Atmospheric Variations in Energy

When we consider the atmospheric envelope that surrounds our solid Earth, there exists an upper boundary, the edge of outer space, and a lower boundary, the Earth's surface. Given the spherical nature of Earth, energy can only enter the atmosphere from the upper and lower boundaries.

Energy that enters the atmosphere through the upper boundary originates from the sun and passes through the voids of outer space, reaching the upper atmosphere at an altitude of somewhere near 650 km (kilometers). How much energy passes through the thermosphere (=500-km depth), mesosphere (=85 km), and stratosphere (=50 km) before reaching the troposphere is a function of the mass that exists within these layers. It is important to note that while the sun emits radiation across a large spectral range, the large majority of radiation originates from the ultraviolet, visible, and near-infrared portions of the electromagnetic spectrum. The manner in which this “shortwave” radiation interacts with the atmospheric mass it encounters varies depending on the character of the mass, particularly its size relative to the wavelength of radiation.

As solar radiation passes through the atmosphere, it can be scattered, absorbed, or transmitted. That which is absorbed raises the temperature of the absorbing body, and if a thermal gradient exists away from that body, heat will be radiated (in many cases “re-radiated”) in that direction. Radiation that is scattered will in many cases be scattered multiple times before being absorbed or returned back to outer space. When the scattering occurs back in the general direction from where it came, it is referred to as [Page 162]“backscattering,” sometimes also known as reflection. Radiation that is not backscattered is “forward scattered.”

On average, about 25% of the solar radiation that reaches the troposphere is absorbed and 25% is backscattered, which leaves about 50% that reaches the Earth's surface. About 4% of the radiation that reaches the Earth's surface is backscattered, so only about 47% of the roughly 1,352 W/m-2 (watts per square meter is a measure of radiant flux density using SI units) that reaches the top of the atmosphere (a value commonly referred to as the solar constant) is absorbed by the Earth's surface. If we are considering anything other than a global average, all these values would vary considerably, and the solar angle of incidence would also have to be considered.

To understand that partitioning of local-scale atmospheric energy between its sensible and latent states we would also have to consider adiabatic processes, especially those associated with cloud and precipitation development and dissipation. So the variation of atmospheric energy on anything less than the planetary scale is infinitely complex, and interactions of all the processes is still not well understood. Nevertheless, numerical models continue to be constructed in an attempt to model these processes for purposes of forecasting future states of the atmosphere on a variety of space and timescales. Some of these are used to forecast weather from a couple of days to periods of 2 wks. (weeks), while others attempt to characterize the state of our climate system decades into the future. At this point in time, with our lack of knowledge regarding the interactions of some of these important processes, the difficulty in simulating cloud and precipitation processes is a limiting factor that cannot be overlooked. In fact, for meteorological purposes, there tends to be too much reliance on numerical models in developing weather forecasts. This situation has grown to the point where it has earned the title “meteorological cancer,” since too heavy a reliance on forecast models has resulted in a general deterioration of our forecasting skills.

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