energy_balance

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Mesa Community College *

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122

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Geography

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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.

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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

5 Diurnal Energy Balance • Day: surplus • Surface warms • Night: deficit • Surface cools A generalized diurnal energy balance shows the timing of surplus and deficit and air temperature. When there’s a surplus of radiation, temperatures increase. With a deficit, temperatures decrease. Air temperatures don’t change from incoming shortwave. Air temperatures change from outgoing longwave. The ground has to heat and emit longwave radiation before the air temperature can increase. At night as outgoing longwave is the dominant process, temperatures cool until the coldest time of day, usually around sunrise.

Partitions • Sensible heat (H) • Air temperature • Soil heat (G) • Soil temperature • Latent heat (LE) • Change state El Mirage, CA Pitt Meadows, BC Energy partitioned into sensible heat Energy partitioned into latent heat The net radiation is what is available from incoming and outgoing radiation. Net radiation is partitioned as to how it would be used. The climate of a location is based on how energy is partitioned at that location. Sensible heat is the energy that does the work of changing air temperatures. Soil heat is the energy that changes the ground temperature. Latent heat is the energy absorbed or released during the phase change of water. 6

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7 Latent Heat (LE) • The amount of energy absorbed or released during phase changes of water Latent heat provides a tremendous amount of energy in the earth- atmosphere system. When water changes phase, it either requires energy or it gets rid of energy. Water needs energy to move from a liquid to a gas, so the water absorbs energy. Evaporation is a cooling processes. The water absorbs energy which cools the air. Condensation warms the air because it releases energy. The latent heat of condensation is the energy that powers hurricanes and tornadoes.

Soil Heat (G) • Energy stored in surface • Factors: • Specific heat • Amount of energy required to raise 1gram 1° C • Thermal conductivity Which heats up faster? Substance Specific Heat (cal/gram) water 1.00 wet mud 0.60 ice 0.50 sandy clay 0.33 dry air 0.24 quartz sand 0.19 Soil heat is controlled by the specific heat of the material. Specific heat shows much energy is required to change the temperature of the material. Water has the highest specific heat, which means it takes the most amount of energy before it changes temperature. That’s the reason why a backyard pool and the sidewalk next to the pool have different temperatures. The sidewalk will heat five times faster than the pool. 8

Urban Heat Islands • Urban materials store heat • Releases slowly at night, causing warmer nighttime temperatures • Urban center hotter than surrounding rural areas • Can affect local air currents and weather conditions Night temperatures are warmer in the city core Sky Harbor Airport The Urban Heat Island is found in cities. Research shows that this issue affects all urbanized locations, and the larger the city, the stronger the impact. The city core becomes much warmer than the surrounding countryside because urban materials like cement and concrete store energy as soil heat. These materials store heat during the day and then release it very slowly at night. This makes the nighttime or minimum temperatures warmer than they would be in a natural landscape. The Phoenix Urban Heat Island is one of the strongest in the world, with nighttime temperatures in Phoenix up to 15 degrees warmer than the rural landscape. 9

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