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K. Fraedrich, E. Ruprecht, and U. Trunte

Abstract

Certain methods are tested to estimate the divergence of the outflow anvil of tropical cloud clusters. These methods are based on the change of digitized brightness values given by a sequence of satellite pictures. From five consecutive pictures of the geostationary satellite ATS 1 a magnitude of the divergence is deduced which is compatible with the results of other investigations.

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D. Wagner, E. Ruprecht, and C. Simmer

Abstract

A semistatistical retrieval technique is presented to derive humidity profiles over the oceans from passive microwave measurements. The procedure is based upon the vertical empirical orthogonal functions (EOFs) of the specific humidity extracted from a large sample of radiosonde measurements over the North Atlantic Ocean during the period from April to October 1979 (FGGE-year). The North Atlantic is divided into seven regions and the EOF-analysis is carried out for each region separately. The first three eigenvectors of the EOF-expansion explain up to 90% of the total variability within each region and it is shown that they are statistically significant and stable. The eigenvectors of the first order mainly describe variations of the total precipitable water (W), while the second and third-order EOFs are related to the ratio WG/W, with WG as the precipitable water of the planetary boundary layer, and the sea surface temperature (SST), respectively. This fact is used to develop a technique for the estimation of atmospheric moisture profiles assuming that W, WG, and SST can be retrieved from satellite observations.

A comparison of the SMMR (Nimbus-7) derived humidity profiles using only W as input data yields a retrieval accuracy of 0.9 g kg−1 in the surface layer and 1.4 g kg−1 at 800 hPa, which corresponds roughly with the top of the PBL. Above this layer the retrieval error falls off rapidly. Case studies demonstrate the capability of the algorithm to resolve typical structures of the humidity field, e.g., synoptic scale disturbances in midlatitudes or the intertropical convergence zone (ITCZ) in the tropics.

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W. B. Rossow, F. Mosher, E. Kinsella, A. Arking, M. Desbois, E. Harrison, P. Minnis, E. Ruprecht, G. Seze, C. Simmer, and E. Smith

Abstract

The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive an experimental climatology of cloud radiative properties from these radiances. A pilot study to intercompare cloud analysis algorithms was initiated in 1981 to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying six different algorithms to the same satellite radiance data. The results show that the performance of all current algorithms depends on how accurately the clear sky radiances are specified; much improvement in results is possible with better methods for obtaining these clear-sky radiances. A major difference between the algorithms is caused by their sensitivity to changes in the cloud size distribution and optical properties: all methods, which work well for some cloud types or climate regions, do poorly for other situations. Therefore, the ISCCP algorithm is composed of a series of steps, each of which is designed to detect some of the clouds present in the scene. This progressive analysis is used to retrieve an estimate of the clear sky radiances corresponding to each satellite image. Application of a bispectral threshold is then used as the last step to determine the cloud fraction. Cloudy radiances are interpreted in terms of a simplified model of cloud radiative effects to provide some measure of cloud radiative properties. Application of this experimental algorithm to produce a cloud climatology and field observation programs to validate the results will stimulate further research on cloud analysis techniques as part of ISCCP.

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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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