New formula supersedes Priestley-Taylor equation for evaporation

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Most people imagine that as temperature rises, evaporation increases. While that is true under specific circumstances, in general this is not the case. This is because when evaporation increases it causes a surface to cool and thereby reducing the temperature.

Currently, practical applications based on the theories/models of wet surface evaporation use the Priestly-Taylor model to get results. This model states that net radiation and surface temperature (or near surface air temperature) are independent inputs that determine the rate of evaporation.

However net radiation and surface temperature are not independent of each other and neither are they independent of evaporation. The complex interaction between radiation, evaporation and surface temperature has been the subject of intense study for 50 years and is of particular importance to the hydrological, meteorological and agricultural communities.

Researchers from the Centre of Excellence for Climate Extremes developed a new theoretical formula that acknowledges the various interactions that occur. For example, as surface temperature increases a larger fraction of net radiation goes into the process of evaporation. However, at the same time, the increase in surface temperature causes the net radiation to decline because of an increase in outgoing long-wave radiation. The consequence of this interaction is that the evaporation that occurs must be the maximum possible value.

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To test this new formulation the researchers used newly available satellite data for radiation to investigate how radiation, evaporation and temperature interact over the global oceans.

The results showed that at global and local scales the observed evaporation from oceans occurs at the maximum possible rate calculated by the new formulation, which means that the evaporation process provided the maximum possible cooling at the ocean surface.

These results suggest that the concept of maximum evaporation reported here is a natural attribute of an extensive wet evaporating surface. This provides a fundamental new insight as to how radiation, evaporation and temperature are interlinked, which opens the way to use these results to investigate how temperature might evolve into the future.

Reference: Yang Y., Roderick M. L. (2019). Radiation, surface temperature and evaporation over wet surfaces. Q J R Meteorol Soc. 2019;1–12. https://doi.org/10.1002/qj.3481

 

Originally published by CLEX, 6 March 2019.