vertical motion theory to work you need the PVA to be above the LND. Above the LND air is rising and diverging. Below the LND...the atmos flow is assumed to be mainly converging. Also...the omega equation states that vertical motions are due to thermal advections and vorticity advections through a layer....or differential vorticity advection (dPVA). Since wind speeds generally get higher as you go up, you can assume the dPVA will be positive (upward vertical motion) and therefore you can use the magnitude of PVA at H5 as a decent proxy for dPVA for say the H7-H5 layer. Some software packages like AWIPs, PCGRIDDs, etc let you look at various layers of explicit dPVA, but if that's not available...the H5 proxy is good to use in most cases depending on the vertical wind shear.

Okay thanks isohume. Is another part of the reason it is used because vertical motions are generally at their greatest there?

[isohume]

Okay thanks isohume. Is another part of the reason it is used because vertical motions are generally at their greatest there?

Yeah that's generally true. The LND separates the levels where air is generally converging and diverging. According to the theory I've heard, the upward moving air parcels are performing work on the atmos as they enter the level of divergence and move away from each other. Therefore, they give up a certain amount of their vertical momentum. Below the LND...the atmos is performing work on the air parcels and giving them more momentum as they rise...thus the LND would be the level at which the air parcels have their highest vertical velocity.

[baroclinic_instability]

Very general (i.e., 500 hpa is the best level to assess upward vertical motion in association with upper waves). Research and modeling has actually shown in many situations the region which correlates the best two DPVA/DCVA (depending on if you use "cyclonic" or "positive") is actually above 500 hpa more around 400-300 hpa. In summer, it is not rare for the wave disturbance to be located completely above 500 hpa. 500 just became the norm a long time ago and we haven't changed, unfortunately. I still think PV is far and away a better method for tracking baroclinic wave disturbances aloft. That said, meteorology is still stuck in the 50s. It is pretty clear since we still teach the simplistic Norwegian front theory to this day.

[H2Otown_WX]

Very general (i.e., 500 hpa is the best level to assess upward vertical motion in association with upper waves). Research and modeling has actually shown in many situations the region which correlates the best two DPVA/DCVA (depending on if you use "cyclonic" or "positive") is actually above 500 hpa more around 400-300 hpa. In summer, it is not rare for the wave disturbance to be located completely above 500 hpa. 500 just became the norm a long time ago and we haven't changed, unfortunately. I still think PV is far and away a better method for tracking baroclinic wave disturbances aloft. That said, meteorology is still stuck in the 50s. It is pretty clear since we still teach the simplistic Norwegian front theory to this day.

That's interesting. Is that because thicknesses in the summer are greater and the jet stream is higher up? What do you mean by PV? Positive vorticity? EDIT: Or is it potential vorticity? That's something I am unfamiliar with.

[baroclinic_instability]

That's interesting. Is that because thicknesses in the summer are greater and the jet stream is higher up? What do you mean by PV? Positive vorticity?

Yeah, pretty much. The retreat of the polar jet northward and the development of the subtropical high means thicknesses are significantly higher and the upper jet is located much higher. Wave disturbances aloft are typically very shallow in depth and relatively weak, but they can still incite significant bouts of severe weather/moist convection, especially across the plains as these shallow waves eject out of the Rockies. Meteorologists should be careful to dismiss these simply because they don't show up at 500 hpa (or look very weak at that level). They can also be enhanced via vertical stretching as they eject out of the Rockies, and it is one of the many reasons why areas such as the high plains are prime regions for severe activity mid summer. And yeah, PV = potential vorticity aka Isentropic Potential Vorticity.

I actually pulled this from my blog...something I wrote this summer. But note the effect of vertical stretching as this upper wave ejects into the plains from Colorado into NE/KS (note the "kink" that develops in the geopotential heights).