EMERALD is a project to conduct an airborne measurement campaign to study cirrus clouds formed by the Polar front Jet Stream over South Australia. Both in-situ sampling and remote sensing were used to investigate the interactions between particle microphysics, dynamics, turbulence, and radiation. Two separate aircraft, carrying complementary instruments, were employed to probe the clouds from below, within and above. These aircraft are the Egrett and the King Air, both supplied by Airborne Research Australia (ARA). The measurement campaign was conducted from ARA headquarters at Parafield airport in Adelaide Australia.

The project was performed as a consortium of three institutions: University of Wales Aberystwyth (UWA), University of Manchester Institute of Science and Technology (UMIST), and Imperial College (IC). UMIST were responsible for measurements of particle microphysics on the Egrett. IC used their TAFTS instrument on the Egrett for spectral measurements of infrared radiation. UWA installed a lidar on the King Air, and a TDL spectrometer for water vapour measurements on the Egrett. Additional instruments, available through international collaborations, provided measurements on the Egrett of water vapour, dynamics/turbulence, temperature and ozone.

In the EMERALD campaign, airborne lidar observations of cloud structure were coincident with in-cloud sampling of particles, dynamics, humidity, and thermodynamics. The cloud structure will be directly related to the in-situ measurements, and thus the interpretation will be much less ambiguous. Also, the variations in particle characteristics and structure will be related to the resulting effect on the spectrum of IR radiation. These observations will provide initialisation, constraint and validation for the numerical simulation of both microphysics and radiative transfer. This modelling will then provide a basis for interpretation of the measurements.

The measurements to be discussed in this paper were from the microphysics probes on board the EGRETT aircraft (a SPEC corporation CPI and a Forward Scattering Spectrometer Probe (FSSP)) related to the turbulence measurements on this aircraft. These were put into the wider context of the overall cloud structure using the data from the LIDAR flown on the King air aircraft below the cirrus clouds. A total of 11 separate case studies were performed over the course of the project in September 2001. It was found that the clouds consisted mostly of irregular ice crystals with spheroids, columns and bullet rosettes also present but in much lower concentrations. A few regions containing supercooled water were observed, mostly at warmer temperatures and close to the base of generating cells. Generally, however, the clouds were completely glaciated. The paper will describe the detail of the cloud microphysical structure as well as observation of the microphysical development including the precipitation formation processes. The factors determining the depth of penetration of the precipitation below the cloud will be investigated. The implications of the results for the computation of the radiative properties of cirrus clouds, together with their influence on water vapour distribution in the upper troposphere will be discussed.