An Airborne Millimetre-Wave Radiometer at 183 Ghz: Receiver Development and Stratospheric Water Vapour Measurements
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An airborne millimetre-wave radiometer at 183 GHz: receiver development and stratospheric water vapour measurements Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakult¨at der Universit¨atBern vorgelegt von Vladimir Vasi´c von Serbien-Montenegro Leiter der Arbeit: Prof. Niklaus K¨ampfer Institut f¨urAngewandte Physik -2 An airborne millimetre-wave radiometer at 183 GHz: receiver development and stratospheric water vapour measurements Inauguraldissertation der Philosophisch-naturwissenschaftlichen Fakult¨at der Universit¨atBern vorgelegt von Vladimir Vasi´c von Serbien-Montenegro Leiter der Arbeit: Prof. Niklaus K¨ampfer Institut f¨urAngewandte Physik Von der Philosophisch-naturwissenschaftlichen Fakult¨at angenommen. Der Dekan: Bern, den 16.12.2004 Prof. Paul Messerli 0 Abstract The subject of this Ph.D. thesis is the redesign of an airborne water-vapour radiometer AMSOS (Airborne Millimetre- and Submillimetre Observing Sys- tem) and measurements conducted with the new receiver. The thesis consists of three parts and four publications. The publications are placed within the thesis after appropriate chapters. The first part is an introduction which addresses the scientific motivation of this thesis: why is water vapour in the stratosphere important and why are we studying it at all? An answer to this is made by emphasising the three main roles of stratospheric water vapour: its role in the ozone chemistry con- nected with ozone depletion, its greenhouse effect contributing to the global warming of the atmosphere and its long lifetime which makes H2O a good tracer. Another question arises logically: how can we measure water vapour in the stratosphere? This is also answered in the first part, where we discuss the physics behind absorption and emission in the millimetre-wave part of the spectrum, radiative transfer and line broadening and how they can be used for retrieval of vertical H2O-profiles. The principles of operation and calibration of a microwave radiometer are also presented here. The second part of the thesis deals with experimental aspects and describes design of the AMSOS radiometer and especially of the radiometer’s qua- sioptics. After a short introduction to Gaussian optics, a new layout of the radiometer frontend is presented. All components are described according to their role in the quasioptics: first, a beam-shaping section which consists of a feed horn, a parabolic and an elliptic mirror; then a new quasioptical λ/4- isolator for reduction of internal reflections (which cause baseline ripple in observed spectra); and finally a Martin-Puplett interferometer for sideband filtering. The theoretical calculations and methods used in the design of ev- ery component are presented together with numerical simulations. However, special attention was paid to measurements of the quasioptical behaviour of every component and their artifacts. Measurements of the amplitude, phase and polarisation were done using the AB Millimetre vector network anal- yser, mainly at the radiometer’s operational frequency of 183 GHz. Several components were also tested at other frequencies. The beam-shaping section generated a regular output beam with a half power beam width of around 1◦, which was one of our main goals. The new λ/4-isolator performed well and ac- 2 cording to our theoretical predictions, attenuated internal reflections by more than 30 dB. Although it is a standard and simple component, widely used in mm- and submm-wave radiometers, the Martin-Puplett interferometer (MPI) and its rooftop mirrors showed very interesting quasioptical behaviour, differ- ent from theoretical expectations. The measured level of internal reflections in an MPI was always around -30 dB. The radiometer backend, system sta- bility, operational control and improvements are also presented. This part of the thesis includes three publications. Publication I (in press in Optics and Lasers in Engineering) addresses issues concerning the quasioptics of the frontend. Publication II (proceedings of the 28th International Conference on Infrared and Millimeter Waves, Otsu, Japan, 2003) deals with non-ideal quasioptical behaviour of a rooftop mirror. Publication III (submitted to IEEE Transactions on Geoscience and Remote Sensing) presents the whole radiometer and first results of water vapour measurements. In the third part we present inversion methods that we used for the retrieval of water vapour altitude profiles, as well as the results from two measure- ment campaigns in November 2003 and February 2004. For the inversion we decided to switch to the retrieval software Qpack, that uses ARTS as for- ward model. Therefore we tested Qpack and ARTS in many simulations of a realistic retrieval which included instrumental parameters and artifacts. At the end we present the profiles obtained from the spectra recorded in the two campaigns. A comparison of AMSOS and balloon measurements from Febru- ary 2004 during the international LAUTLOS/WAVVAP campaign are also presented. In addition, AMSOS measurements were compared with satellite profiles. The thesis ends with a summary of the results achieved and with an outlook towards further development of the receiver and its application. Contents I Stratospheric water vapour and principles of mi- crowave radiometry 6 1 Water vapour in the atmosphere 7 1.1 Water vapour as a greenhouse gas . 7 1.2 Water vapour dynamics and ’Tape Recorder Effect’ . 10 1.3 Water vapour chemistry . 11 2 Microwave gas spectroscopy 13 2.1 Absorption and emission of gases . 14 2.2 Line broadening . 15 2.3 Radiative transfer . 19 3 Microwave radiometry 23 3.1 Heterodyne principle . 23 3.2 Calibration of a heterodyne receiver . 26 II AMSOS radiometer 28 4 Overview 29 5 The quasioptics of AMSOS radiometer 32 5.1 Principles of quasioptics . 32 5.2 Design of AMSOS’s quasioptics . 34 5.2.1 Feed horn antenna . 35 5.2.2 Parabolic mirror . 37 5.2.3 Radiometer’s output beam . 44 6 Quasioptical λ/4 isolator 51 6.1 Introduction . 51 3 4 CONTENTS 6.2 Theoretical description . 52 6.2.1 Detail analysis . 55 6.3 Measurements of the isolator . 63 6.3.1 Measurement setup and methods . 63 6.3.2 Results . 65 6.4 Isolator in AMSOS . 70 6.5 Isolator during flight campaigns . 72 7 Publication I 77 8 Martin-Puplett interferometer 93 8.1 Basics of the MPI . 93 8.2 Reflection measurements . 93 8.3 Sources of reflections . 98 8.4 Non-ideal polarisation rotation . 105 8.5 Lateral offsets of two beams . 106 8.6 Usage of the MPI . 111 9 Publication II 113 10 Signal down conversion and radiometer backend 117 10.1 Down conversion and amplifiers . 117 10.2 System stability . 118 10.3 Radiometer calibration and operation control . 121 10.3.1 Calibration cycle . 121 10.3.2 Operation control . 122 10.3.3 Control software and AMSOS database . 124 10.4 Aircraft window . 125 11 Publication III 127 III Inversion and Results of Stratospheric Water Vapour Distribution 137 12 Inversion 138 12.1 Introduction to inversion process . 138 12.2 Forward model: ARTS; inversion: Qpack . 141 12.3 Validation of retrievals with Qpack . 143 CONTENTS 5 13 Measurement campaigns and results 151 13.1 Ground-based measurements . 151 13.2 Airborne measurements . 151 13.2.1 Test campaign, November 2003 . 151 13.2.2 LAUTLOS/WAVVAP campaign, February 2004 . 153 13.3 Validation of AMSOS: comparison with HALOE and POAM . 155 14 Publication IV 159 15 Conclusions and outlook 163 Part I Stratospheric water vapour and principles of microwave radiometry 6 Chapter 1 Water vapour in the atmosphere The Earth atmosphere consists mainly of nitrogen (N2, 78 %), oxygen (O2, 21 %) and argon (Ar, 0.93 %). Other gaseous species and particles make up the remaining 0.07 %. Although the amount of these remaining component (trace gases) is very small compared to the major ones, they play a crucial role in atmospheric chemistry and dynamics. One of the most important tracers is water vapour. The vertical distribution of water vapour, on contrary to e.g. N2 and O2, decreases significantly with increasing altitude in the troposphere. Figure 1.1 shows the vertical distribution of water vapour according to the U.S. Standard atmosphere, [1]. The difference between the water vapour content at sea level and in the stratosphere is several orders of magnitude. The H2O-abundance in the stratosphere might look negligible compared to the troposphere. However, despite the concentrations that make water vapour a trace gas in the strato- sphere (concentrations similar to the ones of ozone), its role has a key influ- ence in several domains. There are three major fields where water vapour should be studied: its role in the ozone chemistry, its long lifetime affording studies of atmospheric dynamics and its major impact on the greenhouse effect. 1.1 Water vapour as a greenhouse gas A yearly average of the power density radiated by the Sun to the Earth is 342 Wm−2. However, the whole energy will not be absorbed by the Earth sur- face. About 107 Wm−2 will be reflected from clouds, aerosols, air molecules 7 8 Water vapour in the atmosphere 120 100 80 60 Altitude [km] 40 20 0 −1 0 1 2 3 4 10 10 10 10 10 10 H O volume mixing ratio [ppm] 2 Figure 1.1: Vertical distribution of water vapour according to the U.S. Stan- dard Atmospheres. The abundance changes for several orders of magnitude from the lower troposphere to the stratosphere (atmosphere) and from the Earth surface, and returned to space, [2]. Addi- tional 87 Wm−2 is absorbed in the stratosphere and troposphere by ozone, water vapour and clouds. The rest of the 148 Wm−2 is absorbed by the Earth surface. This power density would be enough for an Earth mean sur- face temperature of 255 K. However, the mean Earth temperature is 288 K, which is a result of the greenhouse effect. The surface temperature of the Sun is around 6000 K, which, according to Planck’s law on blackbody radiation gives a maximum radiation in the visible part of the EM-spectrum.