JLH Mark2 – An Improved Opto-Mechanical Approach to Open-Path in situ Water Vapor Measurement in the Upper Troposphere / Lower Stratosphere

Abstract. To improve the accuracy and precision of in situ water vapor measurements from aircraft, a new opto-mechanical design was implemented on the JPL Laser Hygrometer Mark2. The first JPL Laser Hygrometer (JLH Mark1), originally developed in mid-1990s, provided airborne in-situ water vapor measurements for 15 years from several platforms, including the NASA ER-2 and WB-57 aircraft. Due to heavy use over the years and aging of the instrument parts, many of the components in JLH Mark1 have been modified and replaced. This instrument paper reports the redesigned opto-mechanical structure of the instrument, new data retrieval algorithms, and updated data analysis procedures. These improvements are described in this paper, along with recent laboratory and field performance, and a comparison with other water vapor instruments. Key changes in the redesigned instrument have significantly improved the performance, as demonstrated during the NASA Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys (SEAC⁴RS) field mission and eight years of subsequent science flights on the Northrup Grumman Corporation Flying Test Bed (FTB).