Storminess and the Inefficient Atmospheric Heat Engine

November 6th, 2013 by Roy W. Spencer, Ph. D.

There is an aspect of weather generation and storminess that I never see discussed, and which I think could be important to understand when discussing possible changes in weather with climate change.

It has long been known that the atmosphere is a very inefficient heat engine. The rate of kinetic energy generation supporting the atmospheric circulation is only about 1% of the rate of solar heating (e.g. Peixoto J P and Oort A H 1992 Physics of Climate). Since most of what we perceive as weather is related to wind, one way or another, we can roughly say that only 1% of the solar energy absorbed by the Earth goes into the creation of weather systems.

I suspect that when we see periods of greater or lesser storminess on a global basis, we are seeing fluctuations in this efficiency. If air mass temperature differences build up over a period of days or weeks, say with cold winter air masses over N. America or Asia intensifying in the winter, the temperature contrast (available energy) for the creation of storms increases. (I would imagine that storminess was considerably more energetic during the ice age(s)…I’m sure someone has researched this issue before.)

Since it takes time for low pressure systems to form and draw upon this potential energy from the temperature contrast between air masses, there is a time lag involved in the cycles of storminess. The potential energy built up is released as low pressure areas form and their circulations cause warm air to rise up and flow over the cold air masses, and the cold air slides under and displaces the warm air masses.

Global warming theory has traditionally expected that the equator-to-pole gradient in temperature would be reduced during warming. Observations suggest this has indeed occurred, at least over the Northern Hemisphere. So, the energy available for storm formation has decreased. I suspect the effect is small, though. (Storminess is also related to the tropospheric vertical temperature lapse rate…a steeper lapse rate can support more kinetic energy generation).

[By the way, I don’t think the decrease in the equator-to-pole temperature contrast is a fingerprint of human-induced warming…it’s a reflection of the geographic distribution of land, which will warm faster than the ocean no matter the cause of the warming.]

What is interesting about the 1% efficiency is how small that number is, which is related to the fact that weather is driven by relatively small temperature contrasts over relatively large distances: a few degrees over hundreds or thousands of kilometers. In a car engine, which can also be considered a heat engine with about 25% efficiency, the mechanical work that is done is drawing on temperature differences of hundreds of degrees over a few inches.

If the atmospheric heat engine efficiency were to increase from an average of 1% to 2%, that would be a doubling of the kinetic energy involved in weather systems…yet the thermodynamic efficiency would still be very low.

What does all of this have to do with global warming? I don’t know…I just think it’s interesting.

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