Water Vapor Differential Absorption Lidar (WV-DIAL)

WV-DIAL data are available in the EOL Field Data Archive.

Science

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 weather forecasting, atmospheric science, and climate science communities. Two National Research Council studies list high-resolution vertical profiles of water vapor as one of the highest-priority observations that need to be addressed for the next-generation mesoscale weather observation network [1-2].  Additionally, these observations are of high importance to the National Weather Service and other Federal agencies for improving both severe and quantitative precipitation forecasts.

Radiosondes, combined with 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 -- e.g., infrared and microwave radiometers -- are useful close to the surface yet only provide coarse vertical resolutions and are unable to detect elevated water vapor layers. GPS receivers provide only the integrated column of precipitable water vapor. Raman lidars and high-power differential absorption lidar or DIAL (e.g., based on Ti:sapphire laser systems) can provide the continuous high-resolution vertical profiles of water vapor needed; however, they are inherently large and expensive instruments to build, operate, and maintain. To enable large-scale ground-based networks, NCAR and Montana State University have developed a micro-pulse DIAL.

Technical description

The NCAR and Montana State University laser remote sensing groups have worked together since 2011 to develop a compact, field-deployable, micro-pulse DIAL. The instrument continuously monitors water vapor in the lower troposphere at 150 m range resolution and 1-5 min temporal resolution from 300 m to 4 km above ground level in daytime operation with a greater range at night. The instrument design is discussed in Spuler et al. 2015 and its validation is discussed in Weckwerth et al. 2016.  A high spectral resolution channel, capable of providing quantitative aerosol and cloud properties, is now available and is discussed in Hayman and Spuler 2017.

A testbed of five micropulse DIAL units is under construction and scheduled for completion in Sep 2019. A 6-month commissioning phase, including a summer field test, is scheduled to follow. The test network of instruments can be used to advance knowledge in the areas of measuring water vapor concentration and distribution, convection initiation, and land-atmosphere exchange. This will lead to both improving our current understanding and improving our weather and climate forecasting skills.

DLB lidar architecture

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 NCAR diode-laser-based (DLB) lidar architecture 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 (10-20 pm full width half max) optical filters.  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. 

Publications

Researchers at Montana State University (MSU) developed the seminal diode-laser-based (DLB) lidar technology for the purpose of water vapor profiling [Nehrir et al., 2009, 2011, 2012].  A more capable DLB lidar was designed and built by NCAR and MSU [Repasky et al., 2013, Spuler et al. 2015]. The instrument has been shown to deliver accurate retrievals of water vapor in the lower troposphere and produce scientifically significant data [Weckwerth et al. 2016].  A high spectral resolution channel, based on the DLB lidar architecture, was developed by NCAR [Hayman and Spuler 2017] and has been incorporated into one of the instruments.

1. Nehrir, A. R., K. S. Repasky, J. L. Carlsten, M. D. Obland and J. A. Shaw, 2009: Water Vapor Profiling Using a Widely Tunable, Amplified Diode-Laser-Based Differential Absorption Lidar (DIAL). Journal of Atmospheric and Oceanic Technology, 26(4), 733–745, doi:10.1175/2008JTECHA1201.1

2. Nehrir, A. R., K. S. Repasky and J. L. Carlsten, 2011: Eye-Safe Diode-Laser-Based Micropulse Differential Absorption Lidar (DIAL) for Water Vapor Profiling in the Lower Troposphere. Journal of Atmospheric and Oceanic Technology, 28(2), 131–147, doi:10.1175/2010JTECHA1452.1

3. Nehrir, A. R., K. S. Repasky and J. L. Carlsten, 2012: "Micropulse water vapor differential absorption lidar: transmitter design and performance," Opt. Express 20, 25137-25151, doi:10.1364/OE.20.025137

4. Repasky, K., D. Moen, S. Spuler, A. R. Nehrir and J. L. Carlsten, 2013: Progress towards an Autonomous Field Deployable Diode-Laser-Based Differential Absorption Lidar (DIAL) for Profiling Water Vapor in the Lower Troposphere, Remote Sensing, 5, 6241–6259.  doi:10.3390/rs5126241

5. 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

6. 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

7. 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