Abstract
Abstract. 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. Droplets grow to larger size bins in the 10–30 µm radius range. 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. These recirculating parcels, comprising about 6 %–7 % of all parcels investigated, make up 45 % of the rain for the no-radiation simulation and up to 60 % when 3-D radiative effects are considered. The effect of 3-D thermal radiation on rain production is stronger than that of 1-D thermal radiation. Three-dimensional thermal radiation can enhance the rain amount up to 40 % compared to standard droplet growth without radiative effects in this idealized framework. 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. The dynamical feedback is shown to be an enhanced cloud circulation with stronger subsiding shells at the cloud edges due to thermal cooling and stronger updraft velocities in the cloud center. 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.
Highlights
Cloud droplets form in saturated environments by condensation of water vapor on cloud condensation nuclei (CCN)
We focus on the effects of thermal radiation on microphysics, neglecting any changes in cloud development that would occur due to feedbacks within a large eddy simulation (LES) framework
– When thermal radiation is coupled to microphysics, there is a small increase in rain production for 1-D radiative effects (1DD_1DM)
Summary
Cloud droplets form in saturated environments by condensation of water vapor on cloud condensation nuclei (CCN). Harrington et al (2000) used a large eddy simulation (LES) and an independent parcel model, including bin microphysics and radiative effects on droplet growth They showed that only parcel trajectories spending long periods of time at cloud top (10 min or more) can cause the droplet size spectrum to broaden via radiative cooling. They found an earlier onset of drizzle production; this occurred along parcels that would produce drizzle anyhow. The recent theoretical study of Brewster (2015) and direct numerical simulations by de Lozar and Muessle (2016) re-emphasize the hypothesis that thermal radiation might influence droplet growth significantly and lead to a broadening of the droplet size spectra and enhance the formation of precipitation.
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