E . Bösche 1 , P. Stammes 2 , R. Preusker 1 , T. Ruhtz 1 , J. Fischer 1

1. E. Bösche1, P. Stammes2, R. Preusker1, T. Ruhtz1, J. Fischer1 1(Institute for Space Sciences, Free University of Berlin) 2(Royal Netherlands Meteorological Institute, De Bilt, The Netherlands) Simulation of skylight polarization with the DAK model and comparisons with measurements

2. Introduction • Mie-Simulations • DAK-Simulations • First comparison with measurements • Outlook Overview

3. Radiative forcing by tropospheric aerosols depends on their optical properties like optical thickness, single scattering albedo and phase function. • Basic idea: • Spectral resolved VIS and NIR measurements of the diffuse sunlight at various directions in the suns principal plane with inclusion of polarization. • Comparison of measurements and radiative transfer simulations • Comparison allows to draw conclusions on: • the size distribution, • the refractive index • and vertical distribution of aerosols. • From the knowledge of the size distribution and the refractive index the single scattering albedo and the phase function can be derived. Introduction

4. First mie-calculations have been performed with the following standard input parameters: • wavelength 675 nm • real and imaginary part of the refraction index of 1.5 and 0.006 • log normal size distribution with a sigma of 1.8 • mode radius of 0.1 microns • To analyse how sensitiv the mie calculations are to changes of one of these parameters, each parameter was changed in a certain range, while the rest of the parameters remaind constant. • The mie simulations serve as input for the DAK-simulations Mie-Simulations (1/4)

5. Mode radius variation Mie-Simulations (2/4)

6. Abs(Im(m)) variation Mie-Simulations (3/4)

7. Re(m) variation Mie-Simulations (4/4)

8. The DAK-simulations have been performed for multiple scattering and the following standard input parameters: • a surface albedo of 0.15 • an optical thickness of 0.39 • a standard midlatitude atmospheric profile • and the mie-aerosol (output of the mie-simulations) was put in the first layer (first one km) • while varying the optical thickness the standard mie aerosol was used. DAK-Simulations (1/5)

9. Mode radius variation DAK-Simulations (2/5)

10. Abs(Im(m)) variation DAK-Simulations (3/5)

11. Re(m) DAK-Simulations (4/5)

12. Optical thickness variation DAK-Simulations (5/5)

13. For the first comparison of simulations and measurements data from the BBC2 campagne have been used, taken at Cabauw, 8 Mai 2003, the Netherlands. • Measurement geometry: principal plane • Wavelength: 675nm • Sunelevation angle: 17.65° Comparisons with measurements (1/5)

14. FUBISS-POLAR scans, Cabauw, 08 May 2003 Comparisons with measurements (2/5) FUBISS-POLAR zenith, Cabauw, 08 May 2003

15. Comparisons with measurements (3/5)

16. Mode radius variation Comparisons with measurements (4/5) Abs(Im(m)) variation

17. Re(m) variation Comparisons with measurements (5/5) Optical thickness variation

18. Investigate the effect of different size distributions (e.g bimodal) • Implementation of the k-distribution in DAK (O2A-Band) • First sensitivity studies of radiance and polarization to aerosols in the absorbtion bands (O2A-Band) • Sensitivity to aerosol optical thickness • Sensitivity to aerosol altitude • Sensitivity to aerosol size parameters • Size distribution • Effective radius • Effective variance • Sensitivity to aerosol refractive index • Surface albedo effects • Final aim: an retrieval algorithm for aerosol optical properties from polarization measurements. OUTLOOK…

19. ENDE

20. Interaction of a light wave with an optical unit (e.g. polarizer, lense or even a scattering particle) can be described by a linear transformation of the Stokes-Vector • Negligence of the circular component V • Supposition of an ideal sensor • FUBISS-POLAR measures only intensities Measurement-equation ...