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MASTER's THESIS the Lifetimes of Tropical Cloud Regimes

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MASTER's THESIS the Lifetimes of Tropical Cloud Regimes 2010:080 CIV MASTER'S THESIS The Lifetimes of Tropical Cloud Regimes Mattias Häggkvist Luleå University of Technology MSc Programmes in Engineering Space Engineering Department of Applied Physics and Mechanical Engineering Division of Physics 2010:080 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--10/080--SE Preface After five years of studies at the MSc programme in Space Engineering at Lule˚a University of Technology it was time for me to write my thesis. I had recently taken a course in atmospheric physics that I found very interesting. So I asked Professor Sverker Fredriksson if he could help me find a thesis project within this subject. This is how I got in contact with Professor Christian Jakob at the School of Mathematical Sciences, Monash University, Melbourne. He suggested that I could work with studying the lifetime of tropical cloud regimes. Feeling a bit nervous, not knowing if I would be able to manage this with my prior knowledge, I left for Melbourne in the fall of 2009. Once there, any doubt I previously had disappeared. I got a very warm welcome to the university and everyone was very helpful. After three intense months it was time for me to leave and return to Sweden. I did this with a lot of new knowledge and with a feeling of gratitude to all those who helped me. First I would like to thank my supervisor at Monash University, Professor Christian Jakob, for all the time he spent helping me, always with a smile. I am deeply grateful for all your help. I would also like to thank my examiner at Lule˚aUniversity of Technology, Professor Sverker Fredriksson, for helping me to get this thesis project and for all his good advice. I would also like to thank Sven Molin at Lule˚aUniversity of Technology for his help with the trip to Melbourne. i Abstract This MSc thesis in Space Engineering is about the lifetime of cloud regimes over the tropical and subtropical regions of Earth. The International Satellite Cloud Climatology Project D1 dataset and the cloud regimes, derived using a cluster algorithm on cloud top pressure and optical thickness, form the basis of the analysis of the lifetime. The cloud regimes are measusered how long they appear at a certain location. Significant differences were found, among other things between convectives cloud states and suppressed ones. Transitions be- tween the eight regimes were also examined and it was found that some regimes transformed to regimes with similar physical properties whereas others didn't, suggesting that some regimes are transition regimes and others represent a more stable or unstable end state. iii Contents 1 Introduction 1 2 Theory of the atmosphere 3 2.1 Physics of the atmosphere . 3 2.2 Cloud formation . 4 2.3 Cloud properties . 4 2.4 Climate . 6 3 Tropical cloud regimes 9 3.1 ISCCP D1 dataset . 9 3.2 Cluster regimes . 10 3.2.1 CTP-τ diagram . 10 3.2.2 Anderberg k-means clustering algorithm . 12 3.3 The eight regimes . 13 4 Cloud regime lifetime 17 4.1 Global results . 17 4.2 Tropics . 39 4.3 Subtropics . 40 4.4 The Pacific . 42 4.4.1 Tropic Pacific . 42 4.4.2 Subtropic Pacific . 45 5 Summary and final thoughts 47 5.1 Conclusions . 47 5.2 Future work . 49 Bibliography 52 Appendix A Lifetimes plots for the entire region 53 Appendix B Probability for the entire region 55 Appendix C Result for the tropics 59 v Appendix D Pacific plots 61 vi Chapter 1 Introduction All of us have probably at one point in our lives laid on our back in the green grass, staring up at the blue sky covered with white clouds in differents shapes, letting our imagination run wild, fantasizing of what the clouds look like. Maybe we have looked with fascination at noctilucent cloud shimmering in the evening, awaited with anxiety as a wall cloud approaches from the distance or gazed from our airplane window over a seemingly endless white ocean of clouds. But clouds are not just beautiful to look at, they affect us in our everyday life as well. Clouds play a major role in Earth's climate. The Sun is the driving force behind our climate and life here on Earth. Around 30 percent (Goode et al., 2001) of the incoming solar radiation is reflected from Earth, a part of this is due to clouds. But the atmosphere is a complex system and in order to enhance our knowledge of it, further studies must be conducted, to gain a better understanding of the Earth's climate. One of the hottest, and most controversial, political issues today is that of global warming. But no matter in what political camp one stands, learning more about the climate will help us with our everyday lives. The climate is not just something that will happen in our future, it is something that is happening today. The weather is something that affects us everyday. It affects the business man flying to his meeting, it affects the farmer growing his crops and the family spending the weekend by the sea. Getting a better understanding of Earth's atmosphere and climate will help getting better weather and climate models and hence better predictions and forecasts. In this thesis the lifetime of tropical cloud regimes will be investigated. An- alyzing cloud data deduced from satellite observations and sorting them into groups using clustering techniques have privous been used by Jakob and Tse- lioudis (2003). Using data from ISCCP (International Satellite Cloud Clima- tology Project), they showed that one can identify four physically interpretable cloud regimes over the tropical West Pacific. In later studies (Jakob and Schu- macher, 2008; Jakob et al., 2005) the cloud regimes' precipitation, radiative and thermodynamical properties have been examined. In Rossow et al. (2005) and Singh and Jakob (2010) the region was first expanded to the latitude band 1 15◦ S−15◦ N and later to 35◦ S−35◦ N and eight regimes were then identified. It is the time scale of these cloud states that will be investigated in this thesis. The data used will be derived from the ISCCP D1 dataset, and the same region as in Singh and Jakob (2010) will be examined. Different aspects of the time scale will be examined, such as for how long time the clouds regimes live and the probability of the transition from one regime to another. Chapter 2 gives a brief overview of fundamental atmospheric physics relevant to clouds. An introduction to the parcel concept and how clouds are formed will be described. A brief description of different cloud types will also be included in this chapter. Since the data used in this thesis are gathered from tropical and subtropcial areas, the climate in these regions will be discussed as well. This is meant to give a basic understanding of the atmosphere and familiarize the reader with some of the terms that will be used. Readers already familiar with these concepts can skip this chapter and go on to Chapter 3. Here the data used will be presented. The chapter will give a look into how clouds are analyzed in ISCCP from satellite measurements. Then the cluster algorithm, as well as its application to the ISCCP data, will be described. The regimes themselves and their physical interpretation will be examined. What other information is included with the cluster data will also be described, helping with the analysis in Chapter 4. The analysis will be carried out in several steps. First a global view of the lifetime will be provided. The lifetime will be examined in several different ways and with different goals in mind, such as how long a regime occurs at a certain spot or what regime is most likely to follow it. This will highlight different aspects of the lifetime of the tropical cloud regimes, but also tell something about what the physcial properties of the regimes are, and thus how these two different parts are connected. Since the regimes all have different physical properties, they are expected to be more common in smaller regions. To examine if this influences the lifetime, a comparison will be made between the tropical and the subtropcial region on Earth as well as some selected ocean basins, such as the Pacific. 2 Chapter 2 Theory of the atmosphere 2.1 Physics of the atmosphere The Earth's atmosphere stretches out from the surface and roughly 100 km up into space. It is made up of several layers with the one closest to the ground called the troposphere. The extent of the troposphere, which varies depending on the current conditions, is around 10 − 15 km. It is in the troposphere where most of the weather takes place and it is thus the most important part of the atmosphere for this thesis. In order to get a better comprehension of what might affect the lifetime of the cloud regimes, let us start by looking at the fundamental physics of the atmosphere. A simple way of explaing the physics, and more specifically circulations and uplifting, of the atmosphere is the so-called parcel concept (Andrews, 2005). This can be visiulized as a small volume of air, a parcel, which is affected by the surrounding air but does not effect its surrounding environment. This will make it easy to understand how air reacts in differents physical environments, e.g., a parcel that is warmer than its surrounding, will due to a buoyancy effect be lifted until it reaches equilibrium. It should be noted that the parcel concept is merely a way of understanding what happens in the atmosphere rather than an exact description of it.
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