Cloud droplet growth in shallow cumulus clouds considering 1-D and 3-D thermal radiative effects

Radiation is a key driver for the development of clouds. Solar radiation heats the surface and causes updrafts to rise, thus initializing cloud formation. When a cloud is formed, absorption and emission of thermal radiation at the cloud itself cause heating and cooling rates of several hundred K/d at the interface between cloud and cloudless sky. The magnitude of the cooling rates, compared to the commonly known clear sky cooling of 1-2 K/d, can alter cloud dynamics and microphysics and thus cloud development or lifetime.

The effect of 1-D and 3-D thermal radiation on cloud droplet growth in shallow cumulus clouds is investigated using large eddy simulations with size-resolved cloud microphysics. A two-step approach is used for separating microphysical effects from dynamical feedbacks. In step one, an offline parcel model is used to describe the onset of rain. The growth of cloud droplets to raindrops is simulated with bin-resolved microphysics along previously recorded Lagrangian trajectories. It is shown that thermal heating and cooling rates can enhance droplet growth and raindrop production. The main effect in terms of raindrop production arises from recirculating parcels, where a small number of droplets are exposed to strong thermal cooling at cloud edge.

In the second stage, fully coupled large eddy simulations show that dynamical effects are stronger than microphysical effects, as far as the production of rain is concerned. Three-dimensional thermal radiative effects again exceed one-dimensional thermal radiative effects. Small amounts of rain are produced in more clouds (over a larger area of the domain) when thermal radiation is applied to microphysics. It is shown that an evaporation–circulation feedback reduces the amount of rain produced in simulations where 3-D thermal radiation is applied to microphysics and dynamics, in comparison to where 3-D thermal radiation is only applied to dynamics.

Reference:
Klinger, C., Feingold, G., and Yamaguchi, T.: Cloud droplet growth in shallow cumulus clouds considering 1-D and 3-D thermal radiative effects, Atmos. Chem. Phys., 19, 6295-6313, https://doi.org/10.5194/acp-19-6295-2019, 2019.