Accurate observations of near-infrared solar spectral irradiance and water vapour continuum Submitted for the degree of Doctor of Philosophy Department of Meteorology University of Reading Jonathan David Charles Elsey September 2018 Declaration I confirm that this is my own work and the use of all material from other sources has been properly and fully acknowledged. - Jonathan Elsey i Abstract This thesis contains analyses of the solar spectral irradiance (SSI) and near-infrared water vapour continuum from high-resolution observations by a ground-based, sun-pointing Fourier transform spectrometer in the wavenumber region 2000-10000 cm-1 (1-5 µm). This was performed primarily using the Langley method on observations during 18 September 2008. Particular focus was placed on a detailed assessment of the uncertainty budget for each of these analyses. The solar spectral irradiance was found to be ~8% lower than the commonly-used satellite-based ATLAS3 SSI in the region 4000-7000 cm-1 (where ATLAS3 is most uncertain). This disagreement with ATLAS3 is in line with several other modern analyses. There is good agreement with ATLAS3 and other spectra in the 7000-10000 cm-1 region (where these spectra are considered more accurate). This thesis contains the first published results of water vapour continuum absorption in the atmosphere in the 1.6 and 2.1 µm atmospheric windows (in which laboratory measurements show some significant disagreement) with robust uncertainties. The derived water vapour continuum in these windows is stronger than the widely-used MT_CKD model (v3.2) by a factor of ~100 and ~5 respectively. These results also show that MT_CKD is a reasonably accurate representation of the continuum in the 4 µm window. These results are broadly consistent with laboratory measurements of the foreign continuum, but are inconsistent with the highest such measurements of the self-continuum. The effect of the self and foreign continuum in atmospheric conditions is assessed, with comparisons to the Langley-derived spectra from this work. These results show that the difference between MT_CKD and this work in the 1.6 and 2.1 µm windows may come primarily from the observed differences in the foreign continuum, with a smaller contribution from the self-continuum. These results show inconsistency with several sets of laboratory room temperature spectra within the experimental uncertainties. ii Acknowledgements First and most obviously is my supervisor Prof. Keith Shine, whose endless patience and insight was instrumental in getting the thesis to the state its in. I’m proud to call you my mentor; if I can be a tenth as expert and generally nice as you are I’ll be doing well! I’d also like to thank Drs. Tom Gardiner and Marc Coleman at the National Physical Laboratory, with whom I worked extensively and who directed the field campaign from which most of the work in this thesis derives. I always enjoyed our meetings, and I hope to keep in contact with you after the PhD ends. The work presented in this thesis would not have been possible without the groundwork laid by Liam Tallis and Kaah Menang during their respective PhDs, and I thank Kaah in particular for our useful meetings and emails. Additional thanks of course go to my monitoring committee, Prof. Richard Allan and Dr. Christine Chiu; your discussions in the committee meetings always helped ground and centre my work and always resulted in fruitful ideas which undoubtedly improved the manuscript. I am also indebted to Dr. Nicolas Bellouin; while the project ultimately went in a different direction to that expected our occasional meetings were always insightful, and your suggestion to use a physical method to obtain aerosol optical depth (the Mie scattering code) was an immense boost to the thesis. There are a million other people I’d like to thank for their love and support. My family, particularly my parents have always been the most supportive people I know and have stopped at nothing to help and encourage me to get where I am today. I love you all, and hope to repay you in kind down the road. I’d like to thank my friends, particularly those who had the misfortune of sharing office space with me on Lyle 3 down the years. The endless Sporcle quizzes and lunchtime board games probably delayed my PhD by a good few weeks, but definitely contributed to keeping me sane when the work wasn’t going so well! I’d also like to thank everyone who lived in House with me over the years, for putting up with me in the bad times and making the good times some of the best I’ve ever had. The last few months of the PhD were definitely some of the most challenging of my life so far, and most of all I thank Sammie (she’s a star) for doing so much to help me get through the thesis and viva, especially in those dark days when it felt like the chances of getting anything submitted were a million to one. Table of Contents: Chapter 1: Introduction 1 1.1: Introduction 1 1.2: Earth’s energy balance and solar radiation 2 1.3: Absorption of solar radiation in Earth’s atmosphere 4 1.4: The water vapour continuum 8 1.4.1: Introduction 8 1.4.2: Theoretical and experimental perspectives on the continuum 9 1.4.3: Impacts of the continuum for atmospheric science 12 1.5: Objectives of this work 14 Chapter 2: Radiative processes in the terrestrial atmosphere 17 2.1: Molecular spectroscopy 17 2.1.1: Rotational spectroscopy 19 2.1.2: Vibrational transitions 21 2.1.3: Rotational-vibrational transitions 22 2.1.4: Line broadening 24 2.1.4.1: Natural broadening 25 2.1.4.2: Collision broadening 26 2.1.4.3: Doppler broadening 27 2.1.4.4: Voigt and Hartmann-Tran lineshape profiles 28 2.1.5: Water vapour near-infrared spectrum 30 2.2: Solar radiation 32 2.2.1: Solar structure 33 2.2.2: Sun position and distance from Earth 34 2.2.2.1: Solar zenith angle 35 2.2.2.2: Orbit eccentricity 35 2.2.3: Models and observations of solar radiation 36 2.3: Radiative transfer in the terrestrial atmosphere 38 2.3.1: Blackbody radiation 39 2.3.2: The Beer-Bouger-Lambert law and atmospheric transmission 40 2.3.3: Absorption by atmospheric gases 42 2.3.4: Rayleigh scattering 44 2.3.5: Mie scattering and the effect of clouds and aerosols 46 2.3.6: Radiative transfer models 48 2.3.6.1: The Reference Forward Model 49 2.3.6.2: Spectroscopic line databases 52 2.3.6.3: Model atmospheric profiles 53 2.3.6.4: Mie scattering models 54 2.4: Near-IR water vapour continuum absorption 55 Chapter 3: Experimental methods in the CAVIAR field campaign 59 3.1: Introduction 59 3.2: Fourier transform infrared spectroscopy 59 3.2.1: Introduction 59 3.2.2: The Michelson interferometer 60 3.2.3: Optical path difference and interferograms 61 3.2.4: Computational methods in FTIR spectroscopy 63 3.4.2.1: Apodization 64 3.2.4.2: Zero filling 66 3.2.4.3: Phase correction 66 3.3: Retrieval methods 68 3.3.1: Radiative closure 68 3.3.2: Langley method 68 3.3.3: Advantages and disadvantages of Langley and closure methods 69 3.3.4: Calculation of Rayleigh scattering 70 3.3.5: Ångström extrapolation and Mie scattering codes 71 3.3.5.1: Ångström extrapolation 71 3.3.5.2: Mie scattering codes 72 3.3.6: Airmass correction 75 3.3.7: Monte Carlo uncertainty evaluation 76 3.4: CAVIAR field campaigns 77 3.4.1: Camborne campaign 77 3.4.2: Jungfraujoch campaign 90 3.4.3: Details of the CAVIAR FTIR 81 3.4.3.1: The Bruker 125 FTS and optical setup 81 3.4.3.2: Calibration procedure 84 3.4.3.4: FTIR uncertainty budget 88 3.4.3.5: Smoothing and filtering of FTIR data 92 3.4.4: Updates to calibration since Gardiner et al. [2012] 94 3.4.4.1: TSARS stability 94 3.4.4.2: Mirror reflectance calibration 96 3.4.4.3: Phase correction 97 3.4.5: Supplementary measurements 98 3.4.5.1: Radiosonde ascents 99 3.4.5.2: Microtops sunphotometer measurements 101 3.4.5.3: HATPRO microwave radiometer 104 3.4.5.4: FAAM aircraft data 105 3.4.5.5: Model reanalyses 105 Chapter 4: Accurate measurements of NIR solar spectral irradiance between 4000-10000 cm-1 106 4.1: Introduction 106 4.2: Paper manuscript 109 4.2.1: Introduction 109 4.2.2 Current state of measurements 110 4.2.3 Analysis methods 115 4.2.3.1 Langley method 115 4.2.3.2 Radiative closure method 116 4.2.4 Experimental methods 116 4.2.4.1 Calibration 117 4.2.4.2 Spectrometer measurement uncertainty 117 4.2.4.3 Atmospheric state analyses 118 4.2.4.4 Filtering of observed spectra 118 4.2.5 Results and discussions 119 4.2.6 Conclusions 126 4.3: Additional analysis 128 Chapter 5: Observations of the near-IR water vapour continuum in the atmosphere 134 5.1: Introduction 134 5.2: Laboratory measurements and MT_CKD 135 5.2.1: CAVIAR and Tomsk continua 137 5.2.1.1: Self-continuum 137 5.2.1.2: Foreign continuum 139 5.2.2: Grenoble CRDS continua 141 5.2.1.1: Self-continuum 141 5.2.2.2: Foreign continuum 142 5.2.3: Other measurements 142 5.2.4: Synthesis 142 5.2.4.1: Self-continuum 142 5.4.2.2: Foreign continuum 145 5.2.5: Discussion of observation techniques 146 5.2.6: Temperature dependence of the observed self-continuum 147 5.2.7: Atmospheric observations of the near-IR continuum 151 5.3: Derivation of continuum using Camborne measurements 154 5.3.1: Introduction 154 5.3.2: Retrieval of continuum optical depth from observed optical depth 156 5.3.2.1: Observed optical depth 156 5.3.2.2: Optical depth from gaseous absorption 157 5.3.2.3: Optical depth from Rayleigh scattering 159 5.3.2.4: Optical depth from aerosol extinction 160 5.3.2.5: Continuum absorption 162 5.4: Uncertainty budget 164 5.4.1: Observational uncertainties 165 5.4.2: Uncertainties in the model-derived