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Elizabeth E. Ebert, Michael J. Manton, Philip A. Arkin, Richard J. Allam, Gary E. Holpin, and Arnold Gruber

Three algorithm intercomparison experiments have recently been conducted as part of the Global Precipitation Climatology Project with the goal of (a) assessing the skill of current satellite rainfall algorithms, (b) understanding the differences between them, and (c) moving toward improved algorithms. The results of these experiments are summarized and intercompared in this paper.

It was found that the skill of satellite rainfall algorithms depends on the regime being analyzed, with algorithms producing very good results in the tropical western Pacific and over Japan and its surrounding waters during summer, but relatively poor rainfall estimates over western Europe during late winter. Monthly rainfall was estimated most accurately by algorithms using geostationary infrared data, but algorithms using polar data [Advanced Very High Resolution Radiometer and Special Sensor Microwave/Imager (SSM/I)] were also able to produce good monthly rainfall estimates when data from two satellites were available. In most cases, SSM/I algorithms showed significantly greater skill than IR-based algorithms in estimating instantaneous rain rates.

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Urs Neu, Mirseid G. Akperov, Nina Bellenbaum, Rasmus Benestad, Richard Blender, Rodrigo Caballero, Angela Cocozza, Helen F. Dacre, Yang Feng, Klaus Fraedrich, Jens Grieger, Sergey Gulev, John Hanley, Tim Hewson, Masaru Inatsu, Kevin Keay, Sarah F. Kew, Ina Kindem, Gregor C. Leckebusch, Margarida L. R. Liberato, Piero Lionello, Igor I. Mokhov, Joaquim G. Pinto, Christoph C. Raible, Marco Reale, Irina Rudeva, Mareike Schuster, Ian Simmonds, Mark Sinclair, Michael Sprenger, Natalia D. Tilinina, Isabel F. Trigo, Sven Ulbrich, Uwe Ulbrich, Xiaolan L. Wang, and Heini Wernli

An intercomparison experiment involving 15 commonly used detection and tracking algorithms for extratropical cyclones reveals those cyclone characteristics that are robust between different schemes and those that differ markedly. Extratropical cyclones are fundamental meteorological features and play a key role in a broad range of weather phenomena. They are a central component maintaining the global atmospheric energy, moisture, and momentum budgets. They are on the one hand responsible for an

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Herb A. Winston and Larry J. Ruthi

Computer-generated volumetric radar algorithms have been available at a few operational National Weather Service sites since the mid-1970s under the Digitized Radar Experiment (D/RADEX) and Radar Data Processor (RADAP II) programs. The algorithms were first used extensively for severe-storm warnings at the Oklahoma City National Weather Service Forecast Office (WSFO OKC) in 1983. RADAP II performance in operational severe-weather forecasting was evaluated using objectively derived warnings based on computer-generated output. Statistical scores of probability of detection, false-alarm rate, and critical-success index for the objective warnings were found to be significantly higher than the average statistical scores reported for National Weather Service warnings. Even higher statistical scores were achieved by experienced forecasters using RADAP II in addition to conventional data during the 1983 severe-storm season at WSFO OKC. This investigation lends further support to the suggestion that incorporating improved reflectivity-based algorithms with Doppler into the future Advanced Weather Interactive Processing System for the 1990s (AWIPS-90) or the Next Generation Weather Radar (NEXRAD) system should greatly enhance severe-storm-detection capabilities.

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William P. Elliott and Dian J. Gaffen

Different nations use different algorithms or other techniques to convert temperatures and relative humidities from radiosonde observations to dewpoint depressions. Thus, it is possible for identical measured values to result in different reported dewpoints. On the basis of a sample of conversion methods, we calculate the possible differences among the national practices. In general, the discrepancies are not large and would often be lost in the usual round-off procedures associated with transmission over the Global Telecommunications System, but in cold, dry conditions dewpoints different by more than 1°C could be reported for identical conditions. Some of the methods have been changed over time, so there is also the possibility of inhomogeneities in climate records.

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Phillip A. Arkin and Pingping Xie

The Global Precipitation Climatology Project (GPCP) was established by the World Climate Research Programme to produce global analyses of area- and time-averaged precipitation for use in climate research. To achieve the required spatial coverage, the GPCP uses simple rainfall estimates derived from IR and microwave satellite observations. In this paper, we describe the GPCP and its first Algorithm Intercomparison Project (AIP/1), which compared a variety of rainfall estimates derived from Geostationary Meteorological Satellite visible and IR observations and Special Sensor Microwave/Imager microwave observations with rainfall derived from a combination of radar and raingage data over the Japanese islands and the adjacent ocean regions during the June and mid-July through mid-August periods of 1989. To investigate potential improvements in the use of satellite IR data for the estimation of large-scale rainfall for the GPCP, the relationship between rainfall and the fractional coverage of cold clouds in the AIP/1 dataset is examined. Linear regressions between fractional coverage and rainfall are analyzed for a number of latitude-longitude areas and for a range of averaging times. The results show distinct differences in the character of the relationship for different portions of the area. In general, to the south and east of the mountainous axis of Japan, rainfall and fractional coverage are highly correlated for thresholds colder than 245 K, and correlations can be increased by averaging in space and in time up to the dominant period of the precipitation events. To the north and west of the axis, the correlations between rainfall and fractional coverage, while generally smaller for all scales, are highest for thresholds warmer than 245 K. The proportional coefficients relating rainfall to fractional coverage at cold thresholds, however, differ greatly between the two periods and both differ significantly from those found for the GARP (Global Atmospheric Research Program) AtlanticTropical Experiment. These results suggest that the simple IR-based estimation technique currently used in the GPCP can be used to estimate rainfall for global tropical and subtropical areas, provided that a method for adjusting the proportional coefficient for varying areas and seasons can be determined.

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Jennifer M. Comstock, Robert d'Entremont, Daniel DeSlover, Gerald G. Mace, Sergey Y. Matrosov, Sally A . McFarlane, Patrick Minnis, David Mitchell, Kenneth Sassen, Matthew D. Shupe, David D. Turner, and Zhien Wang

The large horizontal extent, with its location in the cold upper troposphere, and ice composition make cirrus clouds important modulators of the Earth's radiation budget and climate. Cirrus cloud microphysical properties are difficult to measure and model because they are inhomogeneous in nature and their ice crystal size distribution and habit are not well characterized. Accurate retrievals of cloud properties are crucial for improving the representation of cloud-scale processes in largescale models and for accurately predicting the Earth's future climate. A number of passive and active remote sensing retrieval algorithms exist for estimating the microphysical properties of upper-tropospheric clouds. We believe significant progress has been made in the evolution of these retrieval algorithms in the last decade; however, there is room for improvement. Members of the Atmospheric Radiation Measurement (ARM) program Cloud Properties Working Group are involved in an intercomparison of optical depth τ and ice water path in ice clouds retrieved using ground-based instruments. The goals of this intercomparison are to evaluate the accuracy of state-of-the-art algorithms, quantify the uncertainties, and make recommendations for their improvement.

Currently, there are significant discrepancies among the algorithms for ice clouds with very small optical depths (τ < 0.3) and those with 1 < τ < 5. The good news is that for thin clouds (0.3 < τ < 1), the algorithms tend to converge. In this first stage of the intercomparison, we present results from a representative case study, compare the retrieved cloud properties with aircraft and satellite measurements, and perform a radiative closure experiment to begin gauging the accuracy of these retrieval algorithms.

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P.J. Sellers, S.I. Rasool, and H.-J. Bolle

Satellite observations are essential for the global monitoring of climatologically significant interactions between the earth's atmosphere and land surface. In practice, however, interpretation of remote-sensing data requires the use of algorithms—specialized, semiempirical relationships that connect observed radiances with the actual physical variables needed for climate studies and modeling. At issue is the physical/empirical basis for these algorithms, their effectiveness and shortcomings, and the scope for further improvement.

The International Satellite Land-Surface Climatology Project (ISLSCP) Satellite Data Algorithms Workshop, conducted 5–8 January 1987 at the Jet Propulsion Laboratory in Pasadena, California, was organized to address these questions. The introduction to this paper describes ISLSCP and presents an overview of the scientific topics covered in the workshop.

Derivation of the following key surface parameters from satellite data are discussed in detail:

Shortcomings of the various algorithms and actions required to alleviate them are discussed.

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Charles K. Rutledge, Gregory L. Schuster, Thomas P. Charlock, Frederick M. Denn, William L. Smith Jr., Bryan E. Fabbri, James J. Madigan Jr., and Robert J. Knapp

When radiometers on satellites point toward Earth with the goal of sensing an important variable quantitatively, rather than just creating a pleasing image, the task at hand is often not simple. The electromagnetic energy detected by the radiometers is a puzzle of various signals; it must be solved to quantify the specific physical variable. This task, called the retrieval or remote-sensing process, is important to most satellite-based observation programs. It would be ideal to test the algorithms for retrieval processes in a sealed laboratory, where all the relevant parameters could be easily measured. The size and complexity of the Earth make this impractical. NASA's Clouds and the Earth's Radiant Energy System (CERES) project has done the next-best thing by developing a long-term radiation observation site over the ocean. The relatively low and homogeneous surface albedo of the ocean make this type of site a simpler environment for observing and validating radiation parameters from satellite-based instruments. To characterize components of the planet's energy budget, CERES uses a variety of retrievals associated with several satellite-based instruments onboard NASA's Earth Observing System (EOS). A new surface observation project called the CERES Ocean Validation Experiment (COVE), operating on a rigid ocean platform, is supplying data to validate some of these instruments and retrieval products. This article describes the ocean platform and the types of observations being performed there, and highlights of some scientific problems being addressed.

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R. A. Pielke, L. R. Bernardet, P. J. Fitzpatrick, R. F. Hertenstein, A. S. Jones, X. Lin, J. E. Nachamkin, U. S. Nair, J. M. Papineau, G. S. Poulos, M. H. Savoie, and P. L. Vidale

In order to assist in comparing the computational techniques used in different models, the authors propose a standardized set of one-dimensional numerical experiments that could be completed for each model. The results of these experiments, with a simplified form of the computational representation for advection, diffusion, pressure gradient term, Coriolis term, and filter used in the models, should be reported in the peer-reviewed literature. Specific recommendations are described in this paper.

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Francis S. Binkowski
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