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26 April 2013 [Publications]
Condensation-driven winds in ACP

A new paper in a leading journal, Atmospheric Chemistry and Physics (ACP), challenges centuries' old views on what determines dominant patterns of atmospheric circulation. Wind patterns are of crucial importance for regional climates: they determine the temperature regime and bring rain to land from the ocean. But do we really understand how winds form? In our paper we propose a new mechanism of wind formation based on pressure gradients generated by condensation of water vapor. This theory underlines that land-cover, especially forest vegetation, play a far greater role in sustaining wind and rainfall patterns than previously thought.

Air is set in motion by pressure gradients. The conventional view on wind formation considers air motions caused by pressure gradients associated with spatial temperature differences: warm air rises, while cold air sinks. This conventional temperature gradient approach can be used with thermodynamics-based analyses to allow estimation of the maximum possible efficiency of converting heat to work in the atmosphere. But it does not provide information about the actual efficiency of the atmospheric heat engine. Our new theory suggests that this efficiency is likely to be small (close to zero) as most winds are generated not by temperature but by the distinct processes related to moisture condensation.

Our theory relies on the phase changes of water and water vapor. It is well-known that air pressure declines with height, but this pressure gradient cannot generate kinetic energy. When the air rises, the upward pressure gradient force acting on the gas is balanced by gravity. Water vapor changes this balance. In contrast to the dry air, water vapor pressure declines much more rapidly with increasing height due to condensation. This water vapor pressure gradient exceeds the gravity force and can generate kinetic energy. It can be compared to a spring that is being decompressed while the air rises: it is this spring that drives the air motion.Since the condition of hydrostatic equilibrium prevents the formation of strong vertical winds, the dynamics lead to formation of horizontal pressure gradients and winds.We have developed a consistent set of relationships that indicate the magnitude of this effect. The estimate of global circulation power derived from the new theory matches empirical observations. Such estimates had not been possible before.

Our propositions have generated interest as well as criticisms. The paper has been under open peer-review for two and a half years (much longer than normal). During this time it secured many online comments and was intensely discussed in the blogosphere. The Editor Comment accompanying the paper reflects the controversy and alerts the reader that this publication of an "entirely new view of what may be driving dynamics in the atmosphere" should be seen as "a call for a further development" of the ideas. We agree and support this call.

The theory merits deep investigation by the scientific community. The most important implication of the new theory concerns the potential re-evaluation of the role of land cover and especially forests in determining regional winds and moisture transport. Forests represent both a store and a flux of moisture, which create persistent low pressure zones on land. This causes winds to blow from the ocean to land carrying moisture which feeds the terrestrial water cycle. Deforestation represents a significantly greater threat for the regional water abundance and climate stability than previously recognized. Preserving and recovering forest cover may prove to be the cheapest and most reliable means of ensuring regional environmental sustainability. We hope our ideas will be examined objectively and urgently.

Our paper was extensively discussed at Climate Etc. Below is a selection of comments that might serve as entry points to the discussion and provide a brief guide over the paper.We would also recommended to read the post itself (available as PDF). Media coverage of the paper can be found here.

What is the gap that the new theory fills?
What is the power of global circulation?
Circulation power and air velocity: what is being predicted
Windy continents and seas

Condensation-driven winds and the first law of thermodynamics
A brief summary
An extension

Why we needed Section 3.3
Thought experiments

Hydrostatic Equilibrium
Non-equilibrium equation for water vapor
Another aspect

Equation 34
Discussion Summary 1
Discussion Summary 2

Latent heat
Links to several comments

Thread discussing the effect of droplets

Further research
Isothermal case
Double ITCZ

Biotic pump and condensation-driven winds
Amazon rainforest and trade winds
Rainfall distribution over land
Precipitation over Australia