dynamic tropopause analysis; what is the dynamic tropopause? a level (not at a constant height or...
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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)
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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
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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
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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
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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
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Coupling index:Theta at the tropopauseMinus the equivalentPotential temperature atLow levels(a poor man’s lifted index)
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December 30-31, 1993 SLPAnd 925 hPa theta
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An example illustrates the detail of the dynamic
tropopause (1.5 potential vorticity units) that is lacking in a constant pressure analysis
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250 and 500-hPa analyses showing the respective subtropical and polar jets:
250-hPa z and winds 500-hPa z and winds
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Dynamic tropopause map shows the properly-sharp troughs and ridges and full
amplitudes of both the polar and subtropical jets
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QuickTime™ and aGIF decompressor
are needed to see this picture.
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QuickTime™ and aGIF decompressor
are needed to see this picture.
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QuickTime™ and aGIF decompressor
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The dynamic tropopause animation during the 11 May
1999 hailstorm:
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QuickTime™ and aGIF decompressor
are needed to see this picture.
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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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QuickTime™ and aGIF decompressor
are needed to see this picture.
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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
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The 1.5 PVU contour on the 320 K isentropic surface is…
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…identical to the 320 K contour on the 1.5 PVU (tropopause) surface!
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Color Fill Version of Tropopause Map
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Tropopause Map with Jet Streams
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Tropopause Map, hour 00
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Tropopause Map, hour 06
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Tropopause Map, hour 12
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Tropopause Map, hour 18
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Tropopause Map, hour 24
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Tropopause Map, hour 30
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Tropopause Map, hour 36
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Tropopause Map, hour 42
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Tropopause Map, hour 48
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Tropopause Map, hour 48, with jets
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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
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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
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Diabatic Processes: Diagnosis
• Low-level PV increases
• Upper-level PV decreases
• Tropopause potential temperature increases
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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