Boundary Layer Water Vapor Quantification using the Orbiting Carbon Observatory-2 (OCO-2) and the Atmospheric Infrared Sounder (AIRS) by Madison Shogrin A thesis submitted to the University of Colorado at Boulder in partial fulfilment of the requirements to receive Honors designations in Environmental Studies Thesis Advisor: Julie Lundquist, Department of Atmospheric and Oceanic Sciences Honors Thesis Committee: Sebastian Schmidt, Department of Atmospheric and Oceanic Sciences Dale Miller, Department of Environmental Studies John Cassano, Department of Atmospheric and Oceanic Sciences Outside Committee Members: Vivienne Payne, Gregory Osterman, Robert Nelson, at the NASA Jet Propulsion Laboratory / California Institute of Technology c 2020 by Madison Shogrin ii iii Preface This work was supported by the National Aeronautics and Space Administration Jet Propulsion Laboratory / California Institute of Technology as a part of a summer internship through the California Institute of Technology Student-Faculty Programs Summer Internship Program (SFP-SIP) and the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) under mentor Vivienne Payne and co-mentor Gregory Osterman and Postdoctoral researcher Robert Nelson. This study was then continued for the purpose of this honors thesis made possible by JPL remote affiliation. The remainder of this undergraduate honors thesis was supported under Julie Lundquist, Dale Miller, Sebastian Schmidt, and John Cassano at the University of Colorado, Boulder’s Environmental Studies and Atmospheric and Oceanic Sciences departments. PREFACE iv Acknowledgements I would like to express my sincere gratitude to Vivienne, Greg, Rob, and everyone at JPL who made this work possible. My experience at JPL has made a huge positive influence on my desired career path. I am sincerely grateful for the continued support at CU from my thesis commit- tee. I would like to express my sincere gratitude to my primary advisor, Julie Lundquist, and to my committee, Dale Miller, John Cassano, and Sebastian Schmidt. I am so appreciative of your help and support through this process. This thesis experience has inspired me to peruse a graduate degree in atmo- spheric science beginning in the fall. I plan to continue to work with satellite data to further understanding of earth’s climate. I am so excited and could not have gotten to this point without those involved in this thesis. Thank you for the constant inspiration and support. v Abstract Accurate quantification of water vapor in the lower troposphere is critical for nu- merical weather prediction and climate studies. Water vapor is the most important greenhouse gas in the atmosphere with respect to climate feedback. Water vapor’s natural regulation and large variability with space and time makes it difficult to quantify on a global scale. Ground-based in-situ measurements of water vapor are historically accurate, however, lack the global coverage needed to fully understand water vapor’s role in the greater climate system. Space-based satellite observations of water vapor in the lower atmosphere have the potential to improve temporal and spatial resolution of water vapor measurements, allowing for a deeper understand- ing of its role in Earth’s mechanisms of radiative forcing. Here we combine water vapor products from the Atmospheric Infrared Sounder (AIRS) and the Orbiting Carbon Observatory-2 (OCO-2) using a differencing method to quantify water vapor in the lowermost region of the troposphere; the planetary boundary layer. These calculations are then validated against radiosonde-based mergesonde prod- ucts sourced from the Atmospheric Radiation Measurement (ARM) user facilities at the Southern Great Plains (SGP) site. Upon analysis, we see that the valida- tion yielded to have extremely high correlation. Accurate quantification of lower tropospheric water vapor from space is crucial for improving numerical weather prediction and understanding Earth’s mechanisms of heat and energy transfer. vi Contents Preface iii Abstractv 1 Introduction1 2 Background2 2.1 Atmospheric Water Vapor...................... 2 2.2 Planetary Boundary Layer ..................... 6 2.2.1 Types of Boundary Layers ................. 9 2.3 Remote Sensing........................... 11 2.3.1 Remote Sensing: Overview . 11 2.3.2 Electromagnetic Radiation . 15 2.3.3 Interactions with Radiation . 17 2.3.4 Atmospheric Radiative Transfer . 20 3 Data sets 27 3.1 The Orbiting Carbon Observatory-2 ................ 29 3.1.1 Modes of Operation ..................... 30 CONTENTS vii 3.1.2 Measurements ........................ 32 3.1.3 Total Column Water Vapor Product . 33 3.2 The Atmospheric Infrared Sounder................. 36 3.2.1 Measurements ........................ 37 3.2.2 Integrated Total Column Water Vapor . 37 3.3 Water Vapor Number Density.................... 39 3.4 ARM Mergesonding Value-Added Product............. 40 4 Methodology 42 4.1 Partial Column Water Vapor Calculation.............. 42 4.2 Boundary Layer Height....................... 44 4.2.1 Bulk Richardson Number Method . 44 4.3 Validation.............................. 46 5 Results and Discussion 48 5.1 Results................................ 48 5.2 Discussion.............................. 51 6 Conclusions 54 6.1 Future Work............................. 55 Bibliography 56 1 1. Introduction In this study I combine data products from the Orbiting Carbon Observatory-2 (OCO-2) and the Atmospheric Infrared Sounder (AIRS) to quantify water vapor in the lower-most region of the troposphere; planetary boundary layer. Integrated profiles of upper column water vapor from the AIRS instrument are subtracted from the total column water vapor product from the OCO-2 satellite to obtain a boundary layer water vapor product. The height of the boundary layer was calculated using the Bulk Richardson Number Method. The results were then validated using radiosonde integrated boundary layer water vapor products. The purpose of this study is to validate this method of space-based boundary layer water vapor quantification in order to answer the following research question: How accurately can we quantify water vapor in the lower-most portion of the troposphere, the planetary boundary layer, on a global scale using space-based satellite measurements in two different regions of the infrared spectrum? 2 2. Background In this section I will discuss the importance of atmospheric water vapor, significance of the boundary layer, and fundamental information needed to make satellite measurements of water vapor in the atmosphere. This information is necessary for understanding the motivation behind this study and the obtained results. 2.1 Atmospheric Water Vapor Water vapor is the most dominant greenhouse gas in the atmosphere, accounting for about 60 percent of the natural greenhouse effect for clear skies (Kiehl and Trenberth, 1997). Water vapor concentration ranges from 0.1 to 4 percent of total atmospheric composition at any given time and space. Water vapor is under the natural regulation of the hydrologic cycle. This natural regulation attributes to its high variability. Water vapor in the hydrologic cycle is the principle medium of energy exchange between the earth’s spheres: the cryosphere, biosphere, ocean, and atmosphere. Atmospheric water vapor is a major component of the earth’s hydrologic cycle, CHAPTER 2. BACKGROUND 3 despite representing only a small part of earth’s H2O species (Starr, 1991). The transfer of water vapor across the earth-atmosphere interface through evaporation, sublimation, evapotranspiration, condensation, precipitation, and atmospheric transport represents one of the most rapid and effective coupled mechanisms of transport within the earth’s system (Starr, 1991). The residence time of water vapor in the atmosphere ranges anywhere from minutes, to hours, to days. An amount of water vapor equivalent to the total global tropospheric reservoir is exchanged across the earth-atmosphere interface on a 10 day cycle (Starr, 1991). 99.13 percent of atmospheric water vapor resides in the troposphere, where about 80 percent of this is located in the planetary boundary layer (Leinweber, 2010). See figure 2.1 for a visual representation of atmospheric gas profiles. The vertical structure of water vapor in the atmosphere is dominated by a decrease with pressure. The global distribution of water vapor plotted along the Orbiting Carbon Observatory-2 (OCO-2) global ground track is shown in figure 2.2 and shown by the Atmospheric Infrared Sounder (AIRS) in figure 2.3. The global distribution of water vapor is concentrated at the poles and decreases poleward. The Inter-Tropical Convergence Zone (ITCZ) is a region defined by tropical convection and low atmospheric pressure and extremely high water vapor content. This region contains the highest concentrations of global water vapor. CHAPTER 2. BACKGROUND 4 Figure 2.1: Shown are atmospheric profiles of numerous trace gases. It is aparent that water vapor uniquely has a steep exponential decline with height. Image credit: Leinweber, 2010 The concentration of water vapor is under natural regulation. Therefore, the abundance of atmospheric water vapor is under the control of climate feedback and the hydrologic cycle. Water vapor molecules absorb and emit longwave infrared radiation emitted by the earth. Water vapor therefore has capacity for positive feedback. As the climate warms, more evaporation occurs, warmer air has a larger capacity for molecular water vapor and greenhouse trapping of infrared