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My essay on the atmospheric boundary layer

  1. Nov 8, 2006 #1
    3 pages double spaced. Can someone please comment on it before Sunday? That would be nice. Thanks!

    the file located at http://students.washington.edu/achen89/3_boundarylayer.doc . Please feel free to e-mail corrections to hemaalpha@gmail.com

    Human civilization (and biological phenomena) has long been surrounded with the Earth’s atmosphere, a peculiar combination of gases that has made its existence both possible and secure. The atmospheric boundary layer (ABL) occupies the lowest 10-20% of the troposphere, which is the lowest layer of the atmosphere, containing a disproportionately large amount of the mass and kinetic energy of the atmosphere (although the height of the ABL varies). It happens to be the most dynamically active of the Earth’s layers, for the surface’s effects on it, such as friction and heating, are felt on timescales of less than a day. Such effects, involving heat, momentum, and matter, contribute to turbulence, absent in the free atmosphere (the layer of the troposphere directly above the ABL, which is separated by the capping inversion). Due to such turbulence, airplanes fly in the free atmosphere. A study of the ABL can provide insight to the conditions that must be maintained so that the human civilization can continue to live in a hospitable environment.

    The word “boundary layer”, unlike many other scientific words, traces its origin to the study of boundary layers in fluid flows – the boundary layer is the layer of fluid that is impacted the most by friction with a bordering region, so the ABL is the layer of fluid impacted the most by friction with the Earth’s surface. Since the atmosphere is a fluid, methods of fluid dynamics are used to study the atmosphere (along with the ocean). A defining characteristic of the ABL is turbulence, caused by both thermal convection due to thermal buoyancy and wind shear due to frictional forces. Turbulence consists of a number of swirls, called eddies, that interact non-linearly to create chaotic motions. The Navier-Stokes equations of momentum conversation are used to predict the motions of eddies in turbulent fluids. Unfortunately, the equations are impossible to solve under most conditions, and atmospheric scientists are consequently forced to use statistical, rather than deterministic, models in order to predict the eddy motions in the ABL.

    Ironically, such difficulties have instigated an active field of research surrounding the ABL. It is notable to remark that there is an entire journal, “Boundary Layer Meteorology” devoted to its study. When the deterministic models of the ABL are averaged over many eddies, persistent patterns can be measured and used for models. Computer models often used simplified models based on similarity theory to predict turbulence. One of the excitements of ABL research is that the research has coincided with the exponential increase in computing power of the last few decades, allowing for the implementation of more complicated higher-order models to predict the large eddies responsible for ABL dynamics. Furthermore, ABL research has also coincided with the development of equipment that can more accurately measure the variables important in the development of the ABL, such as airplanes, which allow for the measurements of such variables with respect to all four spatial and temporal dimensions.

    Turbulence within the ABL is the atmosphere’s natural response to perturbations in particular variables, the force responsible for mixing up a fluid more efficiently than molecular diffusion, the method of distributing heat, moisture, and chemicals throughout the Earth. When winds are low and the surface receives little thermal heating, ABL turbulence virtually disappears, paving way to a capping inversion with a height much lower than it would be elsewhere. Yet, if the capping inversion is too high, air molecules become dispersed over the Earth’s atmosphere, leaving way for a reduced number of air molecules at lower levels. [check Mars, see if its change in temperature with respect to height is lower than that of Earth’s]. If not for the effects of wind, frictional drag on the ground, the Coriolis effect from the Earth’s rotation, the consistent input and output of heat, and the unique chemical composition of the Earth, the Earth wouldn’t have the ABL it has. The ABL is important for ensuring that the atmospheric composition of the Earth is relatively homogeneous throughout, despite the input of heat and energy from without, and is consequently important for establishing that life can survive over many regions of the Earth.

    ABL research is important for several applications. The applications can be illustrated in its several subfields – urban meteorology, air quality control, agricultural meteorology, and numerical weather predictions. The development of urban environments has changed the ABL around such environments, resulting in surface heating and the development of artificial boundary layers that hem in pollutants. The distribution of pollutants is dependent on boundary layer calculations. Boundary layer calculations can help architects develop urban environments with minimal impact on the boundary layer. It is well known that weather forecasts are not especially reliable, especially long-term weather forecasts. Yet, research on the ABL can help improve such forecast models, which are improving with respect to time.Air masses are mobile ABLs that are responsible for the weather patterns that occur over the regions that they cover, and they form an important part of forecasts. It can be said that research on the boundary layer is important both in short-term weather forecasts and long-time climate models. Without boundary layer research, we do not have the accurate weather forecasts that so many people rely on. Without the boundary layer the way it is now, we lose the unique hospitable conditions that humans are so entitled to.
     
    Last edited: Nov 8, 2006
  2. jcsd
  3. Nov 17, 2006 #2

    Mk

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    Hmmmm. :bugeye: I'm looking hard, but "boundary layer" does not look like a word. :biggrin: What do you mean "unlike many other scientific words"?
     
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