From: kjun@typhoon.atmos.colostate.edu (Pat Fitzpatrick)
Newsgroups: sci.geo.meteorology
Subject: Re: Anti-Hurricane in the North Atlantic?
Date: 27 Aug 92 17:09:48 GMT
Distribution: na
Organization: Colorado State University -- Atmos Sci

In article <1992Aug26.235531.27850@meteor.wisc.edu> stvjas@meteor.wisc.edu (Stephen Jascourt) writes:
>In article <Aug26.181941.41887@yuma.ACNS.ColoState.EDU> kjun@typhoon.atmos.colostate.edu (Pat Fitzpatrick) writes:
>>, and has mid-latitude charcteristics such as being
>>cold-cored (a hurricane is warm-core) and being baroclinic (a hurricane
>>is essentially barotropic). By baroclinic, we mean a sheared environment
>>with temperature contrast. A strictly barotropic atmosphere has no
>>temperature contrast with no wind shear.
>>
>A hurricane is plenty baroclinic, but being warm core, the wind becomes
>more anticyclonic with increasing height. A perfectly axisymmetric storm,
>either cold or warm core, is equivalent barotropic-- the temperature and
>pressure varies only radially, so the thermal shear is parallel to the 
>geostrophic wind at all levels. Spiral bands break up axisymmetry-- the 
>bands have both a radial and azimuthal component and all fields have 
>cross-band gradients.  A strong hurricane like Andrew is probably close 
>to equivalent barotropic because the mean radial gradients are much larger 
>than the local azimuthal gradients (of pot temp, mix.rat., winds, p', etc.). 
>

OK, this is correct. Just some clarifications to avoid confusion

barotropic == no isotherms on a horizontal plane implying no thermal contrast
              which implies no vertical wind shear. Isotherms parallel to
              isobars in the vertical

"equivalent" barotropic == There are isotherms on a horizontal plane, but
             parallel to isobars. Isotherms are NOT parralel in vertical.

baroclinic == Isotherms are not parallel in horizontal or vertical. Implies
              vertical wind shear.

A hurricane forms in the tropics, which is generally equivalent barotropic 
and sometimes almost strictly barotropic. The hurricane's main source of
energy is from the warm ocean waters, which is the "fuel" for thunderstorms
which release latent heat in the air, and if left alone in a non-sheared
environment, can lower surface pressures which enhances convection and cyclonic
rotation, and a feedback mechanism begins. (Of course, this is more complicated
than I'm presenting, just want to show the basic differences; for example,
for a tropical cyclonegenesis to occur, a pre-exising meso-scale vortex is
needed). A hurricane is a warm-core system.

In contrast, a system that is cold-core, requires a different genesis
mechanism. For cyclogenesis of a cold-core cyclone, 
the thermal contrast and wind-shear that occurs
in a baroclinic atmosphere is needed. It's 
a process known as baroclinic instability.

A warm-core system also is strongest at the surface and weakens with height,
and in many occasions actually becomes an anticyclone aloft (like a hurricane).
A cold-core cyclone is weakest at the surface and becomes an increasingly
stronger cyclone with height.

One form of a cold-core system, the TUTT cyclone, occurs aloft initially.
If it becomes especially vigorous, it can burrow to the ground, causing a 
weak cyclone at the surface. If this happens over the ocean and enough
convection forms, the latent heat released by the thunderstorms can
transform this cold core system to a warm core system and poof! a tropical
cyclone forms. This accounts for about 10-15% of all tropical cyclones
in the Pacific, and even fewer in the Atlantic. But, who knows, it may
be happening in the Atlantic now as we speak........

Of course, this is a very simplistic discussion, and these processes are much
more complicated than I have shown here, but it's just to give some
basic ideas to the non or new meteorology folks :-)