The Evolution of Lidar Networks: A U.S. Perspective

Atmospheric aerosols and clouds play an important role in climate by directly scattering and absorbing sunlight. Aerosol-cloud interactions modify the particle properties causing indirect effects, alter aerosol deposition and rainfall, and contribute substantially to the uncertainties in predicting climate effects. Aerosols also affect air quality and both constituents modulate boundary layer dynamics to a certain extent, in turn impacting aerosol transport and aerosol-cloud interactions. The spatiotemporal distribution of aerosol and cloud layers is thus important, and lidar remains the primary instrument for determining the vertical distribution of aerosols and thin clouds. Lidar still provides important information for opaque cloud layers by determining base heights. Combined lidar and radar data provide a comprehensive coverage of aerosol and cloud vertical distributions, and thus locations where and when they interact. Lidar data provide a means of constraining column aerosol loading observations (e.g. AOD) to vertical extents. In addition, lidar has proven effective at determining a proxy for boundary layer height by examining aerosol gradients and/or cloud base heights in lidar profiles. Polarized lidars provide additional information on particle shape, allowing estimates of cloud phase and separation of dust or smoke from sulfate and sea salt aerosols. Given the importance of lidar observations and a critical maturation in technology and retrieval techniques, several organizations began to build operational lidar networks around 2000. The European Aerosol Research Lidar Network (EARLINET) was started as a research focused network of advanced lidars across Europe. The Asian Dust Lidar Network (ADNET) was also developed as a regional network providing lidar profiles of dust and pollution across Eastern Asia. In the US, the NASA Micro Pulse Lidar Network (MPLNET) was created to provide global lidar profiling at key sites in the NASA Aerosol Robotic Network (AERONET). The Network for the Detection of Atmospheric Composition Change (NDACC) pre-dates these networks and many sites have lidar, but it is not strictly a lidar network and at the time focused less on the lower troposphere. Finally, there were already existing ceilometer networks operated by various meteorological agencies, but in particular here in the US the profile data has not been available. The ceilometer networks were used to provide only clouds base heights and estimates of PBL height. Thus the distinguishing feature between lidar and ceilometer networks was historically the ability to actually provide profile information (in addition to differences in wavelength, and advanced retrievals such as the raman technique). As time progressed, each lidar network matured and developed more operational capabilities and data sets, coupled with viable data centers providing DAAC services and access to near-real-time (NRT) data. In 2008 under WMO guidance, the Global Atmospheric Watch (GAW) Aerosol Lidar Observation Network (GALION) was formed as a global network made up of the existing lidar networks. The goal was to share information, best practices, and develop frameworks and techniques for quality data. GALION grew to include several other regional lidar networks, and has led to a vast increase in quality lidar sites worldwide. In addition, the GALION effort has contributed to several meteorological agencies creating or upgrading their existing ceilometer networks to provide the full signal profile, further expanding the availability of global ground based lidar data. Despite these successes globally, here in the US and North America (NA) in general, there remains a lack of dense regional lidar coverage. MPLNET is based in the US, but it is a global network with sparse regional coverage and no funding mandate to provide regional coverage in NA. Environment Canada had built the Canadian Operational Aerosol Lidar Network (CORALNET) as its contribution to GALION, and had lidar coverage from the west to east coast. However, CORALNET was discontinued several years ago due to changes within the department. There are many NWS ceilometers operating in the US but none provide full profile data due to data communication constraints. In addition to these operational efforts, the NOAA CREST project has been funding lidar sites at participating Universities which does provide additional sites and is part of GALION. However, the CREST lidar sites are focused more on research and education, and are not operational in the sense of larger federally funded activities. This talk will present an overview of the history, achievements, and future of lidar networks with a focus on the US role in GALION. Lingering problems will be discussed, as well as plans that have been initiated to overcome these hurdles. As a mature lidar network, MPLNET can provide a framework and resources (e.g. data center assistance) to help coalesce the varied US lidar network activities. GALION colleagues also offer a blueprint to building a mature heterogeneous lidar network across the country.