Dynamic tropopause analysis; What is the dynamic tropopause? A level (not at a constant height or pressure) at which the gradients of potential vorticity

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

QuickTime™ and aGIF decompressor

are needed to see this picture.





The dynamic tropopause animation during the 11 May

1999 hailstorm:



An animation of the dynamic tropopause for the period from December 1, 1998

through February 28, 1999:



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 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

Documents

Dynamic tropopause analysis; What is the dynamic tropopause? A level (not at a constant height or pressure) at which the gradients of potential vorticity