The formation of ice particles in cold clouds is a vital component of the hydrological cycle. These particles redistribute water in the troposphere, as well as reflecting and absorbing visible and infrared light. Recent studies have shown that the evolution of these particles and the way in which they are distributed throughout a cloud layer is important if we are going to correctly simulate the earth's present and future climate. This evolution and distribution is a subject of considerable uncertainty however.

Remote-sensing techniques such as radar are a powerful tool to probe the ice particles in natural clouds. The small amount of power reflected back to the radar by the ice particles contains information on the mass, shape and dimensions of the particles, while changes in the phase of the reflected wave contain information on how fast those particles fall (the Doppler spectrum).

In this project we will develop a new technique to derive the properties of ice particles from radar measurements at 3 different wavelengths. While in the past many assumptions would need to be made a-priori when interpreting the radar data, the extra information content of the full Doppler spectrum at 3 wavelengths allows us to straightforwardly resolve these uncertainties.

Once the technique has been developed, we can derive the microphysical properties of the cloud, such as the distribution of ice particles with size, the relationship between a particle's mass and its size, and how fast the particles fall as a function of their size. This information is key to the accurate representation of clouds and precipitation in numerical weather prediction and climate models, and the results will be used to validate/improve those models, in collaboration with the Met Office.

We can also use the microphysical information to develop an improved understanding of the mechanisms by which ice particles grow and evolve in clouds, and use this to constrain currently-unknown parameters such as the aggregation efficiency ('stickiness') of natural ice crystals.

Objectives: The aim of this proposal is to use Doppler radar measurements at 3 wavelengths to measure the characteristics of ice particles in clouds with greater accuracy than has previously been possible, and to use those measurements to understand how the ice particles evolve within a cloud layer. Specifically, we will:

Develop a new technique to probe the microphysics of ice clouds, exploiting the information content in multiwavelength radar Doppler spectra

Measure the size spectrum of ice particles in clouds

Determine the relationship between particle mass, particle fall speed and particle size in clouds

Validate microwave scattering models of ice particles

Test the realism of ice microphysics parameterisations in numerical weather prediction and climate models

Test the a-priori assumptions which underpin simpler single- and dual-wavelength radar techniques

Investigate the evolution of ice particles as they grow and fall in clouds, and constrain their growth rates.