Clear-Sky Shortwave Observational System Simulation Experiments

1. Clear-Sky ShortwaveObservational System Simulation Experiments Dan Feldman, Chris Algieri, and Bill Collins UC Berkeley and Lawrence Berkeley National Lab CLARREO Science Team Meeting May 13, 2009 with help from Z. Jin, A. Berk, and D. Rutan

2. Outline • Description of purpose of the shortwave Observational System Simulation Experiment (OSSE). • Summary of experiment using 21st century data: • Forcing: increasing aerosol loading for first-half of experiment, followed by decreasing loading. • Feedback: surface boundary condition changes. • Albedo vs. spectral reflectance description of signal. • Conclusions. • Future work.

3. Simulation and the CLARREO questions • Objectives of OSSE: • Use models as “perfect worlds” to understand utility of CLARREO for detection and attribution vs. models. • Estimate minimum time to detection for shortwave spectral forcings and feedbacks

4. OSSE Setup Forcing Projection SimulatedForcing Climate Models CLARREO Emulator Forcing Compare FeedbackProjection SimulatedFeedback Climate Models CLARREO Emulator Forcing Model Feedback Compare

5. CCSM A2 Scenario in the OSSE N2O CO2 CH4 SO2 From Kabat, 2008 • IPCC AR4 realization with CCSM provides an ideal test case. • We chose the A2 scenario with minimal emissions reductions in WMGHGs and SO2 emissions that peak mid-century.

6. Aerosol Loading in the CCSM A2 Scenario • Sulfate and carbonaceous aerosol loading have a scaled seasonal cycle • Mid-century peak • End-of-century decline. • Present-day, mid-, and end-of-century results have contrasting amounts of aerosol and WMGHG forcing.

7. Present Day AOD • Present-day aerosol optical depth is governed by: • Saharan dust • Southeast Asian sulfates and carbonaceous emissions • Biomass burning in tropics. • Sea-salt over open ocean.

8. AOD Change in Simulation • Substantial tropical and mid-latitude increases in aerosol optical depth at mid 21st century. • Anthropogenic aerosol forcing nearly eliminated by end of century.

9. Mid 21st Century Snow/Ice Changes • Some sea-ice decrease by mid-century. • Small amounts of snowpack decrease in the summer, more significant decrease at other months.

10. End of 21st Century Snow/Ice Changes • Substantial sea-ice decrease by the end of the century. • Some snowpack decrease in summer months, more substantial decrease in boreal winter across much of the northern Hemisphere.

11. Albedo Change in Simulation • Larger albedo changes follow surface boundary condition changes, at higher latitudes and more severely in 2099 than in 2050. • Aerosols increase albedo moderately at mid-latitudes and in the tropics in 2050; almost no direct impact on albedo in 2099.

12. From Albedo to Spectral Reflectance • CLARREO will measure radiance/reflectance to provide insight into earth’s albedo control. • Deconvolution of SW spectra into forcings and feedbacks is much less obvious than in the LW. • For clear-sky analysis, spatial deconvolution is possible. Anderson et al, 2007 Aerosols and Clouds Frozen Surface Vegetative Red Edge

13. Summary of MODTRAN Calculations (1of2) • Initial calculations are cloud-free. • Monthly-averaged CCSM fields. • 26-layer atmosphere. • Atmospheric state: T, H2O, O3, CO2. • Snow/sea-ice cover. • 4 aerosol species: • Sulfate. • Sea-salt. • Dust. • Carbonaceous (BC & OC). • Spectral resolution: 15 cm-1 band model resolution, convolved to a 15 nm instrument function, covering 300 to 2500 nm. • Spectral downwelling solar and diffuse upwelling fluxes at 15 nm also produced.

14. Summary of MODTRAN Calculations (2of2) • Surface boundary condition: BRDF functionality. • Land: Ross-Li kernel model using annually cyclic MODIS BRDF product. • Non-frozen ocean: Cox-Munk model with surface winds. • Sea-ice or snow: Hapke kernel model, fixed parameters. • Solar boundary condition: Kurucz database (1368 W/m2 total insolation), annually cyclic solar ephemeris. • Sun-synchronous orbit with 1:30 pm local equator crossing time.

15. Reflectance Spectra for Present Day • Reflectance spectra at shorter wavelengths closely follow albedo. • Mid-latitude aerosols and vegetation are evident in simulated spectra.

16. Separating Spectra by Ocean/Land • Separating zonally-averaged reflectance by land and ocean facilitates spectral examination of sea-ice, land surface, and aerosols. • Over land, vegetative reflectance is a prominent signal. • Asymmetry in land reflects influence of aerosols • Over ocean, sea-ice is prominent signal.

17. Change in Reflectance Spectra: Jul. 2050 • Over ocean, sea-ice signal strong at high latitudes, sulfate aerosol can be seen at shorter wavelengths at lower latitudes. • Over land, sulfate and carbonaceous aerosols produce mixed residual signal.

18. Change in Reflectance Spectra: Jul. 2099 • Over ocean, very strong signal from sea-ice change. • Over land, reduction in AOD from 2000 is very evident in the spectra.

19. Conclusions • We have developed the capability of simulating large amounts of clear-sky shortwave CLARREO spectra using CCSM output data, the MODTRAN code, and some ancillary data. • Mid- and end-of-century simulations with the A2 emission scenario provide contrasting amounts of aerosol and WMGHG forcing for testing simulations. • Simulated reflectance spectra clearly detect changes frozen surfaces. • Detection of aerosol signal in the spectra is more involved, but large signals are eminently detectable.

20. Future Work • MODTRAN calculations will be parallelized for NASA HEC facilities. • Global T85 resolution MODTRAN calculations from 300 to 2500 nm will take less than 5 minutes wall clock per time-step. • Interannual and decadal reflectance spectra changes will be estimated. • We will also estimate unforced variability in the reflectance spectra using CCSM pre-industrial and present-day control runs to establish time-to-detection estimates. • Projection of shortwave spatial, and temporal forcings and feedbacks on spectral basis functions. • Detection/attribution case studies. • All-sky calculations will be implemented to explore the feedback of liquid and ice clouds in the shortwave. • Start with independent column approximation. • Analysis of different orbits.