2 questions regarding wind shear.

[BigA]

1) Excluding mid-level shear, the wind shear is usually calculated by subtracting the 850 mb wind vector from the 200 mb wind vector, correct? If so, why does a sheared tropical cyclone appear to have all of its convection removed from the LLCC? Why is it not merely that the convection does not reach as high, and the tops of the convection aroudn 200 mb are blown away, with everything below it remaining stacked with the LLCC?

2)Strong extratropical low pressure systems (Nor-Easters, et cetera) often exist in areas of extremely high shear; in fact, I remember reading that they need the shear provided by the jet stream. Why are extratropical cyclones not torn to bits by wind shear the way that tropical cyclones are?

Much thanks.

[theavocado]

The main reason tropical cyclones are effected by shear is that they require a closed vertical circulation with updrafts and downdrafts and the associated release of latent heat. When they are subjected to shear, it's not the convection being blown off that hurts it, but the disruption of the positive feedback circulations that can kill a TC.



Also, the shear doesn't just kick in at 200mb around the jet, it's that those are two levels that are used quite often to calculate shear. The shear can be at much lower levels, and often is, causing tropical forecasters to look at other levels of sheart. It's not uncommon to have the flow be easterly at 700mb, but the surface flow to be out of the south. This is what keeps many weaker systems from developing.

[weatherwindow]

just an addendum to the alligator pear:)....the tc's energy budget is maintained by creating an artificial temperature disparity between the core and the periphery of the system via the release of latent heat of condensation in the updrafts of the core...the heat differential creates the pressure falls which drive the winds.....the extratropical system is driven by the broad temperature differences across the broad areal extent of the typical mid latitude cyclone...the shear which strengthens/fuels the extratropical cyclone(by maintaining the spatial temperature difference)..kills the tropical systems by destroying the vertical stacking which is neccessary to assist in the release of the latent heat of condensation..which maintains the warm core which drives the pressure falls and the wind field.........rich