* Corresponding author. Journal of Quantitative Spectroscopy & Radiative Transfer 64 (2000) 173}199 Note Deriving terrestrial cloud top pressure from photopolarimetry of re#ected light Willem Jan J. Knibbe, Johan F. de Haan*, Joop W. Hovenier, Daphne M. Stam , Robert B.A. Koelemeijer, Piet Stammes  Department of Physics and Astronomy, Free University, Amsterdam, Netherlands  Royal Netherlands Meteorological Institute, De Bilt, Netherlands Abstract The linear polarization of sunlight re#ected by cloudy areas on Earth is sensitive to the cloud top pressure as a result of molecular scattering above the clouds. We consider the derivation of cloud top pressures using polarimetric data from satellites or aircraft. The inversion method used is based on adding/doubling calculations and a Newton}Raphson iteration scheme developed earlier to analyze the polarization of the planet Venus. A modi"cation of the adding/doubling scheme is presented and used. This approach reduces the execution time for multiple scattering calculations with about an order of magnitude. Two di!erent atmospheric models were used. The "rst model includes multiple scattering by a cloud layer of spherical water drops and a higher layer of molecules and spherical aerosol particles, whereas in the second model the re#ection by the cloud layer is approximated by that of a Lambertian surface and the aerosols in the higher layer are ignored, thereby reducing the necessary computer time by about two orders of magnitude. Using simulated observations, the errors in derived cloud top pressures due to the approximations made in the second model are compared with those due to measurement errors. For a "rst application of our method to real measurements we used some photopolarimetric data obtained over the Atlantic Ocean by the Global Ozone Monitoring Experiment (GOME). It is shown that for the data considered the second model leads to errors in derived cloud top pressures which are typically smaller than 80 mb. The measurement errors in the GOME polarization observations at about 355 and 490 nm lead to errors in derived cloud top pressures which are typically smaller than 100 and 200 mb, respectively, so that the second model is su$ciently accurate to derive cloud top pressures from these observations. It also appears that the measurements at about 355 nm are more suited to derive cloud top pressures than those at about 490 nm. If more precise measurements are made, a more realistic model, such as our "rst model, will be required.  1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Radiative transfer; Polarization; Satellite remote sensing; Clouds 0022-4073/99/$ - see front matter  1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 2 - 4 0 7 3 ( 9 8 ) 0 0 1 3 5 - 6