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 the ARM sites.
It was recognized early in the program that Raman* lidar held promise for fulfilling this dream. Following an initial period of instrument development, the first Raman lidar was delivered in the fall of 1995. The automated nature of the Raman lidar provided multiple-day views of water vapor mixing ratio and aerosol scattering ratio and extinction. Initial aerosol data analysis showed the tremendous promise of the technology, but there were some artifacts in the data. The ARM Raman lidar was designed to measure water vapor first and foremost; aerosol and cloud observations were considered of secondary importance. This ultimately led to some choices in lidar design that would turn out to hinder the derivation of aerosol scattering ratio (and hence backscatter coefficient) and extinction coefficient from the data.
Upgrades in 2004
During the early 2000s, a company in Berlin (Licel GbR) developed a new set of detection electronics that combined both analog-to-digital (AD) and photon-counting (PC) electronics into single package. The two detection systems have different strengths: AD is well suited for large signals (such as those typical for the lower troposphere) but PC is better suited for small signals (such as those in the upper troposphere or from weak scattering processes). Because the combined AD and PC electronics have a larger dynamic range, this allowed some of the neutral density filters in some channels of the lidar, which were originally in place to prevent the saturation of original PC electronics, to be removed. Removing the neutral density filters greatly increased the signal strengths in the aerosol and nitrogen channels by factors of 10–20, which greatly reduced the noise levels of the water vapor and aerosol data products.
During that same time period, other research groups demonstrated two intriguing new measurements that could be made with the Raman lidar: (a) atmospheric temperature measurements using rotational Raman scattering by nitrogen and oxygen molecules and (b) measurements of cloud liquid water using Raman scattering. The decision was made to add three new channels (two for the rotational Raman scattering and one for liquid water) to the Raman lidar.
New Potential for the Raman Lidar
The installation of the Licel detection electronics, and the subsequent reduction in the amount of neutral density attenuation in some of the detector channels, opened up a new area of research for the Raman lidar. 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 Raman lidar to study turbulence is that multiple years of data can be included to build a climatology and investigate relationships between different variables.
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 lidar’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,
- relative humidity,
- aerosol scattering ratio,
- aerosol volume backscatter coefficient,
- aerosol extinction coefficient,
- aerosol optical depth,
- linear depolarization ratio,
- cloud mask and cloud base height, and
- temperature (currently under development).
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 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. There will be challenges running these advanced lidar systems in these harsh environments, but we have confidence that these challenges will be overcome and that the scientific benefit will be huge. This will result in three autonomous water vapor and aerosol Raman lidars operating in the ARM program, which is quite an achievement given the uncertainties surrounding whether a Raman lidar could be made operational in the early 1990s.