Earths Radiation Balance and Cloud Radiative Forcing The

Earth’s Radiation Balance and Cloud Radiative Forcing The Earth’s surface is kept warm through one source: the Sun. It is the primary source for Earth’s energy. Some of the incoming sunlight and heat energy is reflected back into space by the Earth’s surface, gases in the atmosphere, and clouds; some of it is absorbed and stored as heat. When the surface and atmosphere warm, they emit heat, or thermal energy, into space. The “radiation budget” is an accounting of these energy flows. If the radiation budget is in balance, then Earth should be neither warming nor cooling, on average. Clouds, atmospheric water vapor and aerosol particles play important roles in determining global climate through their absorption, reflection, and emission of solar and thermal energy.

Solar Constant measured by satellites at TOA 11 -yr solar cycle 11/25/2020 2

How does the Earth Respond? Forces Acting On the Earth System Response IMPACTS Feedback Of the total forcing of the climate system, 40% is due to the direct effect of greenhouse gases and aerosols, and 60% is from feedback effects, such as increasing concentrations of water vapor as temperature rises.

Major Climate System Elements Carbon Cycle Atmospheric Chemistry Water & Energy Cycle Coupled Chaotic Nonlinear Atmosphere and Ocean Dynamics

Radiative Forcing from 1750 to 2000 Anthropogenic Forcings IPCC, 2001

Human Influence on Climate Carbon Dioxide Trends: 100 yr lifetime Methane Trends Sulfate Trends Global Temperature Trends From M. Prather University of California at Irvine

Global Radiation Budget

Daily mean solar flux at TOA 1) The Sun is closest to the Earth in Jan. So more solar energy received in SH than in NH. 2) At the equinoxes, the solar insolation is at a Max at the equator and is zero at the poles. 8 Sun 3) At 11/25/2020 the SS of NH, daily solar insolation reaches a Max at NP. At the WS of NH, the does not rise above north of about 66. 5 o, where solar insolation is zero.

Top-of-Atmosphere Radiation Budget (Incoming Solar = Outgoing Longwave) A = Planetary Albedo S 0 = Solar Irradiance Te = Earth Radiative Temperature Ts = Equilibrium Surface Temperature 1% relative error in A 1 W m-2 flux error 0. 5 C error in Ts 2 x. CO 2 => +4 W m-2

The Greenhouse Effect Solar Radiation Longwave Radiation

Clouds have been classified as the highest priority in climate change by the U. S. climate change research initiative because they are one of the largest sources of 12 uncertainty in predicting potential future climate change

Cloud Radiative Forcing The effect of clouds on the Earth's radiation balance is measured as the difference between clear-sky and all-sky radiation results FX(cloud) = FX(clear) – FX(all-sky) FNet(cloud) = FSW(cloud) + FLW(cloud) where X= SW or LW Negative FNet(cloud) => Clouds have a cooling effect on Climate Positive FNet(cloud) => Clouds have a warming effect on Climate

Cloud Radiative Forcing (CRF) Since cloud-base temperature is typically greater than the clear-sky effective atmospheric radiating temperature, CRFLW is generally positive. The magnititude of CRFLW is strongly dependent on cloud-base height (i. e. , cloud-base temperature) and emissivity. Conversely, clouds reflect more insolation than clear sky, therefore, CRFSW is always negative over long time averages or large spatial domains. The magnititude of CRFSW cooling strongly depends on the cloud optical properties and 11/25/2020 14 fraction, and varies with season.

235 W m-2 265 W m-2 342 W m-2 57 W m-2 342 W m-2 107 W m-2 285 W m-2 Earth (No Clouds) 235 W m-2 Earth (With Clouds) FSW (cloud) =-50 W m-2 FLW (cloud)= 30 W m-2 => Net Effect of Clouds = -20 W m-2

A brief history of ERB missions

CERES Data Processing Flow CERES Data 6 Months CERES Calibration/ Location ERBE Inversion ERBE Averaging ERBE-Like Products Cloud Imager Data 18 Mo. Cloud Identification; TOA/Surface Fluxes Atmospheric Structure 36 Mo. Surface and Atmospheric Fluxes Geostationary Data 30 Mo. 24 Mo. Angular Distribution Models 36 Mo. Diurnal Models CERES Surface Products 42 Mo. Time/Space Averaging 42 Mo. CERES Time Averaged Cloud/Radiation TOA, SFC, Atmos Algorithm Theoretical Basis Documents: http: //asd-www. larc. nasa. gov/ATBD. html Validation Plans: http: //asd-www. larc. nasa. gov/valid. html

CERES Advances over Previous Missions • • • Calibration Angle Sampling Offsets, active cavity calib. , spectral char. Hemispheric scans, merge with imager matched surface and cloud properties new class of angular, directional models Time Sampling CERES calibration + 3 -hourly geo samples new 3 -hourly and daily mean fluxes Clear-sky Fluxes Imager cloud mask, 10 -20 km FOV Surface/Atm Fluxes Constrain to CERES TOA, ECMWF imager cloud, aerosol, surface properties Cloud Properties Same 5 -channel algorithm on VIRS, MODIS night-time thin cirrus, check cal vs CERES Tests of Models. Take beyond monthly mean TOA fluxes to a range of scales, variables, pdfs ISCCP/SRB/ERBE overlap to improve tie to 80 s/90 s data. CALIPSO/Cloudsat Merge in 2006 with vertical aerosol/cloud Move toward unscrambling climate system energy components

CERES Instrument TRMM: Jan-Aug 98 and Mar-Apr 2000 overlap with Terra: Mar 00 - present planned life: 2006 Aqua: July 02 start Now in checkout Planned life to 2008 NPOESS: TBD: gap or overlap? 2008 to 2011 launch

CERES LW Terra Results - July 2000 CERES Clear-Sky TOA Longwave Flux (W m-2) CERES TOA Longwave Cloud Forcing (W m-2)

CERES SW Terra Results - July 2000 CERES Clear-Sky TOA Shortwave Flux (W m-2) CERES TOA Shortwave Cloud Forcing (W m-2)

CERES Net Cloud Forcing (July, 2000)

Li and Leighton (1993)

Li and Leighton (1993)

Solar Energy Disposition (in percentage) 100 242 0 28 • 30 46 50 The upper values are from satellite, middle 42 ones from GCMs and the bottom from limited surface data

Forces Acting on Climate Forcing (W/m 2) (in Watts per meter 2)

Assessment of Cloud Absorption and Earth’s Radiation Budget • What is going on with recent debate on cloud absorption problem following ARESE ? • What is the most sound value for global surface solar radiation budget at present?

Li et al. (Nature, 1995)

Validation of satellite SRB estimates to check if the difference increases with cloud cover Hypothesis to be tested If CAA exists, satellite retrieval of SRB would not agree with ground-based observations, and the difference would increase with cloud amount

Li (J. Climate, 1998)

Summary of ARESE Studies • Cloud absorption anomaly is not supported by ground-based, nor space-borne measurements. • The central piece of information supports cloud absorption anomaly comes from TSBR aboard Egrett, which are inconsistent with other measurements.

Relatioship between TOA albedo and atmospheric transmittance

A summary of the consistency among the data collected by various instruments

Evidence from the following Investigations 1. 2. 3. 4. 5. Validation of satellite SRB estimates to check if the difference increases with cloud cover Use of TOA satellite and ground-based BB SRB data to determine atmospheric absorption Use of measurements of surface, atmospheric and cloud variables to compute and compare TOA and surface solar fluxes Use of NB satellite spectral data to retrieve cloud optical properties from which BB fluxes are compared and compared with satellite BB fluxes Use of ground-based radiation to retreive cloud optical depth from which TOA fluxes are estimated and compared.

Potential Causes for Apparent CAA 1. NB to BB conversion due to the use of non-calibrated NB operational weather satellite data 2. Calibration in satellite and/or aircraft measurements 3. Inadequate analysis method prone to mis-interpretation: Issues with the slope approach Issues with CRF approach 4. Representative of measurements – surface albedo

Home Work Due on Apr. 6 (email me) • When the earth was formed some 5 billion years ago, the sun was about 30% of today’s brightness. When the sun ceases illuminating, its brightness is estimated to be 3 times brighter. Estimate changes in planet temperature relative to the current. • Based on the global energy balance diagram, summarize the sinks and sources of energy at the top, bottom and inside of the atmosphere.