The atmosphere — modelling cirrus clouds

Cirrus are high-level clouds, which are mainly composed of ice crystals, creating a wispy-like appearance. They appear at altitudes usually greater than six kilometres, and are spatially and temporally well distributed around the globe. At any one time cirrus can cover up to 30% of the Earth's surface.

While lower clouds — such as cumulus — more directly affect our weather, by blocking the sun and bringing conditions such as rain and snow, cirrus chiefly affect climate and global warming. Cirrus can reflect solar radiation back into space, thereby cooling the atmosphere. They can also trap the Earth's outgoing radiation, creating a warming effect. These two opposing processes depend on the cloud's properties, such as the cloud's horizontal or vertical position, ice water content and ice particle microphysical properties.

Understanding cirrus clouds

The measurement of cirrus properties is crucial in improving and validating models, and consequently our understanding of the impact of cirrus on the climate.

We use airborne data from the FAAM (Facility for Airborne Atmospheric Measurement) BAe 146 aircraft to obtain remote sensing and in-situ measurements of cirrus.

The following are used for cirrus studies:

• Visible, infrared and far-infrared spectrometers and a microwave radiometer for radiative measurements
• A full suite of cloud physics instrumentation for in situ measurements of aerosols, liquid water and ice particles
• In situ aircraft and chemistry instrumentation for temperature, pressure, humidity, ozone and CO, and dropsondes to supplement the data gathered during aircraft profiles

Cirrus properties can vary greatly, so it is important to study the clouds using many different wavelength regions, each of which is sensitive to particular parameters. For example, visible and infrared techniques can be used to detect the presence of ice clouds and to determine the cloud-top altitude and optical depth, whereas longer wavelengths can probe into the clouds and are more sensitive to larger particle sizes and ice-water content.