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Centre for Atmospheric Science

EMERALD Project

Introduction

Cloud system observed during EMERALD flight.
Cloud system observed during EMERALD flight.

The EMERALD projects were airborne measurement campaigns designed to study dynamical, microphysical and infra-red radiative properties of cirrus clouds, using both in-situ and remote measurement techniques. In the case of EMERALD 1 this was mid latitude frontal cirrus, and during EMERALD 2 tropical convective outflow cirrus. Both projects took place in Australia, with EMERALD 1 based in Adelaide during September 2001 and EMERALD 2 based in Darwin during November and December 2002. Both these projects involved University of Wales - Aberystwyth (Overall PI), UMIST, Imperial College London, and Airborne Research Australia.

The ACTIVE project followed on from the work of the EMERALD projects.

Methodology and Instrumentation
Two instrumented aircraft were used during these projects, both operated by Airborne Research Australia, a Beech B200T Super King Air, and the Grob Egrett T520. The King Air is a pressurised twin turbo prop aircraft, capable of carrying a scientific payload of 900Kg to an altitude of 10Km. For these projects the King Air was carrying the Aberystwyth vertical pointing depolarisation lidar system, and was flown below the cirrus.

Egrett and King Air working together collecting data.

Egrett and King Air working together collecting data.

The Egrett is a unique high altitude aircraft, capable of carrying a 500Kg scientific payload to 15Km. During the EMERALD projects, the Egrett was equipped with the Cloud Particle Imager and Forward Scattering Spectrometer Probe (UMIST) to measure cloud microphysical properties, turbulence and temperature probes (Aberystwyth/ARA), TAFTS (Imperial) which is a far infra-red spectrometer, an ozone monitor and frost point hygrometer (Aberystwyth/DLR) and a water vapour TDL (Aberystwyth). The Egrett was flown in, above and below the cloud in a co-ordinated flight plan with the King Air, such that the Egrett was always directly above the King Air during measurement legs (at least most of the time). Both aircraft were equipped with sophisticated differential GPS systems to allow accurate position and altitude to be determined. For EMERALD 2, high resolution cloud radar data was also available, provided by the Australian Bureau of Meteorology. The radar and lidar measurements, which were available in the King Air in real time allowed regions of interest within the cloud to be identified and the Egrett to be directed to fly through these regions. For EMERALD 2, The lidar and radar data allowed the in-situ measurements to be interpreted in the context of large scale cirrus structure and evolution. The radar data also provided the necessary information about the location of active convective cells to allow safe operation of the aircraft.

Aircraft used during the EMERALD projects.
Aircraft used during the EMERALD projects.

Results
Results from EMERALD 1 show significant variation in microphysical properties throughout the cirrus cloud, as illustrated here. These variations in microphysical properties are related to the dynamic structure of the cloud, and indicate the processes which are taking place. For example the measurements support ice crystal formation by homogeneous freezing of water droplets, which subsequently grow into Bullet Rosette crystals. This mechanism was typically seen to occur in updraft regions within the cloud where the water supersaturation was large enough to activate cloud condensation nuclei to form droplets. Overall, clearly identifiable crystal types were seen to make up only about 15% of the total ice particle concentration, but this varied greatly throughout the cloud, on length scales of 5-10Km. The vast majority of particles were small spheroidal / irregular particles in the size range 70-90 microns, which are present in all regions of the cloud, but much more numerous in updraft regions. While it is still early stages in the analysis of data from EMERALD 2, the results were quite different, with a significantly greater number of plate-like crystals (very few of these were observed in EMERLAD 1), and chain aggregates of plate-like crystals. Here are a few examples of the crystal types observed during EMERALD 2. It was also seen that the crystal types observed were dependant on the position in the outflow, relative to the convective column. Generally the further away from the convective column, the more the cirrus properties were similar to those measured during EMERLD 1, with small irregulars and Rosettes being observed.