1. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Problems related to absorption dependent retrievals and their validation Marc Schröder 1, Rene Preusker 1, Ralf Bennartz 2, Rasmus Lindstrot, and Jürgen Fischer 1 1 Institut für Weltraumwissenschaften, Freie Universität Berlin, Berlin, Germany 2 Atmospheric and Oceanic Sciences Department, University of Wisconsin, Madison, USA

2. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Overview Introduction Some items affecting absorption dependent retrievals Various LWC profiles Photon path length distributions and their parameterisation Validation and evaluation: Horizontal averaging effects on nadir reflectance and reflectance ratios Results based on casi measurements Results retrieved from radiative transfer simulations Conclusions and outlook

3. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Introduction Photon path length distributions and cloud top height retrievals cloud aircraft / satellite photon path L(761 nm) ~ T L: radiance T: transmission T = exp(-  l)l : photon path  : extinction difference between L for an adiabatic and a cloud top entrained profile (optical thickness = const.)

4. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Introduction Validation and evaluation efforts under different spatial resolutions 0 10 20 MODIS: 1 km, casi: 10 m, Lidar (Polis, MIM, B. Heese): 10 m

5. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Effect of various LWC profiles on absorption intensities: Methods LWC profiles from LES model (F. Chosson, Meteo France, CNRM) conversion of LWC to microphysical parameters: Chosson et al., 2004 Brenguier, 1991 Schueller et al., 2004 Radiative transfer with a Monte Carlo model: Petty, 1994 Schroeder, 2004 constant liquid water path in all cases

6. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Effect on absorption Difference mainly depends on absorption intensity and extinction at cloud top. Layer thickness and mixing scheme are of minor importance.

7. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Parameterisation of mean photon path length In addition to optical thickness, depends significantly on effective radius. N

8. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Horizontal averaging effects: Results based on observations cloudland surface use casi images Calculate reflectance: R =  *L / (F cos(  )) Use reflectance ratio of R(761 nm) / R(755 nm) as a measure for cloud top height Artificially degrade resolution land cloud

9. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Smile Effect temperatures as observed by radiosondes: FL100:-4 to –5°C FL60:+2°C

10. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Overview of cases Stratus / Stratocumulus 1+2 layer clouds broken clouds increasing cloud thickness

11. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Results Stratus/Stratocumulus other (e.g. broken clouds)

12. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 1D versus 2D averaging take time series at left and right side average over each 1 km series average over each 1 km x 1 km area

13. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 1D versus 2D averaging: Results time series at left and right side of image black, red: 1 km averages green: 1 km x 1 km averages Very high spatial variability of clouds possible. Reduction of resolution of time series might not be sufficient. Exact allocation of pixels required.

14. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 2D averaging based on RT simulations cloud droplet number spectra, n(r), from M. Leporini, LaMP, CNRS directly from n(r)homogeneous 2.57 3.20 0.16 0.12 directly from n(r) homogeneous mixing

15. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Conclusions Extinction at cloud top strongly affects the signal at absorbing channels. Significant to strong dependence of the mean photon path on cloud optical thickness AND effective radius.  Potential improvements for retrieval schemes, either through direct simulation or subsequent adjustments.  It may increase the accuracy of gas absorption estimation in GCMs. Stratus and Stratocumulus: possible impact of averaging at scales below 1 km, hardly reaching the significance level

16. Marc Schröder et al., FUB BBC2 Workshop, De Bilt, 10.´04 Conclusions and outlook Complicated cases: significant effects on scales up to 25 km, effect up to 15% A simple reduction of the resolution must be accompanied by an exact allocation of considered observations. The microphysical properties, in particular the phase function, can have strong effects on the overall reflectance and absorption intensity. Validation of cloud top height (continued) Evaluation of forecast models with satellite data Intensify analysis of different microphysical schemes