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International Space Station

International Space Station overflying a region of small-scale cirrus and cumulus clouds. Image source: NASA

Earth Observation and Climate Studies

Staff: Dr Pete Hargrave, Prof Peter Ade
Collaborators: Prof Stefan Buehler (University of Hamburg); Patrick Eriksson (Chalmers University); Dr Clare Lee, Dr Anthony Baran, Dr Ian Rule (UK Met Office); Dr Frank Evans (University of Colorado at Boulder); Dr Tom Bradshaw (Rutherford Appleton Laboratory); Dr Lionel Duband (CEA-SBT)

Ice clouds in the global climate system

Cirrus (ice) clouds play a crucial role in the global climate system and energy budget of the atmosphere, in that they reflect near-infrared radiation back to space (cooling effect), whilst reflecting thermal emission from the Earth’s surface back to Earth (warming effect). The net effect on the atmosphere is very important to understand. It depends on the cloud’s horizontal extent, vertical position, ice water content, and ice microphysical properties, all of which influence the cloud’s optical thickness.

Observation from high altitude

Observation of cirrus clouds from a high altitude aircraft or satellite platform. Some of the upwelling radiation from water vapour in the troposphere is scattered by ice clouds.

Cirrus clouds also affect the atmospheric energy budget by releasing latent heat during the depositional growth of ice particles, and by absorbing heat during their sublimation. Ice particles, if large enough, may also settle through the atmosphere and enhance precipitation generation in lower clouds by the seeder-feeder mechanism.

The importance of clouds in weather and climate processes has been emphasised in the latest Report of the Intergovernmental Panel on Climate Changes (IPCC), stating that “... cloud feedbacks remain the largest source of uncertainty in climate sensitivity estimates”. Particularly large uncertainties are associated with clouds that consist partly or entirely of ice particles.

Measurement of ice cloud properties

radiometric brightness spectra

Model of radiometric brightness for various cirrus formations. (Click to enlarge)

The far-infrared (FIR) wavelength region is ideally suited to probe the properties of cirrus clouds. Ice particles (diameters typically <1mm) in cirrus clouds are strong scatterers in the FIR. The instrumentation and technologies developed by the Astronomy Instrumentation Group in Cardiff can be applied to studies of the upper atmosphere via a high-altitude aircraft or satellite platform. The lower atmosphere is opaque at these wavelengths. Upwelling radiation from the low to middle troposphere is scattered by high-altitude cirrus clouds, and we can measure the suppression in brightness temperature of the transmitted light. The frequency dependence of this scattering gives us information on the particle size distribution and ice water path (IWP), and measurement of the polarization signature will provide information on mean particle shape.

Instrumentation for ice cloud observation

Wide field of view (fore-)optics.

Wide field of view fore-optics, coupled to detector array via re-imaging optics and reflective cold-stop.

Traditionally, instruments proposed for studies of cirrus clouds have been based around a heterodyne scanning radiometer. This type of instrument typically employs a single-pixel detector and a scan mechanism to cover a conical or across-track scan pattern.

We are working on new instrument concepts, enabled by the detector and quasioptics technology developments in Cardiff, and by advances in cryogenic technology by our collaborators. The concept is based around a wide (~25°) instantaneous field of view antenna, coupled to large arrays of Kinetic Inductance Detectors (KIDs) in a pushbroom configuration. Development and evaluation of large arrays for applications in space-based Earth observation is currently funded via a European Union FP-7 grant, “SPACEKIDS”. This instrument configuration avoids the requirements for mechanical scanning, and drastically increases the inherent detector sensitivity, as well as the detector integration time. It also allows observation at frequencies above 1 THz, where the performance and stability of heterodyne receivers are presently poor. Complete coverage of a cloud scene can be obtained using a single line of detectors, aligned in the across-track (perpendicular to flight) direction.

Possible multispectral/hyperspectral imaging configuration

Left: Prototype array of Lumped-Element Kinetic Inductance Detectors. Right: Indication of possibility of multispectral/hyperspectral imaging configuration. Flight direction is indicated by the arrow. Across-track direction is spatial domain, along-track direction is spectral domain.

Using large 2-dimensional arrays of detectors allows us to use each across-track row as a separate spectral channel. This can be implemented using either the world-leading spectral filters developed in Cardiff as simple strips placed across the array, or we could employ a reflective grating in place of the reflective cold stop. Such an instrument will require cooling, but we are working with groups that have successfully flown mechanical cryocoolers and sorption coolers on the Herschel and Planck satellites.