Optical Properties of Cirrus Clouds from Satellite Imagery and Radiative Transfer Calculations

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1 Optical Properties of Cirrus Clouds from Satellite Imagery and Radiative Transfer Calculations by Robert S. Stone and Graeme L. Stephens Department of Atmospheric Science Colorado State University Fort Collins, Colorado

2 OPTICAL PROPERTIES OF CIRRUS CLOUDS FROM SATELLITE IMAGERY AND RADIATIVE TRANSFER CALCULATIONS by Robert S. Stone and Graeme L. Stephens Research supported by NSF Grant ATM Principal Investigator: Graeme 1. Stephens Department of Atmospheric Science Colorado State University Ft. Collins, CO December 1987 Atmospheric Science Paper No. 425

3 ABSTRACT In this study, differences between narrowband near infrared (NIR) and infrared (IR) brightness temperatures are related to cloud optical depth providing a theoretical basis for determining cirrus optical properties from combined satellite images. A simple monochromatic radiative transfer model is developed and utilized to simulate the upwelling radiation field as a function of cloud optical depth for five hypothetical clouds characterized by different particle size distributions, volume densities and temperatures. For simplicity., irregular shaped ice crystals are approximated by equivalent spheres. The radiative properties for each model cloud are calculated at the appropriate wavelengths employing a Mie algorithm and the results compiled into tables. Two case studies are presented to demonstrate the potential of the spectral differencing method. The first involves correlation of bispectral satellite imagery (from the NOAA-9 AVHRR) and ground-based lidar and radiometric (LIRAD) measurements. Validation of the method is accomplished by collating lidar returns with aerological data in order to deduce the physical characteristics of the cirrus layer. In the second case, only bispectral data from the GOES-6 VAS radiometer and rawinsonde data are used. In each case, comparative analysis of satellite derived and simulated cloud brightness temperature differences (NIR-IR) as a function of cloud optical depth are used to select the model cloud which best represents the observed cloud. The radiative properties of the cloud are then assumed to be those listed in the tables for the selected model. Results from the latter study are used to produce a contour map of cirrus cloud optical depth to further illustrate the value of using the spectral differencing method for studying cirrus. This study represents a first step in the development of an objective technique for determining cirrus cloud optical properties. In light of the encouraging results presented in this paper~, recommendations are made to further validate and refine this technique through intercomparisons of in situ and remotely sensed measurements and the use of more radiative transfer models.

4 ACKNOWLEDGEMENTS We wish to acknowledge various people who have contributed in different ways to this research. In the early stages, Alan Lipton and Duane Whitcomb helped one of us (R.S.S.) with the operation of the CSU ground station and assisted in the collection and processing of the GOES imagery used in this study. Nan McClurg assisted with the CSU interactive research imaging system (IRIS). Dr. Garrett Campbell provided software support as did Chi Fan Shih, Dr. Steven Ackerman, Kelly Dean and Bill Davis. BiU Davis also helped to produce photographs from the digital GOES images which are included in this paper. We are particularly grateful to Dr. Martin Platt for supplying data and results from his Aspendale Cirrus Experiments conducted in Australia. Access to Dr. Warren Wiscombe's Mie algorithm on the NCAR CRAY X-MP was of great value in formulating the theoretical basis for this study. We also wish to thank Dr. Don Hillger and Dr. Stan Kidder for their fruitful suggestions which paved the way to meaningful comparisons between satellite and ground-based radiance measurements. We also appreciate the efforts of Susan Lini who helped prepare the manuscript. This research was supported by NSF grant ATM The support of Dr. Tom Vonder Haar is also gratefully acknowledged under grant NA-85RAH ii

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