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How can evaporation of rain slow down cyclone growth?

Evaporation demands energy, condensation releases energy. Both can affect the development of midlatitude cyclones, often associated with clouds and rain. A new study addresses how evaporation of rain contributes to cyclone development. Lead author Kristine Flacké Haualand explains.

evaporation_kristine_haualand
Schematic of midlatitude cyclone without (left) and with (right) the effect of evaporation of rain. Clouds form in the cyclone ascent (arrow) and intensify cyclone development. Evaporation of rain cools the air at low levels. This weakens cyclone ascent and hence also cloud formation and cyclone development.
Photo:
Kristine Flacké Haualand

Content

Midlatitude cyclones are often associated with moist processes related to clouds, ocean, and rain.

In cyclones, there is a lot of moist air that ascends and condenses into clouds. When clouds form, latent heat is released, which fuels midlatitude cyclones and enhances their development.

We find that evaporation of rain opposes this intensifying effect of clouds.

The reason is related to the latent cooling that occurs when rain evaporates on its way down to the surface. While air at lower levels cools, it gets heavier, causing a weakening of cyclone ascent.

The weakened ascent is associated with less cloud condensation, and therefore also less enhancement and cyclone growth. Evaporation of rain is therefore indirectly weakening cyclone growth through its modification of cloud condensation.

Reference

Haualand, K.F. and Spengler, T. (2019): How does latent cooling affect baroclinic development in an idealized framework? Journal of Atmospheric Sciences, https://doi.org/10.1175/JAS-D-18-0372.1

This is also publised at the web-pages of The Bjerknes Centre for climate research.