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 = ASR - OLR • RT: Net radiation at top of atmosphere • ASR: Absorbed Solar Radiation – affected by planetary albedo and cloud cover • OLR: Outgoing longwave radiation – affected by surface temperature and cloud cover
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) How is the energy balance altered by greenhouse gases?
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. 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.
Reaching a New Equilibrium • Higher Surface Temps • Increased es • Increased evaporation • Increased water vapor • Positive greenhouse feedback • Increase in OLR due to differential tropospheric warming • Partially offsets water vapor feedback • Reduction in albedo
New Equilibrium OLR and ASR need to balance for Equilibrium OLR increases due to increasing temperature ASR increases due to reduced albedo From 1950-2000 Decrease in OLR, net warming By 2050, in T&F Increase in OLR due to warming balances greenhouse effect
σT24 σT14 σT04 CO2 H20 Why Does the Stratosphere Cool? http://www.atmosphere.mpg.de/enid/20c.html
For the Visual Learners: 1) More greenhouse gas is added to the tropospheric layer 8) To balance this increased output, the stratospheric temperature must decrease 6) The stratosphere therefore absorbs and re-emits more radiation 3) 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) 7) From the perspective of space, the radiation output from the stratosphere increases 5) From the perspective of the stratosphere, more radiation is emitted from the top of the troposphere 4) Tropospheric temperature must increase in order to balance this Space Stratosphere Ts Troposphere CO2 To Atmospheric Window
The differences between models used in book and in paper Book (Wallace & Hobbs, 2006) Paper (Manabe & Wetherald, 1967)
qualitatively: physical processes involved in the transformation and transfer of the radiation and heat fluxes “know” quantitatively: values of the radiation and heat fluxes TOA (SW, LW) 3 • 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) • Coupled ocean-Atmosphere GCMs: Coupled Ocean-Atmosphere GCMs simulate/predict the terms (Trenberth & Fasullo, 2009) • Remote Sensing: instruments in several platforms, like EOS Aqua (CERES, AIRS, MODIS), Nimbus-7 (Wielicki et al., 1996, 2006) How do we know the energy budget? ATMOSPHERE SW (absorption, scattering, reflection)1, 2, 3 LW 1, 2 (Cloudy, clear sky param.), 3 The global annual mean Earth’s energy budget for the Mar 2000 to May 2004 period (W/m²). (Trenberth, et al. 2009) SURFACE LH=f(E, P) 1, 2 SW 1, 2, 3 LW 1, 2, 3
Global Energy Budget How Well Do We Know? • Top of the Atmosphere Fluxes • Satellite Observations – Balanced within ~3W/m2 (Kiehl et al. 1994) • Solar Irradiance varies from satellite to satellite – 1365 to 1373 W/m (Ardanuy et al. 1992) • - Earth Radiation Budget Experiment (ERBE)* 1985-1989 • - Designed to measure global albedo, fluxes, and solar radiation • Solar Irradiance varies throughout time • Associated with the buildup of greenhouse gases • 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 WellDo We Know? (cont.) • Longwave Radiation Fluxes – Values differ by 21 W/m2 (Kiehl & Trenberth, 1997) • Model Calculations • Depends on temperature and efficiency of gaseous absorber at a given wavelength • (Kiehl & Ramanathan, 1983) • Cloud Forcing = Clear Sky – Cloudy Sky (30 W/m2) • Gaseous Absorbers affect cloud emissivity • (Rothman et al. 1992) • Aerosol Overlap • Shortwave Radiation Flux – • 1. Model Calculations – • 2. Albedo uncertainties are an issue. • How significant a roll does water vapor • presence play? • To what extent do cloud and aerosol • overlaps affect the balance? • Surface Fluxes – • Sensible Heat Flux • = SW – LW – LH IPCC Report (2007) Atmospheric Science Data Center