Radiation

• Group 3: Manabe and Wetherald (1975) and

Trenberth and Fasullo (2009)

– What is the energy balance of the climate system? How is it altered by greenhouse

gases? How does it reach a new equilibrium? What does that new equilibrium look

like? (Jenna and Josh)

– Why does the stratosphere cool? (Peng)

– How does the T&F differ from the simple radiative‐

convective adjustment we discussed in class? (Ryan)

– How do we know and how well do we know the energy budget? That is, can we test these ideas

against observations? (One member of the group may have to do some additional research on this)

(Thania and Shaun)

What is the energy balance of the climate system?

RT: Net radiation at top of atmosphereASR: Absorbed Solar Radiation – affected by

planetary albedo and cloud coverOLR: Outgoing longwave radiation – affected by

surface temperature and cloud cover

RT = ASR - OLR

How is the energy balance altered by greenhouse gases?

Greenhouse gases increase radiative forcing → Higher surface temperatures, More evaporation → More water vapor in atmosphere

With higher surface temperatures comes less snow and ice, reducing the planetary albedo

The role of clouds is examined in the paper by Trenberth and Fasullo (2009)

Trenberth, K. E., and J. T. Fasullo (2009), Global warming due to increasing absorbed solar radiation, Geophys. Res. Lett., 36, L07706, doi:10.1029/2009GL037527.

Changes in net radiation (top), -OLR (middle), and ASR (bottom) from 1960 to 2100 using 13 CMIP3 models. On the right are averages over the period of 1950 to 2100.

Reaching a New Equilibrium

1. Higher Surface Temps2. Increased es

1. Increased evaporation2. Increased water vapor3. Positive greenhouse feedback

3. Increase in OLR due to differential tropospheric warming

1. Partially offsets water vapor feedback4. Reduction in albedo

New Equilibrium

OLR and ASR need to balance for EquilibriumOLR increases due to increasing temperatureASR increases due to reduced albedoFrom 1950-2000

Decrease in OLR, net warming By 2050, in T&F

Increase in OLR due to warming balances greenhouse effect

Why Does the Stratosphere Cool?

http://www.atmosphere.mpg.de/enid/20c.html

H20CO2

σT14

σT24

σT04

To

Ts

For the Visual Learners:

Atmospheric Window

Troposphere

Stratosphere

CO2

Space

1) More greenhouse gas is added to the tropospheric layer8) To balance this increased output, the stratospheric temperature must decrease 7) From the perspective of space, the radiation output from the stratosphere increases6) The stratosphere therefore absorbs and re-emits more radiation5) From the perspective of the stratosphere, more radiation is emitted from the top of the troposphere4) Tropospheric temperature must increase in order to balance this3) From the perspective of space, outgoing radiation is reduced (due to smaller atmospheric window)2) Outgoing radiation is absorbed more efficiently (the atmospheric window becomes smaller)

The differences between models used in book and in paper

Book (Wallace & Hobbs, 2006) Paper (Manabe & Wetherald, 1967)

How do we How do we knowknow the energy budget? the energy budget?

The global annual mean Earth’s energy budget for the Mar 2000 to May 2004 period (W/m²). (Trenberth, et al. 2009)

““know”know”qualitatively: physical processes involved in the transformation and transfer of the radiation and heat fluxesquantitatively: values of the radiation and heat fluxes

TOA (SW, LW) 31. Land based observations: observational

data (vertical distributions of T, humidity (rh and q), P compute the terms by equations (Manabe et al., 1964; Manabe et al., 1967)

2. Coupled ocean-Atmosphere GCMs: Coupled Ocean-Atmosphere GCMs simulate/predict the terms (Trenberth & Fasullo, 2009)

3. Remote Sensing: instruments in several platforms, like EOS Aqua (CERES, AIRS, MODIS), Nimbus-7 (Wielicki et al., 1996, 2006)

ATMOSPHEREI.SW (absorption, scattering, reflection)1, 2, 3II.LW 1, 2 (Cloudy, clear sky param.), 3

SURFACEI.LH=f(E, P) 1, 2II.SW 1, 2, 3III.LW 1, 2, 3

How Well Do We Know? Global Energy Budget

1. Top of the Atmosphere Fluxes1. Satellite Observations – Balanced within ~3W/m2 (Kiehl et al. 1994)

1. Solar Irradiance varies from satellite to satellite – 1365 to 1373 W/m (Ardanuy et al. 1992)

1.- Earth Radiation Budget Experiment (ERBE)* 1985-1989 - Designed to measure global albedo, fluxes, and solar radiation

2. Solar Irradiance varies throughout time2.Associated with the buildup of greenhouse gases 3.Associated with heat changes in heat storage in climate systems (i.e., El Niño events)

NASA’s Modern Era Retrospective-analysis for Research and Applications (MERRA)

Global Energy Budget (cont.)How Well Do We Know? (cont.)

1. Longwave Radiation Fluxes – Values differ by 21 W/m2 (Kiehl & Trenberth, 1997)1. Model Calculations 2. Depends on temperature and efficiency of gaseous absorber at a given wavelength (Kiehl & Ramanathan, 1983)

1. Cloud Forcing = Clear Sky – Cloudy Sky (30 W/m2)2. Gaseous Absorbers affect cloud emissivity

(Rothman et al. 1992)1. Aerosol Overlap

2. Shortwave Radiation Flux –1. Model Calculations –

2. Albedo uncertainties are an issue. 1. How significant a roll does water vapor presence play? 1. To what extent do cloud and aerosol overlaps affect the balance?

1. Surface Fluxes – • Sensible Heat Flux = SW – LW – LH

Atmospheric Science Data Center

IPCC Report (2007)