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

Aug 27, 2026

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A redesigned opto-mechanical structure has been implemented on the JPL Laser Hygrometer Mark2 to enhance the accuracy and precision of in situ aircraft water vapor measurements in the UTLS region. The earlier JLH Mark1, used across multiple airborne platforms for about fifteen years, required extensive component modification and replacement due to aging and intensive operating conditions. This paper documents the opto-mechanical redesign, updated data retrieval algorithms, and revised calibration and analysis workflows, supported by recent laboratory and field performance and comparisons with another water vapor instrument.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages30

Reading Time1 h 16 min

AccessFree to download

Introduction

Importance of UTLS water vapor measurements

Development history of the JPL Laser Hygrometer (JLH Mark1)

Instrument degradation and prior mounting attempts

Overview of the JLH Mark2 opto-mechanical approach

JLH Mark2-An Improved Opto-Mechanical Approach to Open-Path in situ Water Vapor Measurement in the Upper Troposphere / Lower Stratosphere Robert L. Herman 1, Robert F. Troy2,3, Kim M. Aaron 1, Isabelle Sanders 1, Kevin Schwarm 1, J. Eric Klobas4, 5 Aaron Swanson4, Andrew Carpenter4, Scott Ozog4, Keith Chin3, Lance E. Christensen 1, Dejian Fu 1, Robert F. Jarnot 1, Robert A. Stachnik3, Ram Vasudev3 1Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California, 91109, USA. 2Robert Troy Engineering, Los Angeles, California. 3Formerly at Jet Propulsion Laboratory. 10 4Northrop Grumman Systems Corporation. Correspondence to: Robert L. Herman ([Robert.L.Herman@jpl.nasa.gov](Robert.L.Herman@jpl.nasa.gov)) Copyright 2025 All Rights Reserved Abstract. To improve the accuracy and precision of in situ water vapor measurements from aircraft, a new opto-mechanical design 15 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 fifteen 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 on the redesigned opto-mechanical structure of the instrument, new data retrieval algorithms, and updated data analysis procedures, along with recent laboratory and field performance and a 20 comparison with another water vapor instrument. 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 Northrop Grumman Systems Corporation Flying Test Bed (FTB) . 1 Introduction 25 Atmospheric measurements of water vapor in the upper troposphere and lower stratosphere (UTLS) are important for better understanding Earth’s radiative balance because UTLS water has a significant impact on both radiative forcing and stratospheric ozone despite its low concentrations. Through the radiative impact of water vapor (Solomon et al., 2010) and cirrus clouds, climate is strongly affected by atmospheric water. Clouds continue to be one of the biggest sources of uncertainty in climate prediction (Boucher et al., 2013, Davis et al., 2007). An additional climate contribution from clouds is due to aircraft. In the upper troposphere, 30 significant fractions of the area are ice supersaturated regions (e.g., Troy, 2007; Gettelman et al., 2006; Heymsfield et al., 1998) . In such an environment, aircraft engine combustion elevates the relative humidity high enough for homogeneous ice nucleation in persistent contrails and aircraft induced cirrus clouds (e.g., Testa et al., 2024, and references therein) . Burkhardt and Kärcher (2011) found that aircraft induced cirrus are the largest net (warming) effective radiative forcing term from aviation, larger even than the radiative forcing term from CO2 emissions from aircraft (Lee et al., 2021) . There is a renewed interest in minimizing contrails in 35 commercial aviation, which has led to the aircraft campaign coordinated by the Northrop Grumman Systems Corporation (NGSC) and described herein. 40 45 50 55 60 65 70 The JPL Laser Hygrometer (JLH) for in situ UTLS water vapor measurements has been developed and tested over many years. The initial version JLH Mark1(a) instrument provided airborne measurements from the NASA ER-2 aircraft (May, 1998) . Several years later, a similar JLH Mark1(b) was developed by R. D. May at JPL for the NASA WB-57F aircraft. Both instruments are open-path near-infrared tunable diode laser absorption spectrometers designed to accurately measure atmospheric water vapor concentrati

FAQs

What is the main improvement introduced in the JLH Mark2 instrument?

JLH Mark2 implements an entirely new opto-mechanical approach, along with a ruggedized design and updated data retrieval, calibration, and analysis procedures.

Why were components in JLH Mark1 modified or replaced?

Heavy use and aging of instrument parts, including deterioration of mirror coatings from rocket plume exposure and failures of epoxy bonds under low-temperature conditions.

How was the redesigned system evaluated in the field?

Performance improvements were demonstrated during the SEAC4RS field mission and through eight years of subsequent science flights on NGSC’s Flying Test Bed (FTB), with comparisons to another water vapor instrument.

Aug 27, 2026

Upload by Kyle