Ground-Based Eye-Safe Networkable Micropulse Differential Absorption Lidar (DIAL) for Thermodynamic Profiling in the Lower Troposphere

The importance of thermodynamic profiling has been underscored by two National Research Council reports^1,2 as well as a report to the National Science Foundation and National Weather Service³. A recent review article⁴ details the state of remote sensing of lower tropospheric thermodynamic profiles. In this review paper, it was demonstrated that huge observational gaps exist with respect to thermodynamic profiling in the lower troposphere, and low-cost ground-based passive and active remote sensing systems are suggested as the best means to close these observational gaps.

In an effort to develop low-cost ground based active remote sensing instruments to address the needs of the science community, researchers at Montana State University (MSU) and the National Center for Atmospheric Research (NCAR) are actively developing diode laser-based lidar instrumentation for thermodynamic profiling in the lower troposphere. Diode lasers, tapered semiconductor optical amplifiers, and single photon counting modules based on avalanche photodiodes cover a broad spectral range from 650 – 1000 nm allowing for the development of multiple lidar instruments based on a common instrument architecture.

Researchers at MSU and NCAR have developed an eye-safe diode laser-based micro-pulse DIAL (MPD) for water vapor profiling in the lower troposphere⁵. The MPD utilizes two distributed Bragg reflector lasers to injection seed a tapered semiconductor optical amplifier to create a 7 kHz pulse train of 5 μJ, 1 μs pulses. A shared telescope configuration allows the outgoing beam to be expanded for eye-safe operation while providing stability for unattended operation. Careful spectral filtering in the MPD receiver allows operation in most atmospheric conditions using single photon counting modules to monitor the return signal. Currently, two MPD instruments are operational. The first unit has been deployed at the Front Range Air Pollution and Photochemistry Experiment (FRAPPE), the Plains Elevated Convection at Night (PECAN) experiment, and the Perdigão experiment. The second unit has been deployed at the Land Atmosphere Feedback Experiment (LAFE). For each of these field experiments, the MPD was run unattended and provided near-continuous water vapor profiles from 300 m above the ground level to 4 km (or the cloud base) with 150 m vertical resolution and 5-minute temporal resolution. Three additional MPD instruments are currently under construction and will result in a network of five MPD instruments for ground-based weather and climate research experiments which will be made available to the research community through NCAR.

Taking advantage of the broad spectral coverage and modularity of the MPD architecture, a high spectral resolution lidar (HSRL) has been developed at NCAR for aerosol profiling⁶. The HSRL uses a two channel receiver, one for monitoring the total backscatter signal and a second, which incorporates a rubidium vapor cell, for monitoring the molecular backscatter signal. The HSRL has been incorporated into the second MPD instrument and provides range resolved aerosol backscatter and water vapor profiles.

Current MPD development work is focused on adding temperature profiling capabilities based on measuring the temperature-dependent absorption coefficient of diatomic oxygen (O₂). Using a recently developed perturbative retrieval technique that utilizes ancillary measurements of the aerosol and molecular backscatter provided by the HSRL, it is expected that temperature profiles with ±1 K are achievable below 4 km at 20-minute temporal resolution⁷.

In this talk, an overview of the MPD technique will be presented. The talk will include a discussion on the development and current status of the network of five units for water vapor profiling. Data from recent field experiments will be presented as a demonstration of the instrument capabilities, including the latest water vapor and calibrated aerosol measurements. Finally, the status of the development of the temperature profiling MPD will be discussed.

References

1. National Research Council (U.S.). Committee on Developing Mesoscale Meteorological Observational Capabilities to Meet Multiple National Needs., Observing weather and climate from the ground up: a nationwide network of networks (National Academies Press, Washington, D.C., 2009), pp. xvi, 234 p.
2. National Research Council (U.S.). Committee on Progress and Priorities of U.S. Weather Research and Research-to-Operations Activities., When weather matters: science and services to meet critical societal needs (National Academies Press, Washington, D.C., 2010), pp. xvi, 181 p.
3. Carbone, E., Serafin, J., Hoff, M., Hardesty, M., Carr, F., Weckwerth, T., Koch, S., Benedetti, A., Crewell, S., Cimini, N., Turner, D., Feltz, W., Demoz, B., Wulmeyer, V., Sisterson, D., Ackerman, T., Fabry, F., Knupp, K. "Thermodynamic profiling technologies workshop report to the National Science Foundation and the National Weather Service." (2012).
4. Wulfmeyer, V., Hardesty, R. M., Turner, D. D., Behrendt, A., Cadeddu, M. P., Di Girolamo, Schlussel, P., Van Baelen, J., and Zus, F. "A review of the remote sensing of lower tropospheric thermodynamic profiles and its indispensable role for the understanding and the simulation of water and energy cycles." Reviews of Geophysics 53.3 (2015): 819-895.
5. Spuler, S. M., Repasky, K. S., Morley, B., Moen, D., Hayman, M., & Nehrir, A. R. "Field-deployable diode-laser-based differential absorption lidar (DIAL) for profiling water vapor." Atmospheric Measurement Techniques 8.3 (2015): 1073-1087.
6. Hayman, Matthew, and Spuler, Scott. "Demonstration of a diode-laser-based high spectral resolution lidar (HSRL) for quantitative profiling of clouds and aerosols." Optics express 25.24 (2017): A1096-A1110.
7. Bunn, C. E., Repasky, K. S., Hayman, M., Stillwell, R. A., and Spuler, S. M. "Perturbative solution to the two-component atmosphere DIAL equation for improving the accuracy of the retrieved absorption coefficient." Applied optics 57.16 (2018): 4440-4450.