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. 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 improvment of both severe weather forecasts 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.
The laser remote sensing groups at NCAR and Montana State University have worked together since 2011 to develop a compact, field-deployable, micro-pulse DIAL. The instrument provides continuous monitoring of water vapor in the lower troposphere at 150 m range resolution and 1-5 min temporal resolution from 300 m to 4 kmabove ground level in daytime operation with 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 comissioning 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 skill.
