Micro-Pulse Differential Absorption Lidar (DIAL) Network for Measuring the Spatial and Temporal Distribution of Water Vapor in the Lower Atmosphere

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. Researchers at the National Center for Atmospheric Research (NCAR) and Montana State University (MSU) have collaborated to develop a compact, field-deployable, micro-pulse DIAL (MPD).  The instrument provides continuous monitoring of water vapor in the lower troposphere at 150 m range resolution to support monitoring, forecasting, model verification and data assimilation.

The water vapor MPD instrument design and specifications are described in Spuler et al. 2015 [1].  In general, the instrument is a diode-laser-based lidar with 150 m vertical range resolution and 5 min temporal resolution from 300 m to 4 km above ground level in daytime operation with greater range at night. The instrument laser transmitter allows for low-cost, long-term unattended eye-safe operation – which is a key feature for the multi-unit deployment needed to characterize horizontal moisture variability. To date, the NCAR water vapor MPD has been deployed at two field campaigns –  The Front Range Air Pollution and Photochemistry Experiment (FRAPPE ,Colorado USA) in the summer of 2014, and the Plains Elevated Convection At Night Experiment  (PECAN ,Kansas USA) in the summer of 2015 within a 6.1m long intermodal shipping container equipped as mobile lidar laboratory.  The MPD was up operationally >95% in both field projects resulting in a high continuity of water vapor and aerosol profile time series. Following the PECAN deployment, a more portable field enclosure was constructed to simplify fielding the water vapor MPD.  The enclosure was designed to 1.) be reasonably small and lightweight – so that it could be moved with a forklift and not require a crane for lifting on/off transport vehicles, 2.) provide a weatherproof and thermally stable environment for the instrument, and 3.) provide instrument vibration isolation, to minimize the amount of disassembly and packing needed for shipment to field projects.  The enclosure has external dimensions of 1 m depth, 2 m width, and 2 m height and was designed to have extra space around the instrument to allow easy alignment access and allow for future changes and upgrades.  For example, we are in the processes developing a diode-laser-based high spectral resolution lidar (HSRL) channel and temperature measuring capabilities based on the existing MPD architecture.

The current objective is to build a network of five water vapor MPDs to better characterize horizontal moisture variability and to assess their value in data assimilation for improving mesoscale forecast and quantitative prescription forecast skill.  Two existing water vapor MPD instruments will be modified and three additional units will be built.  The units will include design modifications – such as a NASA SBIR funded effort to develop a compact seed laser package with integrated optics to provide a fiber coupled and optically isolated output, to eliminate complex alignments during field deployments.  The instrument environmental enclosure will be based on the current design and allows for long term deployments and includes adequate temperature control and integration with surface monitoring of temperature, pressure and humidity. Finally, the current instrument control and analysis software will be modified to improve reliability, incorporate instrument monitoring, alarm, and control features, and provide the data in an appropriate format for ease of data assimilation. With the construction phase complete, each MPD instrument will be tested using radiosondes for validation. Once the testing phase is complete, the MPD testbed will be made available to the science community through the NCAR Earth Observing Laboratory (EOL) Facilities and Instrument Program.  The initial small network of devices is anticipated to be completed in 2019.

[1] Spuler, S. M., K. S. Repasky, B. Morley, D. Moen, M. Hayman and A. R. Nehrir, “Field-Deployable Diode-Laser-Based Differential Absorption Lidar (DIAL) for Profiling Water Vapor,” Atmos. Meas. Tech., 8, 1073-1087. (2015) doi:10.5194/amt-8-1073-2015C.