Water vapor is one of the fundamental thermodynamic variables that define the state of the atmosphere. It is highly variable in space and time and influences many important processes related to weather and climate. The ability to continuously measure water vapor in the lower troposphere with high vertical resolution has been identified as a priority observation needed by the weather forecasting, atmospheric science, and climate science communities.
Currently, radiosondes and satellite-based measurements form the backbone of observations used for weather forecasting. However the limited spatial and temporal observations prohibit forecasting of mesoscale high-impact weather events like thunderstorms. Passive sensors like radiometers can provide useful measurements of temperature and water vapor close to the surface but their coarse vertical resolution make them unsuitable to detect elevated water vapor layers. GPS receivers can only provide the integrated column water vapor amount. Raman lidars and high-power water vapor Differential Absorption Lidar (DIAL) can provide the high spatial and temporal measurements of water vapor that are desired, however, they are inherently large and expensive instruments to build, operate, and maintain.
Researchers at Montana State University (MSU) and NCAR/EOL pioneered, demonstrated, and validated a micropulse, diode-laser-based, differential absorption lidar for water vapor profiling in the lower troposphere. This development effort was initially just called by the name of the technique: Water Vapor Differential Absorption Lidar (WV-DIAL). The instrument design is discussed in Spuler et al. 2015, and a validation study was done by Weckwerth et al. 2016.
NCAR demonstrated that the same low-cost, high-reliability diode-laser-based architecture can be used to build a high spectral resolution lidar (HSRL). This lidar technique directly retrieves the optical properties of aerosols and clouds without assumptions about particle scattering properties, coupling of retrieval errors between altitude regions, or significant correction factors for low altitude returns. The diode-laser-based HSRL design is discussed in Hayman and Spuler 2017. This work was a critical step toward the development of temperature profiling as a further extension to the diode-laser-based lidar architecture. A demonstration of the temperature profiling has been done as discussed in Stillwell et al. 2020.
Building on the proven design, we developed five compact field-deployable units, now called MicroPulse DIAL (MPD). These instruments are capable of measuring water vapor in the lower troposphere with the appropriate vertical range, resolution, and measurement time needed for monitoring, verification, and data assimilation. The MPD instrument design is discussed in Spuler et al. 2021. This five-unit testbed is enabling the first tests of larger-scale ground-based water vapor profiling networks.
Use of a diode laser (semiconductor active medium) in a lidar has distinct benefits. The lasers are considerably more compact, reliable and less expensive than typical lasers used for lidar instrumentation.
The diode-laser-based (DLB) lidar architecture developed by NCAR in collaboration with MSU uses continuous wave seed lasers that are amplified into pulses (5-10 µJ/pulse) at high repetition rates (5-10 kHz). For high quality daytime operation, suppression of the solar background is achieved with a narrow receiver field of view (100 µrad) and extremely narrow-band optical filters (10-20 pm full width half max). The transmitted laser beam is eye-safe and invisible (Class 1M) and the receiver uses single photon counting detectors.
The differential absorption lidar (DIAL) technique uses two separate laser wavelengths: an absorbing wavelength (online) and a non-absorbing wavelength (offline). A ratio of the range-resolved backscattered signals between the online and offline wavelengths is proportional to the amount of water vapor in the atmosphere. The technique requires knowledge of the absorption feature (obtained from molecular absorption database) and estimates of the atmospheric temperature and pressure (obtained from surface measurements and standard atmosphere models). The technique also requires the laser wavelength to be stable and confined to a narrow band or “single frequency” so some type of diffraction grating is used for feedback to the seed laser.
1. Spuler, S. M., K. S. Repasky, B. Morley, D. Moen, M. Hayman, and A. R. Nehrir, 2015: Field-deployable diode-laser-based differential absorption lidar (DIAL) for profiling water vapor, Atmos. Meas. Tech., 8, 1073-1087, doi:10.5194/amt-8-1073-2015
2. Weckwerth, T. M., K. Weber, D. D. Turner, and S. M. Spuler, 2016: Validation of a Water Vapor Micropulse Differential Absorption Lidar (DIAL). J. Atmos. Oceanic Technol., 33, 2353-2372, doi: 10.1175/JTECH-D-16-0119.1
3. Hayman M. and S. Spuler, 2017: Demonstration of a diode-laser-based high spectral resolution lidar (HSRL) for quantitative profiling of clouds and aerosol. Opt. Express, 25(24) doi: 10.1364/OE.25.0A1096
4. Repasky, K. S., C. E. Bunn, M. Hayman, R. A. Stillwell, and S. M. Spuler, 2019: Modeling the Performance of a Diode Laser-Based (DLB) Micro-Pulse Differential absorption Lidar (MPD) for Temperature Profiling in the Lower Troposphere. Opt. Express, 27(23), 33543-33563. doi: 10.1364/OE.27.033543
5. Stillwell, R. A., S. M. Spuler, M. Hayman, K. S. Repasky, and C. E. Bunn, 2020: Demonstration of a combined differential absorption and high spectral resolution lidar for profiling atmospheric temperature. Opt. Express, 28(1) doi: 0.1364/OE.379804
6. Spuler, S. M., M. Hayman, R. A. Stillwell, J. Carnes, T. Bernatsky, and K. S. Repasky, 2021: MicroPulse DIAL (MPD) – a diode-laser-based lidar architecture for quantitative atmospheric profiling. Atmospheric Measurement Techniques, 14(6), 4593–4616. doi: 10.5194/amt-14-4593-2021
