From the earliest days of the Atmospheric Radiation Monitoring (ARM) program, measurements of water vapor profiles at high temporal and vertical resolution was deemed to be critical for both the radiative-transfer and cloud-processes studies that the ARM program would undertake. The dream of the ARM program founders was that ground-based remote sensors would measure these profiles routinely, and that the program would be able to move away from the routine launching of radiosondes to characterize the thermodynamic profile above ARM sites. (Read more about Raman lidar development here.)
Early Raman* Lidar Successes
Automated measurements of aerosol extinction and water vapor mixing ratio were new for the scientific community. The ARM Raman lidar allowed a large dataset to be quickly collected, which led to the first climatologies of aerosol extinction and water vapor from Raman lidar. Furthermore, because Raman lidars simultaneously measure aerosol extinction and backscatter coefficients directly (the latter is easily derived from the ASR profile), studies on the variability of the “lidar ratio,” which is the ratio of the extinction to backscatter coefficients, could be performed—a capability not provided by simple backscatter lidar systems.
Raman lidar measurements have been used in many different studies, which testifies to the utility and value of the Raman lidar aerosol and water vapor products.
- Ferrare et al. (2006) showed that the daytime water vapor measurements were about 5%–10% moister than other observations, and retrievals of single scatter albedo that used Raman lidar ratio values typically agreed well with in situ observations.
- Schmid et al. (2006) demonstrated that the lidar observed aerosol extinction and optical depth was still larger than in situ and sun photometer observations.
- Pahlow et al. (2006) used Raman lidar data to characterize the hygroscopic growth of aerosol particles as the relative humidity increases.
New Potential for the Raman Lidar
The installation of new detection electronics, and the subsequent reduction in the amount of neutral density attenuation in some of the detector channels, opened up new Raman lidar research areas. Before the 2004 upgrade, the maximum temporal and spatial resolutions for the water vapor mixing ratio and aerosol scattering ratio (and hence backscatter coefficient) were 1 min and 75 m; after the upgrade, the resolutions were improved to 10 sec and 7.5 m.
This resolution is fast enough to resolve turbulent eddies in the convective boundary layer—sufficient to derive higher-order moments of turbulence in the boundary layer. The advantage of using an automated system like the ARM Raman lidar to study turbulence is that multiple years of data can be included to build a climatology and investigate relationships between different variables; this is true for many other processes beyond turbulence. Some new exciting research areas have also resulted from the higher temporal resolution, especially when observations from other instruments are included in the analysis
- deriving water vapor flux observations using coincident Raman and Doppler lidar measurements and
- characterizing entrainment in cumulus clouds using Raman lidar, an atmospheric emitted radiance interferometer, a cloud radar, a microwave radiometer, and surface measurements.
The ARM program’s primary goal for the Raman lidar was to provide routine measurements of water vapor through the boundary layer across the diurnal cycle. The system’s unique and powerful measurements have been used in an extremely wide range of research—a much larger range of research than was originally anticipated. Value-added Raman lidar data products include time-resolved profiles of
water vapor mixing ratio,
This success of the system, both in terms of its operational uptime and its potential to open up new areas of study and contribute to others, led to the decision to build and deploy a new almost identical Raman lidar in Darwin, Australia (December 2010).
The ARM program took a chance, and invested heavily to advance the Raman lidar technology from a research-only tool into the world’s first operational water vapor and aerosol Raman lidar. The success of the ARM Raman lidar enterprise is perhaps best captured in a quote from Reichardt et al. (2012):
The Cloud and Radiation Testbed Raman Lidar at the Atmospheric Radiation Measurement program’s Southern Great Plains site in Oklahoma, USA, stood out because it added another layer of complexity, i.e., it was monitoring tropospheric humidity and clouds continuously and autonomously. Its success enticed meteorological services around the world to develop and operate similar instruments.
Since that time, the community has seen other automated Raman lidars developed (e.g., in Germany, Switzerland, and the Netherlands), but the multiple-year record of near continuous observations made by the ARM system is totally unique and unprecedented and is clearly one of the shining achievements of the ARM program.
The ARM program constructed an additional Raman lidar system that was deployed at the new ARM site at Okiktok Point along the northern slope of Alaska in late September 2014. Furthermore, the ARM program recently elected to close its Tropical Western Pacific sites, and the Raman lidar at Darwin will be relocated to the new ARM site in the Azores in 2015.
Data are available at: http://www.arm.gov/.