From: men@bjerknes.colorado.edu (Matthew Newman)
Subject: Circulation cells in the atmosphere (was Re: Why do Hurricanes Travel West?)
Organization: NOAA Climate Research Division
Date: Wed, 26 Aug 1992 16:00:24 GMT

In article <17farjINNpgi@darkstar.UCSC.EDU> gross@maxwell.ucsc.edu (Mike Gross) writes:

>  In <1992Aug26.035349.3990@physics.ucla.edu> reynolds@physics.ucla.edu (Reynolds) writes:

>  >Seriously, if the earth were not rotating, the prevailing winds would
>  >be from north to south (the hot air at the equator rises, and the
>  >cold air at the poles sinks) in the Northern Hemisphere, and from
>  >south to north in the Southern Hemisphere (i.e., from the poles to
>  >the equator.

>  >But the earth does rotate, and the coriolis force breaks this 
>  >convective cell into three cells: 1 - near the equator, the surface
>  >winds are from east to west.  2 - at mid-latitudes, the surface
>  >winds are from west to east.  3 - near the poles, the prevailing
>  >surface winds are again from east to west.

>  >Now, why the number is three and not five or even ten, I have no
>  >idea.  Perhaps someone could enlighten us as the the current thinking,
>  >if there is any.

>  >Anthony Reynolds
>  >reynolds@physics.ucla.edu

>  This is certainly well out of my field (I'm a cosmologist), but I thought
>  the number of cells had to do with the radius of curvature of the planet
>  surface. For example, a glance at the Voyager pictures of Jupiter will 
>  reveal that that planet  has on the order of a dozen different bands,
>  each circulating in the opposite direction of its neighbors. Saturn and
>  Uranus also show banded structure, but the contrast between the bands
>  is not so pronounced, so the pictures aren't quite so spectacular.

>  There are clearly visible eddies in between the bands, and this is a sign
>  that there is a large velocity gradient.

>  Corrections/clarifications are requested.

>  Mike Gross
>  gross@maxwell.ucsc.edu 


It is still unclear what determines the number of jets (and cells) in
the atmosphere of a planet, but the planetary radius (which I assume
is what you mean by the radius of curvature) is not really the
determining factor.

In fact, Gareth Williams at GFDL had an article in Nature about 
ten years ago in which he took an Earth-like model atmosphere and 
tried to simulate all other planetary circulations by varying 
rotation rate and axial tilt only.  One can disagree with his 
interpretation of his results, but he did get a circulation with 
several cells for the appropriate Jovian parameters.

He had earlier suggested that the number of jets is determined by a
combination of the effects of turbulence, surface drag, and the
Coriolis effect.  The idea basically is that the Coriolis effect
varies with latitude (moving polewards also means moving closer to the
axis of rotation) but not with longitude.  This allows the turbulence
to eventually extend in the east-west direction (to become a jet) but
restricts its north-south width.  

Alternatively, it has been suggested by others, including Andy Ingersoll 
at Cal Tech, that the Jovian atmospheres may behave fundamentally different
because they are 'deep' atmospheres (i.e., the atmospheric depth is a
significant fraction of the planetary radius) as opposed to 'shallow'
(i.e., the atmospheric depth is very small compared to the planetary
radius), terrestrial atmospheres.  Also, for both Jupiter and Saturn,
most of the energy that drives the atmospheres comes from within (as
opposed to coming from above, from the Sun).  Ingersoll suggests that
this could result in an atmospheric structure that consists of a series of
nested cylinders, rotating about the planetary axis of rotation.
Thus, the banded structure results from the intersection of these
cylinders with the cloud tops.  This is pretty counterintuitive, but
there are observations suggesting that it may be possible.  (See
Ingersoll's article in Science, a few years ago, for more information).


Matt Newman
men@noaacrd.colorado.edu







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