This question is related to atmospheric thermodynamics. A. Explain the reasons w

Question

This question is related to atmospheric thermodynamics.

A. Explain the reasons why potential temperature is a preferred variable to study thermodynamic processes in the lower atmosphere.

B. Describe the difference between potential temperature and virtual potential temperature.

C.  Identify the regions of the world where (i) the virtual potential temperature and potential temperature can be the same, and (i) the virtual potential temperature and potential temperature can be substantially different. Explain your answers.

D. Describe why most atmospheric scientists prefer to employ the virtual potential temperature when examining the variation of temperature with altitude.

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Explanation / Answer

A. Potential temperature is preferred variable to study thermodynamic processes in the lower atmosphere because : potential temperature is not at all affected by the sinking or physical lifting associated with the  large-scale atmospheric turbulence or the flow over obstacles. Any parcel of air that is moving over a small mountain will automatically expand and cool as it ascends up the slope, then compress and finally warm as it descends down on the other side of the slope but the potential temperature will not change in the absence of condensation, heating, evaporation or cooling (processes that exclude these effects are all referred to as dry adiabatic). Thus, potential temperature is very useful and is a method of comparing temperature of air at any two different heights. If the potential temperature is constant or it is increasing with height then we may conclude that the atmosphere is stable.

B. The potential temperature may be defined as the temperature that an unsaturated parcel would have if it is lowered (or raised) to a level of 1000 mb that is 100 kPa. It is conserved that is it remains the same for a parcel of air that is unsaturated and it remains unsaturated throughout as it rises and sinks. On the other hand, the virtual potential temperature is the virtual temperature that any substance would have if it changed adiabatically from its actual state to the standard pressure or P0. It is actually the theoretical potential temperature of a given parcel of dry air that would have the same density as a moist air.

C.  i) The virtual potential temperature and the potential temperature can be the same in the tropics

ii) The virtual potential temperature and the potential temperature can be the different in the polar regions.

D.  The troposphere is much hotter near the Earth's surface because the heat it recieves from the Earth warms up this air. As the height or altitude increases the number of air molecules decreases and so the average of their kinetic energy decreases. The results in a decrease in the air temperature with an increase of height/altitude. But potential temperature is a conservative quantity for adiabatic process in the atmosphere. Potential temperature is not at all affected by the sinking or physical lifting associated with the  large-scale atmospheric turbulence or the flow over obstacles. Any parcel of air that is moving over a small mountain will automatically expand and cool as it ascends up the slope, then compress and finally warm as it descends down on the other side of the slope but the potential temperature will not change. Thus, to calculate the variation of temperature with altitude, virtual potential temperature is preferred because unlike potential temperature, it changes with altitude.

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