The EMERALD-2 experiment

The EMERALD-1 study took place in September 2001 over South Australia. In EMERALD-2, the investigation of cirrus cloud continued using basically the same “toolkit”, but a very different geographic area and atmospheric environment. The “toolkit” again were two ARA aircraft, the B200T Super King Air and the G520T Egrett fitted with an identical set of instruments as flown in EMERALD. While EMERALD took place in the basically benign climate over South Australia, in EMERALD-2 the focus was on the cirrus outflow from a severe thunderstorm, nicknamed ‘Hector’, which occurs nearly daily in November and December over the Tiwi Island to the north of Darwin in tropical Northern Australia. The overseas experiment team was the same as in EMERALD, scientists, students and technicians from the University of Wales at Aberystwyth/UK (UWA), the University of Manchester Institute of Technology (UMIST)/UK, Imperial College in London/UK (IC), the German Aerospace Centre (DLR). On the Australian side, apart from the ARA team, the Bureau of Meteorology Research Centre contributed observations and assistance to the project, mainly in terms of radar observations and forecasts. With Darwin being one of the intense observation sites for the long-term US-funded ARM program (‘Atmospheric Radiation Measurement – www.arm.gov’), scientists from ARM also contributed to EMERALD-2 in preparation for a planned major ARM field experiment to be conducted in 2005 or 2006 in Darwin.

As in EMERALD, the acronym stands for “Egrett Microphysics Experiment with Radiation Lidar and Dynamics”. Below is an excerpt from the application for funding for EMERALD-2 outlining the scientific background and objectives:

One of the most challenging problems in climate change prediction is to quantify the influence of deep convection on the distribution of water, in vapour and condensed phase, in the tropical upper troposphere. The main focus of this project will be on the cirrus cloud systems that form as a direct consequence of deep convection. A cirrus cloud is an active regenerating dynamical system in which there is strong coupling between humidity, particles, thermodynamics, dynamics, turbulence, and radiation. This complexity illustrates a large part of the difficulty in accounting for the influence of cirrus clouds in the Earth’s radiative balance and climate. There is presently only a limited understanding of the microphysical-dynamical-radiative interaction, and the cloud structure that manifests from these processes is extremely sensitive to changing environmental conditions. Thus, the uncertainty in climate prediction that is related to cirrus clouds requires an advancement in our understanding of cirrus as a strongly coupled regenerating dynamical system.

The overall aim of EMERALD-II is to advance our understanding of how deep convection influences the distribution of water vapour and the properties of cirrus clouds in the tropical upper troposphere. This will be achieved by conducting an airborne campaign with a variety of complementary measurements that together will reveal the interaction between particle microphysics, humidity, thermodynamics, dynamics/turbulence, and radiation. The scientific issues to be addressed in EMERALD-2 can be divided into the following strongly connected categories.

i) Water Vapour. What is the effect of deep convection on the distribution of water vapour in the tropical upper troposphere? How is this related to the intensity and mesoscale structure of the convection? Does deep convection moisten or dry the upper troposphere? The Egrett will carry two instruments for measuring water: one for vapour, the other for total water content. Flight patterns and timing will be designed for sampling around an entire circumference of a convective system at different stages in its evolution and also following the outflow. This will include vertical profiles. Water vapour measured with radiosondes launched at the Darwin ARM site will also be available. Another issue is to determine how the dynamics of cirrus clouds redistributes water. The measurements of cloud structure, dynamics and humidity will be used to observe how water vapour is redistributed by convective cells, turbulence and precipitation within the cirrus.

ii) Microphysics. What controls the microphysical structure of tropical cirrus clouds? Measurements within the cirrus clouds will provide the phase, concentration, size distribution and crystal habit. The variations in microphysical properties will be put into the context of the overall cloud structure by comparison with the lidar measurements. The lidar depolarisation ratio will be related to the direct measurements of particle phase and shape, so that the lidar can give a more unambiguous view of cloud microphysical structure. The cloud structure will also be related to the dynamics/turbulence measurements within the cloud. These measurements will provide input to detailed microphysical modelling that will be applied to understand the processes controlling the life cycle of cirrus particles: from nucleation through to precipitation and sublimation.

iii) Dynamics. What are the characteristics of the dynamical processes that are important for the generation, regeneration and dissipation of tropical cirrus clouds? How do environmental conditions effect the dynamical activity that is important for maintaining the clouds that are detached from the convective system? The turbulence probes carried on the Egrett will detect a wide spectrum of motions: e.g. gravity waves, Kelvin-Helmholtz instability, convection, and turbulence. This will include processes normally occurring in clear air, as well as the activity induced by cirrus generation and regeneration. Comparison with the lidar measurements will show how the dynamical activity is influencing the cloud structure. Comparison with measurements of particle microphysics and thermodynamics will reveal the interaction between dynamics and particle generation. Ozone measurements on the Egrett will be used as an indicator of airmass origin for mesoscale and long range transport.

iv) Relationship with the Mesoscale Convective Systems. The basic underlying theme of the experiment is to determine the influence of the convection on each of the above processes. It is in this respect that the ground based measurements form the Darwin ARM site will be indispensable. For example the ARM radars will map the mesoscale structure of the convection and also the in-cloud dynamics. These measurements will also provide a guide for flight planning.

v) Radiation and Climate Context. How is the spectrum of IR radiation related to the distribution of water vapour and the microphysical structure of cirrus clouds? In EMERALD-2, the radiative properties of cirrus will be measured in the far IR to improve existing models and provide a basis for the specification of future satellite instruments. These measurements will be related to in-cloud sampling of droplets and ice crystals, and also the cloud geometrical structure that is being revealed by lidar measurements. The radiative properties of clear-sky will also be measured in order to provide a data set from which the uncertainty in the water vapour continuum can be assessed.

Of all field experiments ARA aircraft have taken part in, the environmental and climatic conditions prevailing in Darwin at the start of the wet season were the most difficult ones. The very high humidity, coupled with the frequent thunderstorms was an immense challenge for flight planning, as well as for the instrumentation in – and indeed the crews of – the research aircraft, particularly in the Egrett. For the Egrett, the flights led from getting ready and taking-off in tropical condition to extremes at the other end of the scale at altitude, with outside air temperatures as low as -60˚C and in-cockpit temperatures of around 0˚C. To cope with such environmental conditions was also a major challenge for the instrumentation and other electronics. Despite these challenges, few failures were encountered and a very unique and comprehensive data set was collected during 50 mission hour over 14 flights with both aircraft.

The operations base for EMERALD-2 was an open hangar/shelter on the RAAF Base Darwin plus two office containers which proved to be an ideal and perfect environment.

As in EMERALD, the basic flight pattern consisted of both aircraft flying the same flight tarck at the same time with the Egrett below, within or above the cirrus cloud at altitudes between 7 and 15km, and the King Air flying approximately 4 to 6km below.

Flight strategy used in EMERALD-2.

As the experiment were only taking December, not many results can be shown here. The latest examples and information about EMERALD-2 is available on the website at the University of Wales at http://users.aber.ac.uk/ccc97/darwin/Emerald_II.htm.