Niebaum, Nils (2024) Constraining Rain Evaporation during the EUREC4A Campaign with the Super-Droplet-Model CLEO. (Master thesis), Christian-Albrechts-Universität, Kiel, Germany, 54 pp.

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Abstract

The evaporation of rain below the cloud base is a crucial process and affects surface precipitation as well as the energy balance of the sub cloud layer of shallow convective trade
cumulus clouds and their mesoscale organization. We estimate rain evaporation and its
associated latent cooling, using the newly developed Super-Droplet-Model CLEO in an 1D
rainshaft implementation, based on thermodynamic profiles and Droplet Size Distributions
(DSD) from 111 individual clouds, measured during the EUREC4A campaign.
Our estimated inter-cloud mean latent cooling of 32.7W m−2 and our rain evaporated fraction of 7.1 % are lower than in previous studies, most likely due to our underestimation of
cloud base liquid water content (LWC) and differing DSDs. We find the cloud base LWC to
be the primary constrain of cloud specific column integrated latent cooling, explaining approximately 50 % of its variability. We identify the gradient of relative humidity to determine
the shape of the cloud’s latent cooling profiles.
With our novel approach we are able to incorporate collisional coalescence, breakup and rebound of droplets. Coalescence systematically reduces laten cooling by up to 10 % near the
surface, while breakup enhances it due to fragmentation of large rain droplets into smaller
ones. However, these effects are small compared to the inter-cloud spread. We hereby give
confidence, that also methods excluding droplet-droplet interactions can yield good estimates of rain evaporation.
Further sensitivity experiments and comparisons with other models and radar-retrieval methods will be necessary to give more robust estimations of rain evaporation and associated
uncertainties.

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