Opening a bigger window into the clouds

Cloud droplets form, grow, and evaporate in turbulent clouds at scales considerably smaller than what conventional weather or climate models can resolve. The use of particle-based direct numerical simulations (DNS) has become an important tool to examine the microphysics and turbulence of clouds, as it can resolve all the scales of turbulence to produce a close-up look.

Since DNS models are small and cannot account for natural convection — which sustains turbulence in atmospheric flows — researchers have relied on momentum forcing to sustain turbulence in microscale DNS models. But modeling of atmospheric clouds requires that thermodynamic fluctuations be sustained in addition to velocity fluctuations.

Sharfuddin et al. applied thermodynamic forcing in a particle-based DNS model to study turbulence-cloud-aerosol interactions while tracking the dynamics of sixteen million aerosol particles and droplets. The authors implemented low-wavenumber forcing, infrequently repeating cycles over a distance, for both the momentum and thermodynamic fields.

“In this study, temperature and moisture fluctuations are maintained while being fully coupled with aerosol activation into cloud droplet and condensational growth,” said author Foluso Ladeinde.

The model also accounts for evaporation and droplet deactivation back into aerosol particles.

“The impact is striking,” said Ladeinde. “Without thermodynamic forcing, the cloud settles into a saturated state. With forcing, supersaturation fluctuates, droplets grow and shrink at different rates, and the droplet size distribution broadens.”

The team also found that thermodynamic forcing enables a DNS model to reproduce the three regimes of cloud activation and deactivation, stages that had been reported in earlier studies.

“These details are key to determining the lifetime of clouds,” said Ladeinde.

Source: “The role of thermodynamic forcing in direct numerical simulations of turbulence-cloud microphysics interactions,” by Abdullah Al Muti Sharfuddin, Foluso Ladeinde, Yangang Liu, Satoshi Endo, Fan Yang, Mohammad Atif, Meifeng Lin, and Wenhai Li, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0349785 .