Cloud processes

Boundary-layer clouds

• Cloud microphysical and macrophysical structures
• Cloud-aerosol-precipitation-radiation interactions

Ice/mixed-phase clouds

The formation and development of ice particles in cold clouds is fundamental to understanding how precipitation is formed, and how these clouds interact with radiation. But our understanding of these ice-phase processes is very poor. Some topics we are working on are:

• Characteristics of ice crystal aggregates: most precipitating ice particles are clusters of several ice crystals stuck together. We are using radar measurements at 3 wavelengths (3, 35 and 94 GHz) from Chilbolton to characterise the geometry and size/fall-speed distributions of aggregates. Current work includes analysis of the detailed 3-wavelength Doppler spectra to resolve the behaviour of particles in different parts of the size distribution to better understand the aggregation process
• Vapour growth of ice crystals. The early growth of ice in clouds occurs through diffusion of water vapour to the crystal surface. The problem boils down to calculating a characteristic length scale for the ice particle (its capacitance). The difficulty lies in the complex shapes of ice particles, which make analytical solutions impossible, and traditional numerical calculations error-prone. We have developed a new technique to sample water molecule trajectories, which is very accurate and completely flexible with regard to the geometry. We are currently working on extending this analysis to a wide range of particle types, and investigating how best to parameterise this for models.
• Mixed-phase clouds. Clouds containing supercooled liquid water droplets and ice crystals are a common occurrence in the atmosphere. We have shown that at temperatures above -20°C ice is formed almost exclusively in supercooled clouds. However these clouds are very challenging to simulate, requiring that the nucleation of ice, its subsequent growth and sedimentation, and the dynamics of the cloud all be correctly captured. We are working on ways of representing the particle size spectrum and sub-grid structure of ice growth rates in GCMs, and have used in-situ observations to argue that the singular model of ice nucleation cannot capture the formation of ice in long-lived mixed-phase clouds. We are also looking at dual-polarisation radar techniques to identify mixed-phase regions in deep layer clouds which lidar cannot penetrate.
• Fall speeds and orientations of ice particles. The sedimentation of ice particles is key to understanding their evolution, and has substantial knock-on effects on the growth and evaporation/melting of the particle. However this is very difficult to observe. We are reviving the technique of producing models of ice crystals, and dropping them through viscous fluids in controlled lab conditions to better understand their aerodynamics and drag behaviour.

Convective storms

• Processes that govern the morophology and evolution of convective storms