Dynamic tropopause analysis; What is the dynamic tropopause?

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