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Analysis of the potential vorticity budget of a tropopause polar
cyclone
Steven M. Cavallo
and Gregory J. Hakim
University of Washington
Department of Atmospheric Sciences
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Outline•Tropopause polar vortices (TPVs)
•Ertel potential vorticity (EPV)
•Tropopause maps
•November 2005 TPV
•PV budget of November 2005 TPV
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Waves and vortices
Vortex
Wave
•Consider a materially conserved field such as potential vorticity (PV):
Linear solutions are waves, nonlinear are vortices
•Observations tell us that upper level disturbances are more wave-like near jet stream and vortex-like away from jet stream
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Waves and vortices
• Closed contours in a materially conserved field:Fluid parcels are bound by closed contours of that field
•Using potential vorticity (PV), changes in vortex strength can be assessed by changes in fluid properties within these closed contours
These vortices often drift into mid-latitudes, sometimes triggering surface cyclogenesis
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Tropopause polar vortices (TPVs)Tropopause polar vortices (TPVs) are:
•Vortices that occur well poleward of the jet stream
•Based on the tropopause
•Cold core
Although there is considerable understanding about the life cycles of surface extratropical cyclones, relatively less is known about the upper-level disturbances governing them
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Radiational cooling at cloud top
Radiational heating at cloud base
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Heating Profile EPV Changes
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Heating Profile EPV Changes
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Potential Vorticity & Isentropic Surfaces
PV surfaces (black) in PVU, isentropic surfaces (red) in Kelvin
1 PVU = potential vorticity unit = m2 K kg-1 s-1
(Adapted from Hoskins 1990)
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November 2005 TPV
GFS analysis tropopause pressure Coral Harbour, NT sounding
21 November 2005 at 00 UTC
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November 2005 TPV
GFS analysis tropopause pressure Coral Harbour, NT sounding
22 November 2005 at 00 UTC
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November 2005 TPV
GFS analysis tropopause pressure Coral Harbour, NT sounding
23 November 2005 at 00 UTC
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November 2005 TPV
GFS analysis tropopause pressure Coral Harbour, NT sounding
24 November 2005 at 00 UTC
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November 2005 TPV
•Horizontal grid spacing 30 km, 31 vertical levels
•5-class microphysics, RRTM longwave radiation
•GFS analysis and boundaries updated every three hours
WRF simulations:
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Siberia
Averages within 285 K closed contour
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Siberia
Averages within 285 K closed contour
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Siberia
Averages within 285 K closed contour
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Hudson Bay
Averages within 280 K closed contour
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Hudson Bay
Values at center of vortex
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Siberia
EPV terms Diabatic components
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Hudson Bay
EPV terms Diabatic components
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Summary
•TPV strengthening from cloud-top radiational cooling
•TPV weakening processes not as clear, but weakening appears to occur when latent heating effects dominate the radiational effects
•What is the contribution of the frictional component? To what degree is implicit model diffusion effecting the budget closure?
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