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I am reading about the interior temperature structure of Jupiter. It says in various texts that there are regions in Jupiter's atmosphere which are well-approximated by a dry adiabatic lapse rate.
Does this mean that in these regions there is convection happening? Is adiabatic just a general synonym for convection?
An adiabatic process in physics is at first a process where a system of limited size does not exchange any heat with the outside world, or where the energy exchange timescale is very long compared to the interesting timescale under which the adiabatic process occurs.
It is also loosely used as a term meaning 'constant in the framework of a certain approximation'. One sees this often used in quantum mechanics where it essentially means 'quasi-static', or in plasma physics where particles can gyrate in a field gradient. If the field gradient is much smaller than the gyration radius, such that every individual particle gyration experiences a near uniform field, then the particle experiences an adiabatic change of the underlying field.
Coming back to planetary/stellar atmospheres, here the two concepts are used interchangeably. Should a piece of atmosphere experience a super-adiabatic temperature gradient, then this situation is usually unstable towards buoyant instabilities. As soon as convective motion sets in, the temperature gradient will be brought down towards the adiabatic one.
This is a very general process and happens in stars as well as planets, if no other physics is present. Thus it is used interchangeably, while people using it are aware that this is a bit sloppy.
I would say that an adiabatic lapse rate is indeed synonymous with convection, though what is not known when you just say there is convection is the turnover timescale. The timescale could be so long that it wouldn't look much like convection, but then, that can happen in convective atmospheres like that of the Sun-- in some layers, the convection can be so slow you wouldn't notice it on a sound crossing time across the scale of gradients. But the reason I'm claiming they are synonymous is that convection is about the only reason you can think of why there would be an adiabatic lapse rate for the overall gradient in the atmosphere.
The way adiabatic convection works is, you can think of every parcel of gas in the convective region as if it was the same parcel, just found at a different time. In short, every parcel goes everywhere eventually. What's more, as the parcel goes everywhere, it remains insulated, so it's just the same parcel expanding and contracting with no heat exchange. That's what sets the adiabatic gradient everywhere. Another way to say that is, the entropy per gram is the same everywhere, because entropy doesn't change if there is no heat transport, and if the entropy of each parcel stays the same, and if each parcel goes everywhere, then the entropy per gram is constant everywhere. That's what an adiabatic lapse rate means.
Now, if there is not convection, so parcels just stay put, how would they know to have the same entropy as each other? So it is generally convection that induces an adiabatic lapse rate, though the convection could still be rather slow if the heat transport rate is also very slow.
No doubt there are complications I am leaving out, like what happens when there are changes in ionization or phase which alter what we mean by the adiabatic lapse rate, so I'm just talking about the simplest situation where we are not considering such details. We are basically assuming the parcel is always in thermodynamic equilibrium, even though its environment is slowly changing.
CONVECTION is a PROCESS: It is the vertical movement of matter (due ultimately to gravity) as a result of its relative density. A density change is often (but not only) caused by heating, which causes expansion of a parcel of fluid (gas or liquid) or even of some plastic solids. When a suspended solid or parcel of fluid is heated, the resultant expansion of that parcel leaves it less dense (d=m/v) than its surroundings, which tend to 'squeeze' it, causing it to move 'upward' in the gravitational field.
ADIABATIC is an adjective, describing a phenomenon of temperature variation which is NOT due to the exchange of heat. An example is the increasing temperature in the TROPOSPHERE (the lower reaches of the earth's atmosphere). The tropospheric temperature at 10,000m above the earth's surface is naturally cooler than its temperature at a height of 5,000m; and this is due almost entirely to the relative compression - due to gravity - of the gases at these levels.
