Steven M. Cavallo |
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Tropopause polar vortex (TPV) observations
A particularly long-lived tropopause polar cyclone occurred in November 2005. Using potential vorticity
diagnostics, vortices are defined here by closed contours of potential temperature on the dynamic tropopause (the
2 PVU surface here).
The vortex was identified on the tropopause November 1, 2005 in northern Siberia.
The cyclone moved over the Arctic Ocean and into the Canadian Arctic over the course of the next several weeks, before
it could no longer be identified it on the tropopause December 7, 2005 off the Canadian Maritime coastal region.
In the following animation, the first image shows the vortex tracks on the tropopause using Global Forecasting
System (GFS) analyses. Colors are the values of tropopause potential temperature, so that colder colors (lower values) imply a
stronger vortex. The second image shows a time series of the minimum tropopause potential temperature value of the vortex.
Day numbers are days from November 1, so that Day 0 is November 1. Due to the complex track, only the first 25 days are shown for clarity in both images..
The gray shadings on the second image represent the range of values between the 1.75 and 2.25 PVU surfaces, indicating
the sensitivity to choosing the 2 PVU surface as the tropopause.
Click here to see the relation of potential temperature to potential vorticity surfaces.
The horizontal scales of TPVs typical range from about 300-1100 km, making it difficult to sample the vortex core
with radiosondes. In this case, the TPV was larger than typical, and it moved over a relatively dense radiosonde
network in the Canadian Arctic. The following animation sequence shows tropopause potential temperature from November 20 at 12 UTC
to November 24 at 12 UTC using the GFS analyses. All radiosonde locations are
denoted by a white `+' symbol, and station YZS (Coral Harbour) located in Nunavut, Canada denoted by the large `+' in red.
The corresponding radiosonde stations at Coral Harbour are shown below:
Apparent from this sounding (and typical of soundings from near the vortex cores), are that the atmosphere is nearly
saturated from the ground to the tropopause. This prompts the question of whether diabatic processes play in important
role in the vortex evolution. Vortex strength can change due to diabatic or frictional processes by the Ertel potential vorticity
theorem. In a study we performed, we saw that the competition between radiational cooling, which tends to create PV near the tropopause,
and latent heating, which tends to destroy PV near the tropopause, are the most important. The following shows
a sequence of time-height sections when the vortex was strengthening over Siberia around November 5, 2008.
The values are all averaged approximately within the last closed contour of potential temperature on the tropopause.
The first image is the EPV tendency due to all diabatic processes, the second is the radiational component, the third
is the latent heating component, and the fourth is the remaining component.
Finally, taking a time average (~120 hours), the vertical averages of the same segment of the vortex as above is below,
with the first image showing all components of the EPV tendency, and the second image showing the diabatic components
of the EPV tendency.
For more information, see the corresponding paper:
or email me. Thanks
for your interest!
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