Untangling microphysical impacts on shallow convective clouds

Narodowe Centrum Nauki, OPUS

Project type: Research project

Affiliation: IGF, ZFA

Project realisation: Sept. 29, 2017 - Sept. 28, 2019

prof. dr hab. Hanna Pawłowska Project leader
dr Piotr Dziekan Investigator

Research project objectives

Impact of atmospheric aerosols on cloud and precipitation processes continues to attract attention of the atmospheric science community. The overall goal of the proposed research is to study the influence of microphysical processes on macroscopic properties of shallow warm convective clouds (e.g. cloud cover, mean precipitation, liquid water path) with the use of the state-of-the-art numerical model and a novel assessment methodology. Despite significant progress, the impact of cloud condensation nuclei (CCN) on macroscopic cloud properties remains poorly quantified. We will include the impact of turbulence on collision-coalescence that enhances the drizzle/rain formation, an aspect that has to be treated jointly with the aerosol effects.

Assessing microphysical impacts is important for climate projections. Extracting the impacts with high confidence requires substantial effort – for instance, applying an ensemble of simulations. In the proposed research we will apply a novel methodology, referred to as the piggybacking, that allows assessment with unprecedented fidelity and enables separating purely microphysical effects from the impact of the cloud dynamics.

Research project methodology

A Large Eddy Simulation (LES) framework with a novel particle-based aerosol-cloud microphysics (called the super-droplet method) that is recently being developed at the Institute of Geophysics (Faculty of Physics, University of Warsaw) will be used in the proposed research. The new microphysics is a unique method to represent all particles involved in cloud formation and drizzle/rain development. Using super-droplet method to represent microphysics makes the LES model a perfect tool to study micro- and macrophysics of shallow convective clouds and aerosol- cloud-precipitation interactions. The model will be supplemented with new functionalities to take into account the additional physical processes such as subgrid-scale coupling between turbulence and droplet diffusional and collisional growth. We will apply a novel modeling methodology to provide a confident assessment of the effect of cloud microphysics in large-eddy simulations of cloud fields. The method, developed by Prof. Wojciech Grabowski from the U.S. National Center for Atmospheric Research is referred to as the piggybacking. The idea is to use two sets of thermodynamic variables driven by different microphysical schemes or by a single scheme with different parameters. The first set is coupled to the dynamics as in the standard LES simulation, and the second set is applied diagnostically — that is, driven by the flow but without the feedback on the flow dynamics. Having the two schemes operating in the same flow pattern allows for extracting the impact with high confidence. Because of high variability of the convective cloud field, the traditional approach (i.e., parallel simulations with modified microphysics) provides an uncertain estimate of the impact of cloud microphysics on macroscopic cloud field properties. The piggybacking method can detect minuscule changes of the macroscopic cloud field properties despite their large temporal fluctuations and different evolutions in parallel simulations.

Expected impact of the research project

The proposed research focuses on warm (ice-free) shallow convective clouds. These clouds are abundant in the tropics and subtropics, and their interaction with radiation makes them an important component of the climate system. Representation of those clouds in weather and climate models is uncertain because of their small-scale nature and complex interactions between physical processes involved. These clouds will be the last component of the climate system to be simulated with confidence in global climate models due to the requirement of an extremely high spatial resolution. The proposed project will not only advance our understanding on fundamental physical processes, but will also help assessing their importance for climate and climate change. The project will also significantly contribute to development of the novel simulation framework at the Institute of Geophysics, and help to promote it across the atmospheric modeling community.