The Orbiting Carbon Observatory Mission Effects of Polarization
The Orbiting Carbon Observatory Mission: Effects of Polarization on Retrievals Vijay Natraj Advisor: Yuk Yung Collaborators: Robert Spurr (RT Solutions, Inc. ), Hartmut Boesch (JPL), Yibo Jiang (JPL)
Outline • Introduction • Retrieval Strategy • Radiative Transfer Essentials • O 2 A Band Results • Sensitivity Analysis • Outlook Welcome-2
Introduction Since 1860, global mean surface temperature has risen ~1. 0 °C with a very abrupt increase since 1980. Atmospheric levels of CO 2 have risen from ~ 270 ppm in 1860 to ~370 ppm today. Does increasing atmospheric CO 2 drive increases in global temperature? Do increasing temperatures increase atmospheric CO 2 levels? Welcome-3
Where are the Missing Carbon Sinks? • Only half of the CO 2 released into the atmosphere since 1970 has remained there. The rest has been absorbed by land ecosystems and oceans • What are the relative roles of the oceans and land ecosystems in absorbing CO 2? • Is there a northern hemisphere land sink? • What are the relative roles of North America and Eurasia? • What controls carbon sinks? • Why does the atmospheric buildup vary with uniform emission rates? • How will sinks respond to climate change? • Reliable climate predictions require an improved understanding of CO 2 sinks • Future atmospheric CO 2 increases • Their contributions to global change Welcome-4
Why Measure CO 2 from Space? Improved CO 2 Flux Inversion Capabilities • Global maps of carbon flux errors for 26 continent/ocean-basin-sized zones retrieved from inversion studies • Studies using data from the 56 GV-CO 2 stations • Flux residuals exceed 1 Gt. C/yr in some zones • Network is too sparse • Inversion tests • global XCO 2 pseudo-data with 1 ppm accuracy • flux errors reduced to <0. 5 Gt. C/yr/zone for all zones • Global flux error reduced by a factor of ~3. Flux Retrieval Error Gt. C/yr/zone Current State of Knowledge Rayner & O’Brien, Geophys. Res. Lett. 28, 175 (2001) Welcome-5
OCO Mission • First global, space-based observations of atmospheric CO 2 – high accuracy, resolution and coverage – geographic distribution of CO 2 sources and sinks and variability • High resolution spectroscopic measurements of reflected sunlight – NIR CO 2 and O 2 bands • Remote sensing retrieval algorithms – estimates of column-averaged CO 2 dry air mole fraction (XCO 2) – accuracies near 0. 3% (1 ppm) • Chemical transport models – spatial distribution of CO 2 sources and sinks – two annual cycles Welcome-6
Spectroscopy • Column-integrated CO 2 abundance => Maximum contribution from surface • High resolution spectroscopic measurements of reflected sunlight in near IR CO 2 and O 2 bands O 2 A band Clouds/Aerosols, Surface Pressure “weak” CO 2 band Column CO 2 “strong” CO 2 band Welcome-7 Clouds/Aerosols, H 2 O, Temperature
Retrieval Strategy Welcome-8
Radiative Transfer Essentials Fundamental Equation of RT Beer’s Law Source Function (Emission, Scattering) Welcome-9
Polarization and the Stokes Vector • Electromagnetic radiation can be described in terms of the Stokes Vectors: I, Q, U & V – I - total intensity – Q & U - linear polarization – V - circular polarization • Degree of Polarization (for OCO) Welcome-10
Atmospheric and Surface Setup • 11 -layer plane-parallel atmosphere (4 in stratosphere) • Urban, tropospheric and stratospheric aerosols • Lambertian surface: albedos of 0. 05, 0. 1, 0. 3 • SZA: 10°, 40°, 70° • VZA: 0°, 35°, 70° • Azimuth: 0°, 45°, 90°, 135°, 180° • Aerosol extinction optical depth: 0, 0. 025, 0. 25 Welcome-11
Extinction Processes Welcome-12
Results for O 2 A Band with Rayleigh Scattering 0. 0113 0. 818 103. 539 Welcome-13
Varying Solar Zenith Angle Welcome-14
Varying Aerosol Loading Welcome-15
Varying Surface Albedo Welcome-16
Linear Sensitivity Analysis • Park Falls, Wisconsin – Geometry • Nadir viewing • SZA: 75. 1° (Jan), 34. 8° (Jul) • Azimuth: 210. 9° (Jan), 240. 0° (Jul) – Lorentzian ILS – Resolving Powers • O 2 A Band: 17000 • CO 2 Bands: 20000 – Errors • July: 0. 3 ppm • January: 10 ppm Welcome-17
Outlook • Polarization: significant part of retrieval error budget • Full vector retrieval too time-consuming and not practical • Ways to handle polarization? – Orders of Scattering – Spectral Binning – Look-up tables Welcome-18
- Slides: 18
