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

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.

The Added Value of Large-eddy and Storm-resolving Models for Simulating Clouds and Precipitation

“…Even in the absence of biases horizontal photon transport will lead to a differential heating of the cloud. In idealized large-eddy simulations this effect has been shown to be important for the cloud field development (Jakub and Mayer 2017;Klinger et al 2017;Klinger et al 2019). Whether these effects substantially distort the representation of the cloud field in less idealized situations, and how to capture these effects in parameterizations,…”

Broadening of the Cloud Droplet Size Distribution due to Thermal Radiative Cooling: Turbulent Parcel Simulations

“…The aforementioned problem is called ''the condensation coalescence bottleneck'' (Brewster 2015;Wang and Grabowski 2009). Previous attempts to pass the bottleneck and simulate more realistic droplet distributions investigated the role of giant cloud condensation nuclei (GCCN) (Feingold et al 1999;Houghton 1938;Johnson 1982;Yin et al 2000), turbulence (Fouxon et al 2002;Grabowski and Wang 2013;Pinsky and Khain 1997) and radiation (Brewster 2015;Guzzi and Rizzi 1980;Klinger et al 2019;Lebo et al 2008; Marquis and Harrington 2005;Rasmussen et al 2002;. We would like to highlight the often underrepresented role of radiation in the diffusional growth process in combination with turbulence.…”

“…The thermal radiative cooling is only relevant near the cloud edges, because at the cloud center the emitted radiation is in balance with the absorbed radiation. The distance from the cloud edges at which cooling is relevant depends on the liquid water path and ranges from 50 to 100 m (Klinger and Mayer 2016). The implementation details of the parcel model are based on Grabowski and Abade (2017).…”

Neighboring Column Approximation—An Improved 3D Thermal Radiative Transfer Approximation for Non‐Rectangular Grids

“…This horizontal transport of radiation causes enhanced cloud top cooling and additional cloud side cooling, so‐called 3D effects, in the thermal spectral range. These 3D thermal heating rates can be as high as several hundred K day (Kablick et al, ; Klinger & Mayer, ) and have been shown to modify cloud circulation and cloud field organization (Guan et al, ; Klinger et al, , ) of shallow cumulus clouds.…”