Friday, April 13, 2007

Atmospheric heat: What gets in, what gets out, what stays

The Earth's atmosphere is continually changing. Its temperature distribution is really a snapshot of a continual flow of radiant energy in from the Sun and out from the Earth. How heat flow is distributed is how temperature is determined.

Heat can not only flow by different mechanisms (convection and radiation in the air, conduction and convection in the water and ground), its flow can be split up by different parts of the atmosphere and Earth so that different parts "capture" different amounts of heat at different efficiencies. Extra flows, not directly driven by the Sun, appear as well that remain solely in the lower atmosphere and modify its temperature distribution. The simple picture of solar radiation absorbed and re-emitted is too simple. How heat is captured, concentrated, and moved around in the lower atmosphere is not the same question as how solar energy gets in as light, then escapes as heat, although the two questions are intertwined.

The mean solar radiant energy flux at the Earth's orbit is about 1370 watts per square meter. Because only half the Earth is facing the Sun and the Earth is a sphere, not a disk, geometric factors reduce the effective input by a factor of four, to about 342.5 watts per square meter. About 65% of the Sun's incoming light makes it into the lower atmosphere. Clouds reflect 35% back into space and absorb another 19%.* The 65 - 19% = 46% that remains is absorbed by the Earth's surface. The Earth re-releases that 46% energy flow as infrared radiation, upward convection, and latent heat of water vapor. (The Earth also absorbs another flow of heat directly from the air near the surface that helps to evaporate the water.) The clouds re-emit their 19% share as radiation as well, both upward and downward. The radiative part of this flow is fairly easy to understand. The convection and evaporation part is much harder. That 65% of the total incoming must flow at the same rate back into space. It mostly flows upward by radiation, some by convection; and, once it gets to the upper atmosphere, all by radiation. But pieces of that flow are diverted and re-routed in the lower atmosphere. And the lower atmosphere's air and water are not in complete equilibrium, even locally. Water vapor, condensed water, and air all have somewhat different temperatures.** Water vapor and condensed water droplets are not in chemical (phase) equilibrium, except at saturation (that is, in clouds).

If the Sun were "turned off" and then suddenly "switched on," a very cold Earth would heat up, very gradually, over months, until the radiant energy flow in = radiant energy flow out. This "steady state" (misleadingly called "radiative equilibrium" sometimes) is the foundation of understanding the Earth's climate. Many weather books refer to it as the Earth's "heat balance." It's not really an exact steady state either; rather, it varies over a small range. Sometimes the Earth takes in a little excess radiation, sometimes it lets go a little excess heat.

The atmosphere has natural "oscillation modes" that exhibit oscillations in time and waves in space, in pressure, density, and temperature, accompanied by small surpluses or deficits in the "heat balance." You're familiar with some of these oscillations and waves as hemispheric and quarter-sphere oscillation modes with names like El NiƱo/Southern Oscillation and North Atlantic Oscillation. (Think of the atmosphere as a spherical drum wrapped around the Earth, and you'll get the picture.) They aren't exactly periodic, like a clock. They're a more like a drunken clock that never repeats exactly the same way twice.

The nature and importance of these oscillations will take up a later posting. But next we'll see how the heat flows shape how temperature is distributed. The combination of split-up and secondary flows and variable "heat-capturing" efficiencies is what determines specific temperatures values.
* Not all atmospheric heating comes from below; the 19% represents direct energy input from the Sun into clouds. The upper atmosphere is heated mainly by direct absorption of solar radiation, although it's a small fraction of the total solar energy flow.

** Sometimes we get dramatic reminders of this, as when hailstones fall from very high, very cold altitudes and hit the ground, unmelted - on a hot summer day.

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