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Error Estimates for Retrieval of Cloud-Top Pressure Using Absorption in the A Band of Oxygen

D. M. O'BrienCommonwealth Scientific and Industrial Research Organization, Division of Atmospheric Research, Aspendale, Victoria, Australia

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R. M. MitchellCommonwealth Scientific and Industrial Research Organization, Division of Atmospheric Research, Aspendale, Victoria, Australia

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Abstract

This paper examines the feasibility of remotely sensing cloud-top pressure from observations of reflected sunlight at frequencies in the A band of absorption of oxygen (13 070 cm−1, 765 nm). The data are assumed to consist of several channels within the A band and one reference channel outside the A band. The principal difficulty is that the reflected radiance is attenuated by absorption by O2 not only in the atmosphere above the cloud, but also along photon trajectories within the cloud. The extent of the extra absorption is unknown a priori because the microphysics structure of the cloud is unknown. This paper investigates the possibility of simultaneously retrieving both cloud-top pressure and parameters that determine the distribution of photon pathlength within the cloud. Estimates are derived for the errors in the retrieved parameters induced by instrumental noise and uncertainties in the profiles of temperature and aerosol extinction. The study investigates the sensitivity of the errors to spectral resolution and to both the number and placement of the spectral channels. Accuracies of 5 hPa appear possible over optically thick clouds with an instrument with high spectral resolution (∼1 cm−1).

Abstract

This paper examines the feasibility of remotely sensing cloud-top pressure from observations of reflected sunlight at frequencies in the A band of absorption of oxygen (13 070 cm−1, 765 nm). The data are assumed to consist of several channels within the A band and one reference channel outside the A band. The principal difficulty is that the reflected radiance is attenuated by absorption by O2 not only in the atmosphere above the cloud, but also along photon trajectories within the cloud. The extent of the extra absorption is unknown a priori because the microphysics structure of the cloud is unknown. This paper investigates the possibility of simultaneously retrieving both cloud-top pressure and parameters that determine the distribution of photon pathlength within the cloud. Estimates are derived for the errors in the retrieved parameters induced by instrumental noise and uncertainties in the profiles of temperature and aerosol extinction. The study investigates the sensitivity of the errors to spectral resolution and to both the number and placement of the spectral channels. Accuracies of 5 hPa appear possible over optically thick clouds with an instrument with high spectral resolution (∼1 cm−1).

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