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Dynamic tropopause analysis; What is the dynamic tropopause?• A level (not at a constant height or pressure)
at which the gradients of potential vorticity on an isentropic surface are maximized
• Large local changes in PV are determined by the advective wind
• This level ranges from 1.5 to 3.0 Potential vorticity units (PVUs)
Consider the cross sections that we have been viewing:
• Our focus is on the isentropic cross section seen below
• the opposing slopes of the PV surfaces and the isentropes result in the gradients of PV being sharper along isentropic surfaces than along isobaric surfaces
Dynamic tropopause pressure: A Relatively high (low pressure) Tropopause in the subtropics, and a Relatively low (high pressure)Tropopause in the polar regions; aSteeply-sloping tropopause in theMiddle latitudes
Tropopause potentialtemperatures (contour intervalof 5K from 305 K to 350 K) at12-h intervals (from Morgan andNielsen-Gammon 1998)
The appearance of the 330 K closed contour in panel c is produced by the large values ofequivalent potential temperatureascending in moist convectionand ventilated at the tropopauselevel;as discussed earlier, this is anexcellent means of showing theeffects of diabatic heating, andverifying models
the sounding shows a tropopausefold extending from 500 to 375hPa at 1200 UTC, 5 Nov. 1988for Centerville, AL,with tropospheric air above and extending to 150 hPa.
The fold has descended intoCharleston, SC by 0000 UTC,6 November 1988 to the 600-500hPa layer. The same isentropiclevels are associated with each fold
Coupling index:Theta at the tropopauseMinus the equivalentPotential temperature atLow levels(a poor man’s lifted index)
December 30-31, 1993 SLPAnd 925 hPa theta
An example illustrates the detail of the dynamic
tropopause (1.5 potential vorticity units) that is lacking in a constant pressure analysis
250 and 500-hPa analyses showing the respective subtropical and polar jets:
250-hPa z and winds 500-hPa z and winds
Dynamic tropopause map shows the properly-sharp troughs and ridges and full
amplitudes of both the polar and subtropical jets
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The dynamic tropopause animation during the 11 May
1999 hailstorm:
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An animation of the dynamic tropopause for the period from December 1, 1998
through February 28, 1999:
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The PV Conundrum
• IPV (Isentropic Potential Vorticity) maps– Many isentropic surfaces have dynamically
significant PV gradients– Hard to know which isentropic surfaces to
look at
The 1.5 PVU contour on the 320 K isentropic surface is…
…identical to the 320 K contour on the 1.5 PVU (tropopause) surface!
Color Fill Version of Tropopause Map
Tropopause Map with Jet Streams
Tropopause Map, hour 00
Tropopause Map, hour 06
Tropopause Map, hour 12
Tropopause Map, hour 18
Tropopause Map, hour 24
Tropopause Map, hour 30
Tropopause Map, hour 36
Tropopause Map, hour 42
Tropopause Map, hour 48
Tropopause Map, hour 48, with jets
Cyclogenesis
• Mutual Amplification– Southerlies assoc. w/ upper-level trough
intensify surface frontal wave– Northerlies assoc. w/ surface frontal wave
intensify upper-level trough
• Superposition– Trough and frontal wave approach and
occlude
Diabatic Processes
• Latent heating max in mid-troposphere– PV increases below LH max– PV decreases above LH max
• It’s as if PV is brought from aloft to low levels by latent heating– Strengthens the surface low and the upper-
level downstream ridge
Diabatic Processes: Diagnosis
• Low-level PV increases
• Upper-level PV decreases
• Tropopause potential temperature increases
Diabatic Processes: Prediction
• Plot low-level equivalent potential temperature instead of potential temperature
• Compare theta-e to the potential temperature of the tropopause
• If theta-e is higher:– Deep tropospheric instability– Moist convection likely, rapid cyclogenesis