A thermodynamic question to the pro mets

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Extratropical94
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A thermodynamic question to the pro mets

#1 Postby Extratropical94 » Thu Feb 02, 2017 7:46 am

Hi everyone,

recently my thermodynamics tutor and I came across a problem while discussing one of the exam questions.

Let me introduce you to the task first:

- An air parcel is located at the surface, where the pressure is p = 1000 mbar. The ambient temperature at the surface is T_a = 300K.
The parcel is now heated to a temperature of T_p = 302K, but the pressure of the parcel is still equal to the ambient pressure of p = 1000 mbar.

- The atmospheric lapse rate is 1K / 100 m. The air parcel rises adiabatically because of the temperature difference between the surrounding air and the parcel. There is no humidity involved, so the change in temperature of the parcel is 1K / 100 m, too.

Calculate the vertical speed and the upward acceleration of the parcel due to buoyancy at given heights (z = 2000 m, 5000 m, 10000 m), assuming that the initial vertical motion at the surface is w = 0 m/s.


So far, so good.

The problem lies in the calculation of the acceleration.

The students have tried to solve this in two different ways.

First of all, the buoyancy acceleration is given by this equation:

Image

where the first term is the buoyancy acceleration, ρ_p and ρ_a are the densities of the parcel and the ambient air, and g is the earth's gravitational acceleration.

If you use the ideal gas law:

Image

you can replace the densities with temperatures and you get:

Image

This equations shows that if the temperature difference (T_p - T_a = 2 K) is constant - and we assume that it is as both the parcel and the atmosphere cool by 1 K every 100 m - the buoyancy increases with decreasing ambient temperature or increasing height.

Now, some students used the fact that during dry-adiabatic processes the potential temperature doesn't change. The potential temperature is defined as the temperature a parcel would have if it was lifted or lowered to 1000 mbar adiabatically. Because we start at a pressure level of 1000 mbar, the potential temperature of the atmosphere is equal to its absolute temperature, θ_a = 300 K. The potential temperature of the air parcel is θ_p = 302 K. Due to the given lapse rate and the adiabatic process, those values stay constant even at different height levels.

These students then swapped out the temperature variables from the buoyancy equation by replacing them with the help of this form of the adiabatic equation:

Image

where θ is the potential temperature, T is the absolute temperature, p_0 is the surface pressure (1000 mbar), p the actual pressure and κ is a constant.

They now get the same buoyancy formula, but with θ instead of T.

Image

Maybe you can see where this is going. This equation shows that because both the potential temperature difference (θ_p - θ_a = 2 K) and the ambient potential temperature (θ_a = 300 K) are constant, the buoyancy acceleration stays the same for every height.

This is contrary to the result of the buoyancy equation that relies on the absolute temperature and I fail to see what we are missing or misunderstanding here. The following graphics illustrating the vertical temperature and potential temperature profiles should clarify what the problem with this task is (the height of 5000 m is just an example, the problem occurs at every level).

Vertical temperature profile
Image

Vertical potential temperature profile
Image

Maybe one of you has an idea where we are thinking wrong or why any of those equations cannot be used to solve this problem.

Any help would be greatly appreciated. 8-)
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Re: A thermodynamic question to the pro mets

#2 Postby wxman57 » Thu Feb 02, 2017 9:18 am

I might have had a clue 40 years ago when I was taking dynamic meteorology at A&M. However, as a forecaster, I haven't had any reason to use or think about those equations since I left school in 1980. Sorry...
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Re: A thermodynamic question to the pro mets

#3 Postby wxmann_91 » Wed Feb 22, 2017 6:27 pm

@OP: did you figure this out? I've been stumped by this one myself.

My guess is that the equations that are used in the substitutions are only approximations. For instance, R in the ideal gas equation has dependency on water-vapor concentration, while the exponent kappa in the potential temperature equation is dependent on R and c_p, the latter which varies with ambient pressure. I think in general, meteorologists who use parcel theory in practiec tend to put consider buoyancy in terms of the temperature difference between the parcel and ambient temperature (T_p - T_a), and not consider the temperature difference relative to the ambient temperature itself (T_a - T_p / T_a). I bet if we do some Taylor expansions on the T-based and theta-based equations we'll see the solution works out when considering lower-order terms.

But I know that's not a good enough explanation for the exam.
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