{"id":20,"date":"2016-11-16T09:17:32","date_gmt":"2016-11-16T09:17:32","guid":{"rendered":"https:\/\/expearth.h.uib.no\/wordpress\/?page_id=20"},"modified":"2020-04-22T11:44:17","modified_gmt":"2020-04-22T11:44:17","slug":"atmosphere","status":"publish","type":"page","link":"https:\/\/expearth.uib.no\/?page_id=20","title":{"rendered":"Atmosphere"},"content":{"rendered":"

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MODEL<\/a><\/p><\/div><\/div>\n

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PHYSICS<\/a><\/p><\/div><\/div>\n

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DYNAMICS<\/a><\/p><\/div><\/div>\n

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CHEMISTRY<\/a><\/p><\/div><\/div>

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The all-important thin blue line<\/h2>\n

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Seeing the Earth from space gives us a unique perspective on the eggshell-thin layer of the atmosphere \u2013 the warming blanket of breathable gases on which all life on Earth depends. Credit: NASA<\/figcaption><\/figure>\n

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What happens in the atmosphere has a direct effect on our everyday lives and on every scale. We have local weather forecasts that keep us informed about short-term atmospheric conditions, all the way up to the global-scale climate projections which tell us about future, long-term changes in the climate system as a whole: the atmosphere operates at all scales: from the microscopic to the global; from seconds to millennia.\u00a0 It is one of the most important and most complex of the climate model components and is intrinsically linked to each of the other elements of the climate system.<\/p>\n

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So what is the atmosphere made up of?<\/p>\n

In dry air, 78% (by volume) is made up of nitrogen, 21% oxygen and 1% argon. These numbers are rounded up, which hides the much smaller contribution from CO2<\/sub> (less than 0.04%) and other trace gases such as neon, helium, methane and krypton.<\/p>\n

But don\u2019t let their comparatively small volume deceive you: these trace elements – especially CO2<\/sub> and methane – are some of the biggest players in global climate change, both over geological timescales and over more rapid transitions.<\/p>\n

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The atmosphere is a rapidly changing reservoir in comparison to the other components. Mixing within a hemisphere occurs over a matter of weeks, between hemispheres in the order of about a year, but the different gases themselves can have vastly different residence times, from weeks to thousands of years.<\/p><\/div><\/p>\n

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The layers of the Earth’s atmosphere. Credit: Colourbox.com \/ Panacea.<\/figcaption><\/figure><\/div><\/div>
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Model<\/h2>\n
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As the atmosphere moves across the land surface it encounters obstacles. Lenticular clouds like these form as fast-moving, moist air is forced over a topographic barrier such as a mountain chain, creating an atmospheric wave in its wake where the moisture condenses as clouds. Photo: Colourbox.com \/\u00a0 Dmytro Pylypenko.<\/figcaption><\/figure><\/div><\/div>\n

The atmospheric model used in NorESM is the Community Atmosphere Model (CAM) from the Community Earth System Model (CESM), but NorESM has its own aerosol chemistry, physics and cloud modules developed by the Oslo NorESM community.<\/p>\n

The horizontal grid is based on a typical bipolar latitude-longitude grid. The vertical coordinate is more complicated and uses changes in pressure with height.<\/p>\n

Closest to the ground, the model uses \u201cterrain-following\u201d \u00a0sigma coordinates, which as the name suggests, follow the slopes and shapes of the surface. Conforming to the natural terrain eliminates issues that other coordinate systems can have with matching to the land surface topography. It also allows the scientists to increase the resolution near the ground, where they may wish to look more closely at boundary layer processes such as turbulence or daily temperature changes. The upper atmosphere of the model is purely pressure based, forming layers that are smoother and easier for modelling upper atmosphere processes such as radiative transfers – and again, the resolution here can be increased. Between these two is a hybrid layer combining aspects of the terrain-following sigma and pressure coordinates. Since pressure is itself one of the governing physical properties in the atmospheric equations, keeping the coordinates in this normalised pressure format makes it much easier to use in further mathematical calculations for other properties and processes.<\/p><\/div>

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Diagram of the vertical structure of the atmosphere in NorESM, using CAM. Near the land surface the atmospheric layers conform to the topography, but as you move higher, this influence is reduced, until you reach the upper atmosphere, where the layers are determined by pressure alone. From the NCAR CAM website.<\/figcaption><\/figure>\n

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Physics<\/h2>\n

The Earth\u2019s atmosphere interacts with radiation from the sun in many ways \u2013scattering, absorbing and reflecting off particles and surfaces. The physics of the atmosphere is largely driven by these interactions. There are a lot of processes which occur, but most are summarised in the figure below. This is from the IPCC 5th Assessment Report, 2013, and breaks down the global energy budget into a number of different processes.<\/p>\n

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Global mean energy budget under present-day climate conditions. Numbers state magnitudes of the individual energy fluxes in W m\u20132, adjusted within their uncertainty ranges to close the energy budgets. Numbers in parentheses attached to the energy fluxes cover the range of values in line with observational constraints. From IPCC 5th Assessment Report 2013. (TOA = top of atmosphere).<\/figcaption><\/figure>\n

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Many of the NorESM modellers are based in Bergen, in western Norway. Here, in the summer, the sun only just dips below the horizon for a little while during the night – which leaves plenty of time for long, colourful sunsets in the fjords. Credit: Elizabeth Farmer.<\/figcaption><\/figure><\/div><\/div>\n

You\u2019re probably more aware of some of the physical processes going on in the atmosphere than you realise.<\/p>\n

It\u2019s actually the scattering of this incoming radiation that gives us the blue sky, the colourful sunsets and the greys and whites of clouds.<\/p>\n

Particles in the atmosphere that are smaller than the wavelength of the incoming radiation (so things like gas molecules), cause a type of scattering known as Rayleigh scattering.\u00a0 This affects shorter wavelengths the most \u2013 the wavelengths at the blue end of the spectrum. \u00a0Sunlight travels the least distance through the atmosphere\u00a0when it’s directly overhead, so during the day it’s only the blue end of the spectrum that is affected by this scatter. This results in a\u00a0diffuse blue colour of the sky. As the sun sinks and the angle of the sun decreases, the sunlight has to travel through more of the atmosphere to reach us. This means that more and more of the shorter wavelengths are scattered away, with the only unscattered light left being towards the longer, redder wavelengths \u2013 hence the colourful sunsets.<\/p>\n

When the atmosphere is full of particles that are larger and closer to the wavelength of the incoming radiation (like water droplets, for example), a different form of scattering occurs \u2013 Mie scattering. \u00a0In this case all wavelengths tend to be scattered in a similar manner.\u00a0 Water droplets in clouds create Mie scattering. Since the incoming radiation is scattered equally across all wavelengths of visible light, we see the clouds as grey or white.<\/p><\/div>

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– Radiation budget –<\/h3>\n
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The Sun’s rays strike the Earth at different angles depending on latitude, affecting the amount of energy received in these areas. Image taken from the University of Bergen’s “Causes of Climate Change” MOOC.<\/figcaption><\/figure>\n

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The balance of incoming and outgoing radiation is central to the climate system, and it\u2019s in the atmosphere that this budget is determined.<\/p>\n

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The amount of radiation received by the Earth at the top of the atmosphere depends on the latitude: close to the equator, solar radiation is at its strongest, with the Sun directly overhead, but as you move poleward, the Sun\u2019s rays strike at an angle and the incoming radiation is spread across a larger area. The result: more energy per unit area in the tropics compared to the high latitudes.<\/p><\/div>

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To balance the incoming short-wave radiation, the Earth\u2019s atmosphere emits infrared long-wave thermal radiation. The amount of thermal radiation emitted by an object depends on its temperature \u2013 so for the atmosphere, the discrepancy between the poles and the equator is again evident \u2013 the cold poles and cold cloud tops emit far less than the arid and cloudless areas of the tropics.\u00a0 This is very clear in the NASA visualisation below.<\/p>\n

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