Reading Assignment: A&B: Ch. 3 (p ) CD: tutorial: energy balance concepts interact. ex.: shortwave & longwave rad. LM: Lab.

Transcription

1 Radiation Balance 1 Radiation Balance Reading Assignment: A&B: Ch. 3 (p ) CD: tutorial: energy balance concepts interact. ex.: shortwave & longwave rad. LM: Lab. 5 Radiation = Mode of Energy transfer observes the conservation principle radiation emitted from Earth/atmosphere: terrestrial, longwave radiation radiation emitted from sun: solar, shortwave radiation when solar radiation is absorbed in the Earth/atmosphere, part or most of it is re-emitted as longwave radiation Balance conservation of energy: storage change = input output The Radiation Balance can be expressed in a budget equation, composed of different terms that each represent a radiation transport or conversion process Conservation of energy Q* = (K - K ) + (L - L ) Units: W m -2 = K* + L*

2 Radiation Balance 2 Q* - net all wave radiation net radiative energy that is absorbed and then transformed into a different form (non-radiative) becomes energy available to be partitioned in the energy balance, i.e. to heat the air, heat the ground or evaporate water (see below) K - incoming shortwave radiation. emitted by the sun, transmitted to location of the balance (e.g., top of atmosphere or surface) dependent on solar altitude, transmissive property of the atmosphere above K - outgoing shortwave radiation. outgoing shortwave radiation (reflected!) Depends on K and the albedo (α): K = α K K* - net shortwave radiation (K*= K - K ) Energy that is absorbed may be reradiated at longer wavelengths.

3 Radiation Balance 3 L - incoming longwave radiation: Depends on sky temperature and sky emissivity (ε s ) L = ε s σt s 4 T s ; ε s : summary effect of all layers of the atmosphere; depend on cloud cover, humidity, temperature structure. Can be calculated from radiosonde data L - outgoing longwave radiation: Depends on surface emissivity and surface temperature L = ε 0 σt 0 4 L* - net longwave radiation (L*= L - L ) ε 0 : see list of values in Lab Manual, lab#5 Global Average (i) Shortwave radiation (A&B, Fig. 3-4) total reflected: 30% (= global albedo) total absorbed: 70%

4 Radiation Balance 4 (ii) Long-wave radiation (A&B, Fig. 3-5) total lost to space: 70% (= absorbed solar) L at surface (from atmosphere): greenhouse effect = additional energy source for surface atmospheric net loss: compensated from K (absorbed) & convection from surface (iii) All wave net radiation (A&B, Fig. 3-7) in the atmosphere and at the surface: non-zero net radiation other forms of energy transport must compensate balance can be formed at any level: o top-of-the-atmosphere (TOA) o atmosphere

5 Radiation Balance 5 o surface these numbers are long-term global averages, average cloud cover, temperature, etc. considerable spatial and temporal (weather, seasons, climate!) variations exist Global Distribution (spatial variations) Incoming solar radiation ERBE Video Spatially variable because of Earth-sun geometry Maximum solar radiation receipt at the top of the earth's atmosphere at right angles Solar constant: 1367 W m -2 (i) January incoming solar radiation: K Jan Question: Is this the radiation at the surface or the top of the atmosphere? [Hint: look at isopleths across continents.]

6 Radiation Balance 6 January net shortwave radiation at the surface Note: here, negative values refer to K* taken up by the surface (different sign convention from the one we use) (iii) January net longwave radiation at the surface Note: here, positive values refer to L* deficit at the surface (different sign convention from the one we use)

7 Radiation Balance 7 (iv) Net Radiation: Q* - spatially variable To prevent runaway heating in Q* surplus (net gain) region (latitudes < 40 N/S), and runaway cooling in Q* deficit (net loss) regions (latitudes > 40 N/S), energy is transported from the surplus to the deficit regions (poleward transport) by: ocean currents (~1/3) warm/cold winds (sensible heat) (~1/3) moiisture iin aiir (llatent heat) (~1//3)

8 Radiation Balance 8 5. Temporal Variations Observations at long-term research location: Observations at one location Morgan Monroe State Forest Biosphere-Atmosphere Exchange Project The 46 m (150 ft.) MMSF AmeriFlux Tower with Energy Balance Instrumentation

9 Radiation Balance 9 (i) Clear Summer Day, July Q* Kdn 800 Kup Flux (Wm -2 ) Ldn Lup Time What will happen on a: (ii) Cloudy Summer Day - overcast, stratus Q* Kdn Kup Ldn Lup Flux (Wm -2 ) Time (draw the changes into the figure)

10 Radiation Balance 10 (iii) Cloudy Summer Day - partly cloudy, cumulus Q* Kdn Kup Ldn Lup Flux (Wm -2 ) Time (draw the changes into the figure) (iv) Cloudy Winter Day - what happens if it snows? Q* Kdn Kup Ldn Lup Flux (Wm -2 ) Time (draw the changes into the figure)