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Tomographic Views of the Middle Atmosphere from a Satellite Platform

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Tomographic Views of the Middle Atmosphere from a Satellite Platform Tomographic views of the middle atmosphere from a satellite platform Kristoffer Hultgren AKADEMISK AVHANDLING for¨ filosofie doktorsexamen vid Stockholms Universitet att framlaggas¨ for¨ offentlig granskning den 3 oktober 2014 Department of Meteorology Stockholm University Stockholm 2014 Cover image: Polar Mesospheric Clouds above Vallentuna, Sweden Photographer: P-M Hed´en,www.clearskies.se Tomographic views of the middle atmosphere from a satellite platform Doctoral thesis Kristoffer Hultgren ISBN 978-91-7447-974-4 © Kristoffer Hultgren, Stockholm, 2014 Stockholm University Department of Meteorology 106 91 Stockholm Sweden Printed by US-AB Stockholm, 2014 Summary The middle atmosphere is a very important part of the Earth system. Until recently, we did not realize the importance of the structure of this vaporous shell and of the fundamental role it plays in both creating and sustaining life on the planet. Thanks to the development and improvement of new sounding methods and techniques, our knowledge of the composition of the atmosphere has become more detailed than ever before. We have also learned how to reveal complex interactions between different species and how they react to the incoming solar radiation. The middle part of the Earth's atmosphere serves as a host for the Polar Mesospheric Clouds. These clouds consist of a thin layer of water-ice particles, only existing during the summer months and only close to the poles. These clouds can be used as a proxy for middle atmospheric dynamics. In order to fully utilize Polar Mesospheric Clouds as tracers for atmospheric variability and dynamics, we need to better understand their local properties. The Optical Spectrograph and Infra-Red Imager System (OSIRIS) is one of two instruments installed on the Odin satellite. The optical spectrograph of this instrument observes sunlight scattered by the atmosphere and thus the Polar Mesospheric Clouds. This thesis deals with a tomographic technique that can reconstruct both horizontal and vertical structures of the clouds by using observations from various angles of the atmo- spheric region. From this information, microphysical proper- ties such as particle sizes and number densities are obtained. The tomographic technique presented in this thesis also pro- vides a basis for a new satellite concept - MATS. The idea behind the MATS satellite mission is to analyze wave activity in the atmosphere over a wide range of spatial and tempo- ral scales, based on the scientific heritage from Odin/OSIRIS and the tomographic algorithms presented in this thesis. Contents 1 Introduction 7 1.1 The Earth's atmosphere . 8 1.2 The Noctilucent Clouds . 11 2 Atmospheric Radiation 15 2.1 Electromagnetic spectrum . 15 2.2 Basic radiometric quantities . 16 2.2.1 Solid angle . 16 2.2.2 Radiant flux . 17 2.2.3 Irradiance . 18 2.2.4 Radiant intensity . 18 2.2.5 Radiance . 18 2.3 Concepts of scattering and absorption . 18 2.3.1 Scattering angle and phase function . 19 2.3.2 Scattering cross-section . 19 2.3.3 Index of refraction . 20 2.3.4 Size parameter . 20 2.3.5 Rayleigh and Mie scattering . 20 3 Remote Sensing 23 3.1 Overview . 23 3.2 The inverse problem . 24 4 Odin/OSIRIS 27 4.1 The satellite platform . 27 4.2 The optical spectrograph . 28 4.3 Original retrievals . 30 4.3.1 Measured radiance . 30 4.3.2 PMC brightness . 31 4.3.3 Particle sizes . 33 5 Tomographic Inversions 35 5.1 Background . 35 5.2 Inversion method . 36 5.3 A model of Odin/OSIRIS . 39 5.3.1 Atmospheric grid . 40 5.3.2 Coordinate systems . 40 5.3.3 Model description . 41 6 The MATS Satellite Concept 43 7 Summary of The Papers 45 Bibliography 49 Acknowledgements 53 Paper I: Hultgren, K., Kornich, H., Gumbel, J., Gerding, M., Hoffmann, P., Lossow, S., and Megner, L. (2010). What caused the exceptional mid- latitudinal Noctilucent Cloud event in July 2009? J. Atmos. Solar-Terr. Phys., 73, 2125-2131. Paper II: Hultgren, K., Gumbel, J., Degenstein, D., Bourassa, A., Lloyd, N., and Stegman, J. (2013). First simultaneous retrievals of horizontal and vertical structures of Polar Mesospheric Clouds from Odin/OSIRIS tomog- raphy. J. Atmos. Solar-Terr. Phys., 104, 213-223. Paper III: Hultgren, K. and Gumbel, J. (2014). Tomographic and spec- tral views on the life cycle of polar mesospheric clouds from Odin/OSIRIS. Submitted to J. Geophys. Res. Paper IV: Gumbel, J., Hultgren, K., and the MATS Science Team (2014). Mesospheric Airglow/Aerosol Tomography and Spectroscopy (MATS) - a satellite mission on mesospheric waves. To be submitted to ESA's Living Planet Symposium. Chapter 1 Introduction Why are we interested in the Earth's atmosphere? Earth - a big round planet, yet so small when viewed from space. The atmosphere - a thin gaseous layer, just like the crust of an apple. Just recently, it has been understood that the atmosphere is a very important part of the Earth system. We did not realize the importance of the structure of this vaporous shell and of the fundamental role it plays in both creating and sustaining life on the planet. Up until the mid 20th century, the mean state of the atmosphere was assumed stable and not likely to be noticeably affected by anthropogenic activity. Thanks to the development and improvement of new sounding methods and techniques, our knowledge of the composition of the atmosphere has become more detailed than ever before. We have also learned how to reveal complex interactions between different species and how they react to the incoming solar radiation. In the atmospheric mixture, a wealth of molecular species have been discovered and measured. Together, these gases control the balance between the incoming and outgoing radiation, which, in turn, contributes to the motion of the atmosphere. Some of these gases act as a shield against the harmful ultraviolet radiation from the Sun and as a thermal blanket that traps the heat and insulates the Earth from the cold space. In other 7 words, life on Earth, as we know it, is highly dependent on a sparse layer of molecules floating around above us. Another part of the mixture is present in the atmosphere only as a result of human activities. These gases have an effect that modifies the natural balance of the atmosphere. The long-term consequences of these alterations are yet to be fully evident. 1.1 The Earth's atmosphere The vaporous atmosphere is mainly made up of molecular nitrogen (N2 ∼ 78%) and molecular oxygen (O2 ∼ 21%), but also of a small part of noble gases (Ar, He, Ne, Kr, Xe) and minor constituents (CO2, CO, CH4,N2O, O3,H2O) can be found. All of these gases possess very long lifetimes against chemical destruction and are therefore relatively well mixed. However, even if most of the gases are well mixed, the atmosphere itself is highly stratified with significant vertical variations in composition, temperature, pressure, and density. In addition to gases, the atmosphere also contains various solid and liquid particles, such as water droplets, ice crystals, and aerosols. These particles play an important role in the absorption and scattering of solar radiation, as well as the physics of clouds and precipitation. The ratio of molecular species is controlled by either chemistry, molecular diffusion, or mixing due to fluid motion. Molecular diffusion works together with gravity to try to order the atmosphere in such a way that the lightest molecules are present at the top and the heaviest ones at the bottom. The mixing due to fluid motion of the gases is on the other hand independent of molecular mass. At lower altitudes, the time between molecular collisions is so small that the time necessary for molecular diffusion is many orders of magnitude greater than the time required for turbulent motion. However, at higher altitudes, the collisions between molecules occur so seldom that molecular diffusion is the dominant mechanism. As a consequence, the atmosphere is well mixed at altitudes below 100km (the homosphere) and stratified and well ordered by mass above 100 km (the heterosphere). The magnitude of the atmospheric temperature varies greatly, both vertically and horizontally. However, the vertical structure is qualitatively 8 Figure 1.1: The temperature profile of Earth’s atmosphere. similar everywhere. It is therefore meaningful to think of a typical temperature profile. Figure 1.1 shows a typical example of the vertical temperature profile of the atmosphere. As can be seen, the profile is divided into a set of layers based on the different vertical temperature gradients. These are the troposphere, stratosphere, mesosphere, and thermosphere, each characterized by substantially different chemical and physical processes. The boundaries between these layers are called the tropopause, stratopause, and mesopause. The altitudes at which these boundaries are located are not fixed but vary in time and location, depending on the amount of energy input. The regions are commonly also referred to as the lower atmosphere (troposphere), the middle atmosphere (stratosphere and mesosphere), and the upper atmosphere (thermosphere and above). The focus of this thesis will be on the middle atmosphere. The structure of the temperature profile can be explained by considering the dominant processes occurring at specific altitudes. From a bottom-up approach, a large part of the electromagnetic radiation coming from the 9 Sun at wavelengths in or near the visible part of the spectrum reaches the ground where it is absorbed. From this absorption, the Earth's surface is heated and re-emits the energy as black-body radiation which, in turn, heats the lower atmosphere. This \blocking" of the outgoing radiation by the atmosphere is referred to as the greenhouse effect and keeps the surface of the Earth warmer than it would have been in the absence of an atmosphere.
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