Using high-speed holography to quantify secondary ice processes

Ice and mixed phase clouds have important effects in the atmosphere. They interact with incoming solar radiation and outgoing terrestrial radiation, and generate precipitation which impacts on the surface. Within these clouds, ice can form from one of two major pathways, i) primary ice formation and ii) secondary ice formation.

Primary ice formation occurs when ice forms due the direct deposition of water from the gas to solid phase, or when liquid water freezes. Primary ice formation is mediated by aerosols which may, depending on their chemical composition, facilitate condensation, deposition, and freezing. Much research activity has and continues to focus on developing an understanding of the role of different aerosol types on these processes, with the ultimate goal of improving the accuracy of predicted cloud particle properties (number, size, and morphology).

Despite these crucial advances in understanding primary ice formation, in-situ observations from research aircraft routinely highlight major discrepancies between the anticipated number density of ice particles in clouds versus that measured. These discrepancies have been reported for over 3 decades and have been the subject of much debate. Several theories have been developed to attempt to explain these discrepancies using variety of ambient datasets and laboratory studies. These theories, which typically involve particle collisions or freezing processes, act to increase the number of ice particle in clouds via fragmentation processes, and are termed ice-multiplication processes, or secondary ice processes.

Unfortunately, the various secondary ice theories are highly uncertain, poorly quantified (if at all), and unverified by observations due to the difficulty of direct observation. Various studies which have attempted to assess the importance of these processes, despite the underlying uncertainty, show that there is large scope for impact of secondary ice on cloud evolution, precipitation formation, and lightning generation.

We propose a collection of work to address the longstanding issue of secondary ice. The foundations for this project are based on the use of a new high-speed digital holographic microscope. This system provides the ability to provide high magnification imagery for entire 3-D volumes at high temporal resolution. When coupled with complementary apparatus (e.g. droplet generators, cold stages, environmental chambers), we will recreate and fully observe key microphysical processes of interest in the laboratory. The results of these laboratory experiments will be implemented in a series of numerical models to assess the impact of these processes on cloud properties, cloud lifetime, and precipitation formation.

National weather services (e.g. the Met Office) will benefit from this research through an improved representation of secondary ice processes, and how they should be represented in various numerical models. This benefit will be felt in both numerical weather forecasting and climate prediction, providing further benefits to the government, public and private sector organisations and businesses. As key partners, Met Office involvement has been explicitly incorporated via inclusion in the investigator team and planned research activities.

Hydrological agencies will also benefit from the impacts of the proposed research. This includes governmental and international environment agencies with an interest in flood risk and hydrology. Our research will lead to improved forecasts of precipitation from ice clouds, which can impact on surface hydrological processes and forecasting flood risks.