1. Comparisons of LER/MLER Cloud Pressures with a Model of Mie Scattering Plane-Parallel Cloud Alexander Vasilkov 1, Joanna Joiner 2, Pawan K. Bhartia 2, and Robert Spurr 3 1 Science Systems and Applications, Inc. 2 NASA/Goddard Space Flight Center, Code 613.3 3 RT Solutions Inc. OMI Science Team Meeting De Bilt, June 23-25, 2006

2. OMI Science Team Meeting De Bilt, June 23 –25, 2006 2 Contents Radiative transfer simulations with Rotational Raman Scattering (RRS) for a model of Plane-Parallel Cloud (PPC) Comparisons of the retrieved LER/MLER cloud pressures with true cloud-top pressures Comparisons of OMCLDRR cloud pressures with OMCLDO2, OMTO3 climatology, and MODIS; Identification of problems Conclusions

3. OMI Science Team Meeting De Bilt, June 23 –25, 2006 3 Thin cloud simulation and retrieval of LER/MLER cloud pressure LIDORT-RRS (Spurr) code calculates radiance and RRS filling-in Simulations were carried out for a model of plane-parallel cloud (PPC) with geometric thickness of 1 km at cloud-top heights of 2, 6, and 10 km, cloud optical thicknesses of 3, 5, 10, 20 and 50, and surface albedo of 0.05. The Henyey-Greenstein phase function was assumed with g=0.85. Cloud pressure retrieved from simulated data using lookup tables for  LER model: A single opaque Lambertian surface  MLER model: Clear sky and cloudy radiances, Ig & Ic, are calculated for two opaque Lambertian surfaces with pre-specified reflectivities of Rg=0.11 & Rc=0.40 or Rg=0.15 and Rc=0.80.

4. OMI Science Team Meeting De Bilt, June 23 –25, 2006 4 Errors in LER/MLER Cloud Pressures, High Cloud (10 km) - Deep convective clouds: LER and both MLER models produce small errors. -Cirrus clouds: -Errors in cloud pressure retrieved with LER/MLER models are significant and strongly depend on assumed clear sky and cloud reflectivties. -The MLER model with 15% & 80% retrieves pressures significantly lower than true ones because of high fraction of clear sky sub-pixel. Reflec 0.30 0.47 0.65 0.83 Cl frac 0.20 0.45 0.74 1.0

5. OMI Science Team Meeting De Bilt, June 23 –25, 2006 5 Errors in LER/MLER Cloud Pressures, Middle-Altitude Cloud (6 km) The MLER model with Rg=15% & Rc=80% works best for this case. Errors in cloud pressures retrieved with the LER/MLER models are slightly larger than for high-altitude clouds. Reflec 0.30 0.47 0.65 0.83 Cl frac 0.20 0.45 0.74 1.0

6. OMI Science Team Meeting De Bilt, June 23 –25, 2006 6 Errors in LER/MLER Cloud Pressures, Low Cloud (2 km) MLER cloud pressures are higher than the true cloud- top pressures for all optical depths. For optically thick clouds, errors in the LER/MLER cloud pressures are larger than for high-altitude clouds. The MLER model with Rg=15% & Rc=80% works best for this case. Reflec 0.30 0.47 0.65 0.83 Cl frac 0.20 0.45 0.74 1.0

7. OMI Science Team Meeting De Bilt, June 23 –25, 2006 7 Errors in LER/MLER Cloud Pressures, IPA 80% cloud fraction Neither MLER model works particularly well at the lower values of tau. The Rg=15% & Rc=80% underestimates cloud pressure, but Rg=11% & Rc=40% generally overestimates.

8. OMI Science Team Meeting De Bilt, June 23 –25, 2006 8 Comparison of OMCLDRR with OMCLDO2 Credit to Dr. K. Yang There is an overall bias (OMCLDRR pressures lower) that increases with decreasing cloud fraction. This does not tell the whole story…

9. OMI Science Team Meeting De Bilt, June 23 –25, 2006 9 Monthly mean cloud pressures (Jan 2005 ) OMTO3 clim. R>60% MODIS IR (different scale) OMCLDRR Rc=80% OMCLDO2 Clouds too low Clouds too high R>40% f>60%

10. OMI Science Team Meeting De Bilt, June 23 –25, 2006 10 Glint effect obvious at low reflectivities Glint also appears to increase reflectivities even when the cloud fraction is significant OMCLDRR is extremely sensitive to glint (glint component of radiance will look like an infinitely high cloud; note for OMCLDO2 and other absorbers, glint component will act as a bright cloud at the surface) Therefore, glint can cause cloud fractions to be overestimated and cloud pressures underestimated in OMCLDRR even in moderately cloudy pixels. Solutions: 1) Filter version 1: must eliminate significant portion (~75%!) of swath over much of the ocean 2) Use shorter wavelengths in future versions

11. OMI Science Team Meeting De Bilt, June 23 –25, 2006 11 OMTO3 clim. R>60% OMCLDO2 glint filtered OMCLDRR Rc=80% no filtering OMCLDRR Rc=80% glint filtered Clouds too low Clouds too high (errors due to MLER?) Glint filtering improves cloud pressures in the subsidence regions

12. OMI Science Team Meeting De Bilt, June 23 –25, 2006 12 Conclusions Errors in the LER/MLER cloud pressures are small for optically thick clouds. They slightly increase with decreasing cloud height. Errors in the LER/MLER cloud pressures for thinner clouds strongly depend on assumptions on clear sky and cloud reflectivities and cloud height. The MLER model with Rg=15% & Rc=80% retrieves pressures produces pressures closest to the true cloud-top pressure for PPC (f=1) but not necessarily for f<1. We are considering using shorter wavelength in UV2 (less sensitive to surface effects such as glint) for next release. We are considering a significant change in concept for the next- generation OMCLDRR algorithm (IPA with Mie scattering clouds).