Research project

The way clouds interact with the climate system through precipitation and radiative processes is strongly influenced by their microphysical properties. Cloud microphysical properties are characterized by the droplet-size spectrum, that is, the local (in time and space) size distribution of cloud droplets. The droplet-size spectrum is shaped by activation and subsequent condensational growth of cloud droplets. These microphysical processes are driven by the ambient supersaturation (i.e., the relative degree of water-vapor pressure exceeding the saturation value), which varies in space and time due to small- and large-scale turbulent fluctuations in the cloud environment. This feature has far-reaching implications for the droplet spectrum evolution. Despite intensive effort the problem of droplet spectral evolution in turbulent convective clouds remains unresolved. The overall goal of the present research proposal is to advance our understanding of the fundamental role of multiscale turbulence on the processes that shape the droplet spectrum in convective clouds.

An efficient way to study clouds is through numerical simulations. Cloud turbulence and its effects on all processes shaping the droplet size spectra (e.g., eddy hopping, entrainment at the cloud edge followed by turbulent mixing, and in-cloud activation/deactivation of cloud droplets) pose a formidable challenge and require comprehensive computational models. A promising Lagrangian Cloud Model (LCM), where an Eulerian Large-Eddy Simulations (LES) of the cloud-scale flow is coupled to a Lagrangian aerosol-cloud microphysics based on superdroplets, has been recently developed at the Institute of Geophysics, Faculty of Physics, University of Warsaw (IGFUW). In the course of the project, the present IGFUW’s LCM will be extended to account for turbulence in the full range of spatial and temporal scales. This will produce a novel numerical tool, termed the Stochastic Lagrangian Cloud Model (SLCM). It extends the classical LCM by introducing a physically sound stochastic Lagrangian description of the microphysical processes and turbulent transport occurring at scales that are not resolved by LES of the cloud-scale flow.

The stochastic microphysical schemes underlying the SLCM will be first developed and implemented in simple test frameworks: the idealized cloud parcel model and the kinematic cloud-scale prescribed flow. Subsequently the stochastic microphysical scheme will be coupled to realistic LES of cloud-scale flow. This full SLCM will be applied to simulations of a small cumulus cloud. The main microphysical output will be the droplet size spectrum and its spatial variability at the cloud scale. To contrast the impact of different sub-grid scale models and stochastic microphysical schemes on the evolution of the droplet spectrum, a novel assessment method, called piggybacking, will be used.

The project will apply a robust and physically sound SLCM to the study of the impact of turbulence on the cloud microphysics over a wide range of spatial scales. It brings the most advanced stochastic Lagrangian techniques developed in the context of dispersed multiphase and reactive turbulent flows to the domain of atmospheric cloud physics. This project will not only contribute to the fundamental understanding of multiscale processes that affect growth of cloud droplets by condensation in natural clouds, but will also help to develop improved detailed microphysical representations of shallow convective clouds for modeling weather and climate.