Remote Sounding of High Clouds: I. Calculation of Visible and Infrared Optical Properties from Lidar and Radiometer Measurements

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  • 1 CSIRO Division of Atmospheric Physics, Aspendale, Victoria, Australia
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Abstract

This article describes a method of determining the visible and infrared properties of high ice clouds using a ground-based lidar and infrared (IR) radiometer. A method of calibrating the lidar is described. This is followed by a method for the correction of the cloud backscatter coefficients for pulse attenuation in the clouds, using an experimentally determined backscatter to extinction ratio k. The IR emissivity is then calculated by assuming a value for the ratio between the visible extinction coefficient and the IR absorption coefficient which is invariant with altitude. This ratio is altered until the computed radiance is equal to the measured cloud radiance. Errors in the calculation of the backscatter coefficient, the visible and the IR optical depths and the IR emissivity are assessed for various errors in the backscatter to extinction ratio. It is found that errors in the cloud optical depth become extremely sensitive to errors in k when the cloud visible optical depth becomes large. However, errors in the IR optical depth and IR emissivity are considerably less because they are constrained to agree with IR radiance measurements. For ”typical“ cirrus, having an emissivity of 0.24, and for a standard error in k of 20%, the error in the visible optical depth is 20–30%, whereas the errors in the IR optical depth and emissivity are only 1 and 2%, respectively. The effects of a variable multiple-scattering factor on the above errors appear to be small. However, the variation of this factor is not known well enough yet in high clouds to assess the errors accurately.

Other sources of error, which will be discussed in later papers of this series, include experimental errors in the measured IR radiance and errors in the determination of the calibration factor S.

Abstract

This article describes a method of determining the visible and infrared properties of high ice clouds using a ground-based lidar and infrared (IR) radiometer. A method of calibrating the lidar is described. This is followed by a method for the correction of the cloud backscatter coefficients for pulse attenuation in the clouds, using an experimentally determined backscatter to extinction ratio k. The IR emissivity is then calculated by assuming a value for the ratio between the visible extinction coefficient and the IR absorption coefficient which is invariant with altitude. This ratio is altered until the computed radiance is equal to the measured cloud radiance. Errors in the calculation of the backscatter coefficient, the visible and the IR optical depths and the IR emissivity are assessed for various errors in the backscatter to extinction ratio. It is found that errors in the cloud optical depth become extremely sensitive to errors in k when the cloud visible optical depth becomes large. However, errors in the IR optical depth and IR emissivity are considerably less because they are constrained to agree with IR radiance measurements. For ”typical“ cirrus, having an emissivity of 0.24, and for a standard error in k of 20%, the error in the visible optical depth is 20–30%, whereas the errors in the IR optical depth and emissivity are only 1 and 2%, respectively. The effects of a variable multiple-scattering factor on the above errors appear to be small. However, the variation of this factor is not known well enough yet in high clouds to assess the errors accurately.

Other sources of error, which will be discussed in later papers of this series, include experimental errors in the measured IR radiance and errors in the determination of the calibration factor S.

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