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In 2005, the National Center for Atmospheric (NCAR) completed the retrofitting of a Gulfstream-V (GV) research aircraft, equipping it to take detailed measurements of the atmosphere. Funded by the National Science Foundation (NSF), funds were held in reserve to support development of additional research tools for the plane. Among the instruments deemed useful but not included in the original GV specifications was a lidar system. Similar to radars, lidars identify certain characteristics of the atmosphere, such as the amount of water vapor or aerosols in the atmosphere, by detecting backscatter of electromagnetic radiation, specifically light scattered back from the outgoing laser pulses.
“Lidars are often used to measure cloud-free and thin-cloud regions of the atmosphere; they emit much shorter wavelengths than radars so they measure different atmospheric characteristics. Lidars “see” reflections of objects such as air molecules, aerosols – for example, pollen and black carbon particulates – as well as ice crystals and water droplets in clouds,” Scott Spuler, an EOL research engineer, explains. “If used together, lidar and radar give a more complete picture of the atmosphere.”
In assessing the future trajectory of Earth’s climate, one of the largest remaining scientific uncertainties that is critical for improving climate model capabilities is the proper understanding and treatment of cloud processes and, in particular, the interactions between aerosols and clouds. Aerosols affect cloud formation and evolution, and hence have strong indirect effects on the reflective properties of clouds, which may affect the amount of incoming or outgoing radiation, and even on the timing and magnitude of precipitation.
“Models and field observations have shown that an increase in human-made aerosols can ultimately lead to higher concentrations of cloud drops. In turn, this can enhance the cloud’s reflective properties (albedo), thereby leading to a cooling effect and affecting precipitation,” says Jothiram Vivekanandan (Vivek), a senior scientist in EOL. “Because these processes are highly dependent on the microphysics of the atmosphere, the interaction of aerosols and clouds is currently treated very crudely in the global models used to estimate future climate states. In order to make progress in global modeling, detailed measurements will be necessary to constrain representation of cloud microphysics in global climate models.”
Recognizing the utility of having such a device on the GV, the University of Wisconsin Lidar Group, led by Edwin Eloranta, a pioneer in the lidar field, responded to an NSF solicitation to build innovative instrumentation for use on the GV. The group submitted a winning proposal to create a High Spectral Resolution Lidar (HSRL). The lidar, designed and built for both airborne and ground-based use, provides unique measurements of backscatter and extinction of aerosols and other microscale atmospheric components. Used in combination with radar measurements, which estimate microphysical properties of clouds, the HSRL gives the GV an ability to estimate aerosol characteristics at microscales.
NCAR and UW scientists began working together in 2010 to make the hardware and software changes required to create a GV-compatible lidar system. Not only did the HSRL have to be able to withstand in-flight jostling of the instrument and potentially drastic changes in cabin temperature, but it had to be synchronized to the internal navigation such that if the plane was maneuvering at an angle, the system readouts remained accurate. Additionally, ongoing software design enhancements will result in automation that allows the system to be run remotely, while providing data images to an on-the-ground engineering team in real time.
The HSRL was used in its first field campaign in February 2012, one of a suite of instruments operated as part of TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogen species and Oxygenated volatile organic carbon). The TORERO mission took place in Costa Rica and Chile, with researchers studying the release and transport of gases and aerosols into the atmosphere during the region’s peak period of biologic growth. The HSRL provided a long-distance perspective on the atmosphere, with the laser “looking” either upward or downward from portholes in the NSF/NCAR GV. The data output provided researchers with details on cloud cover, as well as specifics on the scattering and extinction of aerosols in the atmosphere. Not only did this information prove useful for answering essential research questions, but it was also used to guide the aircraft into regions of interest such as cirrus clouds and aerosol layers. Flying at varying heights from the boundary layer to 50,000 ft, the GV, equipped with the HSRL and other TORERO instruments, collected data along a broad cross-section to sample the atmosphere at a variety of heights, observing the layers of clouds, aerosols, and gases.
In addition to providing the HSRL for use by researchers, data collected by the instrument, such as that from the TORERO experiment, are available to the science community from EOL’s online data catalog (catalog.eol.ucar.edu/torero). Another important component of EOL’s service to the community is educating scientists about the potential and capabilities of tools such as HSRL. For the HSRL, EOL staff is helping identify how research might benefit from the lidar’s use, as well as showing how the instrument complements radar capabilities. Through this education process, NCAR/EOL hopes to inspire new and interesting research questions, says Spuler.