Infrared CIrrus CLoud Experiment (ICICLE).

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The High Resolution Dynamics and Limb Sounder (HIRDLS) is a joint NERC-NASA funded instrument which will measure atmospheric limb emission in 21 spectral channels at very high vertical (~1 km) and spatial resolution (9000 profiles per day). In preparation for launch we have developed the Monte Carlo Cloud Scattering (McClouds) algorithm to retrieve cirrus cloud properties (ice crystal effective radius, cloud top height and cloud optical depth). A very strong scientific driver for this programme is the speculation that cirrus cloud properties are modified by nitric acid originating from anthropogenic pollutants. Tantalizing evidence of this nitric acid effect has been made in aircraft studies but no definitive global observations have been made that would unambiguously relate cirrus properties to the source regions of nitrogen containing gases. This proposal will fund the retrieval of cirrus cloud properties from global HIRDLS data. Near simultaneous measurements of temperature, water vapour and nitric acid made by the MLS instrument will be used to characterize the cirrus clouds in terms of the temperature and gas vapour pressures.