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R. A. Raschke and S. K. Cox


The feasibility of using a photodiode radiometer to infer optical depth of thin clouds from solar intensity measurements is examined. Data were collected by a photodiode radiometer which measured incident radiation at angular fields of view of 2, 5, 10, 20 and 28°. Values of normalized annular radiance and transmittance were calculated from the observations and compared to similar calculations from a Monte Carlo radiative transfer model. The Monte Carlo results were for cloud optical depths of 1 through 6 over a spectral bandpass of 0.3 to 2.8 μm.

Eight case studies including high, middle and low clouds were examined. Experimental values of cloud optical depth were determined by comparing plots of transmittance versus field of view with the model calculated curves and from the average of the five optical depths calculated for each field of view. Analysis of the case study results indicates that the photodiode radiometer can be used effectively to determine the optical depth of thin clouds.

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E. Raschke, J. Meywerk, K. Warrach, U. Andrea, S. Bergström, F. Beyrich, F. Bosveld, K. Bumke, C. Fortelius, L. P. Graham, S.-E. Gryning, S. Halldin, L. Hasse, M. Heikinheimo, H.-J. Isemer, D. Jacob, I. Jauja, K.-G. Karlsson, S. Keevallik, J. Koistinen, A. van Lammeren, U. Lass, J. Launianen, A. Lehmann, B. Liljebladh, M. Lobmeyr, W. Matthäus, T. Mengelkamp, D. B. Michelson, J. Napiórkowski, A. Omstedt, J. Piechura, B. Rockel, F. Rubel, E. Ruprecht, A.-S. Smedman, and A. Stigebrandt

The Baltic Sea Experiment (BALTEX) is one of the five continental-scale experiments of the Global Energy and Water Cycle Experiment (GEWEX). More than 50 research groups from 14 European countries are participating in this project to measure and model the energy and water cycle over the large drainage basin of the Baltic Sea in northern Europe. BALTEX aims to provide a better understanding of the processes of the climate system and to improve and to validate the water cycle in regional numerical models for weather forecasting and climate studies. A major effort is undertaken to couple interactively the atmosphere with the vegetated continental surfaces and the Baltic Sea including its sea ice. The intensive observational and modeling phase BRIDGE, which is a contribution to the Coordinated Enhanced Observing Period of GEWEX, will provide enhanced datasets for the period October 1999–February 2002 to validate numerical models and satellite products. Major achievements have been obtained in an improved understanding of related exchange processes. For the first time an interactive atmosphere–ocean–land surface model for the Baltic Sea was tested. This paper reports on major activities and some results.

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