energy_balance

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School

Mesa Community College *

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122

Subject

Geography

Date

Oct 30, 2023

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pdf

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Uploaded by CommodoreDove531

Energy Balance This lecture discusses the energy balance. 1

Radiation Laws • Stefan-Boltzmann • The total amount of radiation from a blackbody is proportional to the 4th power of its absolute (K) temperature • Wien’s • The wavelength of maximum radiation is inversely proportional to its absolute (K) temperature Not every surface perfectly absorbs or emits at every wavelength. The atmosphere is a good example. The atmosphere doesn’t really absorb shortwave radiation, but it does absorb longwave radiation. Since we need to know what wavelengths we’re dealing with, we can use two other laws to help us understand. The Stephan-Boltzmann law tells us that hotter objects emit more radiation than colder objects. Wien’s law tells us that the hotter the object, the shorter the wavelength it emits. The sun is much hotter than the earth, and following Wien’s law, we know the wavelengths emitted by the sun are shorter than those emitted by earth. 2

3 Q* = (SW↓ + LW↓) – (SW↑ + LW↑) INCOMING ↓ OUTGOING ↑ Energy Balance • Net Radiation (Q*) • Incoming - outgoing This chart shows incoming and outgoing radiation during one day A budget approach to energy is an application of the First Law of Thermodynamics. If energy is incoming it’s being added to the budget. If energy is outgoing, it’s being lost from the budget. Incoming shortwave is emitted by the sun, and outgoing shortwave is the surface reflectivity or albedo. Incoming longwave is counter-radiation from clouds, and outgoing longwave is emitted from earth surfaces. The net radiation balance is a calculation of the incoming radiation minus the outgoing radiation. A positive net balance is a surplus which warms. A negative net balance is a deficit which cools.

Top of atmosphere = 1317 W/m 2 , SW incoming at surface only 900 W/m 2 . What happened to the other 417 W/m 2 ? Hourly Radiation Fluxes - Barnwell, SC, July 24 0 100 200 300 400 500 600 700 800 900 1000 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Hour Radiation (W/sq.meter) SW incoming SW outgoing LW incoming LW outcoming outgoing There are imbalances in the energy budget at different latitudes and seasons. Surplus and deficit can be measured during the day. This chart shows a simple energy budget for one summer day in Barnwell, South Carolina. Incoming shortwave is at its highest around noon. When the sun sets, incoming shortwave decreases to zero. Outgoing longwave increases during the day as surfaces get hotter. The hotter the surface, the more outgoing longwave emitted from the surface. At the top of the atmosphere over Barnwell, the satellite measured 1317 W/m2. However, at the surface, incoming shortwave was only 900 W/m2. The decrease occurred due to scattering, reflecting, and absorption of the incoming shortwave. If the incoming shortwave doesn’t reach the surface, then it’s considered lost and gone from the energy balance. 4

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