How can evaporation of rain calm down the weather?

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Illustration picture of evaporation of rain, modified from Alastair Rae.

Midlatitude weather is highly dominated by cyclones that typically form over the ocean and intensify toward land. When hitting land, these cyclones can have a high impact on infrastructure and people’s daily lives. Within the cyclones, several complex processes and interactions occur, such as moist effects related to clouds and rain. To accurately represent the puzzling picture of midlatitude cyclones, we therefore need a good understanding of how the moist processes influence the cyclones’ development and intensification.

One of the most important ingredients for cyclone intensification is cloud condensation. When cyclones move over open water, moist air is transported into the cyclone before it rises and cools at higher altitudes. As the moist air cools, it condenses into cloud droplets. During the phase change from vapour to liquid form, latent heat is released, and the cyclone thereby intensifies. But clouds are also often associated with rain, which may have opposite effects on the cyclone development.

While rain falls toward the ground, the surrounding air usually gets warmer and some rain evaporates. Since the phase change from liquid to vapour is the opposite of condensation, the ambient air is now cooled instead of heated – just as wet clothes make you cold while they dry on your body. But how does the below-cloud cooling associated with evaporation of rain influence cyclone development?

Since cold air is heavier than warm air, evaporative cooling at low levels weakens the upward transport of moist, maritime air from the surface to the cloud layer. The consequence is less cloud formation. Since clouds enhance cyclone growth, a reduction of clouds means less cyclone intensification. Hence, evaporation of rain slows down cyclone development by weakening the intensifying effect of clouds.

Schematic of a midlatitude cyclone without (left) and with (right) the effect of evaporation of rain. Clouds form in the ascent region of the cyclone and intensify cyclone development. Evaporation of rain cools the air at low levels, which weakens cyclone updraft and hence also cloud formation and cyclone development.

 

Evaporative effects on midlatitude cyclones are weak compared to the direct effects of cloud condensation. But with high ambitions for the advances of atmospheric sciences, we also need to know the smaller pieces of the complex puzzle of midlatitude weather. Evaporation of rain is one of these pieces, and there are still many aspects of it we do not yet understand.

Reference: Haualand and Spengler (2019)

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Kristine Flacké Haualand

I am a PhD student in meteorology at the Geophysical Institute at the University of Bergen. My research focuses on weather dynamics in the midlatitudes, including air-sea interactions and storm development.
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