CWVC Results and findings

Almost a third of frontal clouds forming over the UK (that have parts colder than -10 degrees Celsius) consist of water in a mixture of both ice and liquid phases. These 'mixed phase' clouds are more common than previously believed.

The programme developed new ways of detecting mixed phase clouds remotely with radar, and developed new equipment to fly on aircraft to study the very small ice crystals that are present.

Detailed microphysical modelling, combined with actual measurements, shows there are regions of 'embedded convection' present in frontal clouds, where extra (or secondary) ice particle production (produced through collisions between drops and falling snow) is important. The programme identified this for the first time. The process affects the formation of precipitation and heat transfer in these clouds and so it is important that it is properly represented in models used for forecasting weather and future climate.

Researchers produced and used synthetic analogues of ice crystals in the laboratory to better understand how small ice crystals found in cirrus clouds scatter light.

Researchers gathered important new information that helps us understand how sunlight is transmitted through high altitude Cirrus clouds. They investigated jet stream cirrus forming ahead of fronts over South Australia, and compared this with clouds in the anvil outflow from deep tropical convective storms forming over northern Australia. In jet stream cirrus, small irregular and bullet rosette ice crystals formed mainly due to haze particles freezing. A relatively small number of dust particles acted as ice nuclei. In the tropical anvil cirrus, close to the convective turret, ice crystals were mainly aggregate crystals, probably detrained from the main cloud. Further away, haze drops freezing again produced new ice crystals.

Modelling studies have shown that gravity waves affect the microphysics of cirrus clouds, and that ice nuclei can deter haze droplets from freezing. This affects the size distribution of the ice crystals, and optical properties of the cirrus.

Scientists have developed a highly efficient model of tropical cirrus, suitable for use within global climate models. The model has been used to investigate how water vapour is transported into the lower stratosphere. It found good agreement with satellite measurements.

Models of how water vapour is transported into the tropical stratosphere have shown that the air does not simply rise equally at all longitudes. Instead, the air entering the lower stratosphere preferentially moves through a cold region over the West Pacific, where low temperatures freeze water out of the air. More water vapour is transported during the northern hemisphere summer. El-Nino also alters this transport.

Researchers used a unique instrument, the Tropospheric Airborne Fourier Transform Spectrometer, to study the strength of the water vapour continuum in the far-infrared part of the spectrum where the properties of water vapour are particularly badly understood. These results were the first of their kind, and demonstrated that models were overestimating the foreign-broadened water vapour continuum by 20 per cent.

For the first time, researchers with the programme were able to show experimentally that water vapour dimers (two water molecules bound together) have distinct absorption features at wavelengths where solar radiation is absorbed, producing a significant 2 per cent increase in the absorption of solar radiation.