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r / Plan for U.S. Participation in the GARP Atlantic Tropical Experiment Report of the Ad Hoc Tropical Task Group to the U.S. Committee for the Global Atmospheric Research Program National Research Council NAS-NftE SEP 2 71972 LIBRARY National Academy of Sciences Washington, D.C. 1971

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The activities of the U.S. Committee for the Global Atmospheric Research Program leading to this report have been supported by the National Oceanic and Atmospheric Administration and the National Science Foundation under Contract NSF-C310, Task Orders No. 9 and No. 197. Available from U.S. Committee for the Global Atmospheric Research Program 2101 Constitution Avenue Washington, D.C. 20418

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U.S. Committee tor the Global Atmospheric Research Program Jule G. Charney Massachusetts Institute of Technology, Chairman Alfred K. Blackadar Pennsylvania State University John W. Firor National Center for Atmospheric Research N. P. Fofonoff Woods Hole Oceanographic Institution Richard M. Goody Harvard University Cecil E. Leith, Jr. National Center for Atmospheric Research Richard S. Lindzen University of Chicago Bruce Lusignan Stanford University Owen M. Phillips The Johns Hopkins University Richard J. Reed University of Washington Herbert Riehl Colorado State University Joseph Smagorinsky National Oceanic and Atmospheric Administration, Vice Chairman Henry M. Stommel Massachusetts Institute of Technology Verner E. Suomi University of Wisconsin, Vice Chairman John M. Wallace University of Washington Douglas H. Sargeant National Research Council, Executive Scientist John R. Sievers National Research Council, Executive Secretary Ex-officio Members Robert F. Fleagle University of Washington Thomas F. Malone University of Connecticut Invited Participants Homer E. Newell National Aeronautics and Space Administration Edward P. Todd National Science Foundation John W. Townsend National Oceanic and Atmospheric Administration Robert M. White National Oceanic and Atmospheric Administration iii

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Liaison Representatives Hugh W. Albers Morris Tepper Department of Defense National Aeronautics and Space Rudolf J. Engelmann Administration U.S. Atomic Energy Commission Robert T. Webber Richard E. Hallgren Department of State National Oceanic and Atmospheric Fred D. White Administration National Science Foundation Joseph F. Sowar Department of Transportation iv

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Ad Hoc Tropical Task Group Richard J. Reed University of Washington, Chairman George F. Carrier Joanne Simpson Harvard University National Oceanic and Atmospheric Noel E. LaSeur Administration Florida State University John M. Wallace Yoshimitsu Ogura University of Washington, Rapporteur University of Illinois Michio Yanai Herbert Riehl University of California, Los Angeles Colorado State University Edward J. Zipser National Center for Atmospheric Research

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Foreword It has been recognized for some time that the meteorology of the tropical oceans constitutes one of the weakest links in our understanding of the gen- eral circulation of the atmosphere and, therefore, in our ability to model weather and climate. Routine synoptic data have never been adequate to define the mesoscale motions, and what detailed knowledge exists has been gained from special observational experiments, the most recent being the Line Islands Experiment, the Barbados Oceanographic and Meteorological Experiment (BOMEX) and the Atlantic Trade Wind Experiment. Analysis of the data from such experiments has led gradually to a reliable classification of tropical motions and to preliminary attempts to ascribe mechanisms to the main interactions. Planning for a definitive tropical experiment has gradually gained momen- tum from the time of its initial stimulation by United Nations Resolutions 1721 and 1802, which recommend international programs to improve weather- forecasting capabilities and advance our knowledge of the physical forces affecting climates, to the present. The Tropical Oceanic Experiment became a major recommendation of the ICSU-IUGG Committee on Atmospheric Sciences and its successor, the WMO-ICSU Joint Organizing Committee for the Global Atmospheric Research Program (GARP). International agreement was reached at the Brussels Planning Conference on GARP, March 1970, to conduct this experiment in the Atlantic Ocean during 1973-1974. In its first report Plan for U.S. Participation in the Global Atmospheric Research Program, the U.S. Committee for GARP considered an observa- tional study of cumulus-synoptic scale interaction as the most important and demanding of all its recommended field experiments. The central and western vii

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equatorial North Pacific was deemed the most suitable region for the experi- ment. However, when the Committee learned that ground facilities and logistical and satellite support for this area would not become available in the internationally proposed time, the alternative of conducting an experi- ment in the Atlantic was examined, and it was decided that an Atlantic experiment would be a scientifically justifiable alternative. Since the publication of the U.S. GARP Committee's plans, analysis of information from the Line Island and BOM EX experiments (particularly the fourth phase of the latter), geosynchronous satellite data, as well as further analysis of earlier observations in the western North Pacific led to a sharpen- ing of the requirements for the Atlantic Tropical Experiment. A task group, under the Chairmanship of Richard J. Reed, its former Executive Scientist, was appointed by the GARP Committee to review the new scientific material as well as the documents of a series of international planning conferences that had been held and to prepare a plan for U.S. par- ticipation in the GARP Atlantic Tropical Experiment. The group performed its task outstandingly well, and after careful review by the full U.S. GARP Committee, it has been decided to publish the new plan as an addendum to the earlier publication. We wish to express our gratitude to Drs. Reed (Chairman), Wallace (Rapporteur), Carrier, LaSeur, Ogura, Riehl, Simpson, Yanai, and Zipser, members of the Tropical Task Group, for the excellent work they have done in preparing the new plan. Me G. Charney, Chairman U.S. Committee for the Global Atmospheric Research Program viii

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Preface The development of plans for U.S. participation in the Global Atmospheric Research Program (GARP) has been viewed from the start as an evolutionary process. As scientific objectives of proposed experiments become more pre- cisely defined, as available resources become better specified, and as new concepts and constraints are introduced, modification and updating of earlier plans become necessary. The present report represents a revised and expanded version of the tropical experiment outlined in the earlier compre- hensive Plan for U.S. Participation in the Global Atmospheric Research Program. It takes account of the change of location of the experiment from the Western Pacific area, originally proposed by national and inter- national study groups, to the Atlantic Ocean area and of the international planning activities that have taken place since the issuance of the earlier document. In June 1970, a group of U.S. experts in tropical meteorology assembled at Miami for a Workshop on Tropical Experiments. This report is based largely on ideas and recommendations developed in sessions held at the Workshop and in the position papers that keynoted the sessions. These papers, and a summary of the proceedings of the Workshop, are available upon request from the U.S. GARP Committee. The plan contained herein is consonant with planning documents issued by ICSU/WMO following the international conferences held in Miami (December 1969), Brussels (March 1970), and London (July 1970). Also contributing substantially to the present document were comments and suggestions received from a large ix

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number of U.S. scientists in response to a letter of invitation from the chair- man of the task group charged with preparation of the report. The report was drafted by John M. Wallace with the advice and guidance of the ad hoc Tropical Task Group. Richard J. Reed, Chairman Ad Hoc Tropical Task Group

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U. S. GARP Recommendations The scientific basis for the proposed GARP Atlantic Tropical Experiment has been carefully reviewed by the U.S. GARP Committee. The central unsolved problem for the tropics is the mechanism or mechanisms by which deep cu- mulus convection is organized in the Intertropical Convergence Zone (ITCZ) and the synoptic-scale convergence zones, waves, and vortices over the tropi- cal oceans. Because of the importance of the resultant release of latent heat in weather and climate, this problem calls for a large-scale effort aimed at describing, analyzing, and explaining the phenomena. It is essential that the observational experiment be carried out in such a manner that the under- lying physical mechanisms will be exposed and defined. For this purpose cer- tain minimum observational requirements must be met. These requirements are in part restated below as drawn from Chapters 3 and 4 of this report, and certain extensions of the basic requirements are set forth as matters to which additional major attention should be given. 1. The program plan of the GARP Atlantic Tropical Experiment (GATE), having the scientific objectives as described in the report, and as stated in re- lated documents of the Joint (ICSU/WMO) Organizing Committee, should be carried out. 2. The GARP Atlantic Tropical Experiment should be carried out during the months of June, July, and August 1974, in the Eastern Atlantic. The pre- cise location will be dependent upon logistic considerations and the number of satellites, vessels, and aircraft that become available. 3. A geostationary satellite having both day and night imaging capability is an indispensable element of the observing network. A daytime-only imaging xi

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capability would seriously compromise both the scientific and operational effectiveness of the experiment. 4. Adequate observations need to be made to define the ITCZ and wave- scale phenomena. Satellite-viewed cloud motions will be helpful in defining the wind field at the trade-wind cumulus and cirrus levels. Polar-orbiting satellites with infrared sounding capability will be adequate for defining the major synoptic features in subtropical regions. In addition, it will be neces- sary to have a certain number of upper-air wind soundings in the tropical regions. These might either be conventional sondes released from ships or dropsondes released from high-flying aircraft. The extent of this large-scale network has yet to be determined. 5. Monitoring of the bulk properties of convective ensembles will re- quire a closely spaced array of ships with a full sounding capability and cali- brated radar. (Ideally, one might prefer to look at the whole network with one calibrated radar—see item No. 10.) Intensive monitoring of the subcloud layer will require special instrumentation, such as the tethered balloon, which must be able to take observations during periods of disturbed weather. Obser- vations of the state, temperature, and radiation budget of the sea surface should also be made from the ship-based network. 6. The number of aircraft pledged at the Brussels meeting must be in- creased in order to ensure that the experiment will obtain representative samples of the life history, distribution, internal structure, and immediate environment of convective elements. It will be necessary to increase the num- ber to approximately 10-12 aircraft if the performance and instrumentation of the additional aircraft are comparable with those pledged at Brussels. In order to follow the full-life history of convective ensembles, some nighttime observations will be needed. 7. It would be desirable to have infrared temperature soundings from geostationary satellites, even if only on an experimental basis, since these would supply unique temperature and moisture data continuously in time. In addition, this ir sounder would complement the polar-orbiting ir sounders that are required for the First GARP Global Experiment (FGGE) in 1976. 8. Supplementary observations or tests from research satellites might be scheduled to occur during the period of the GATE. Among these observations are superpressure balloon experiments, wind profiles from carrier balloon dropsondes, moisture and temperature observations using the microwave technique from polar-orbiting satellites, and high-resolution stereophotog- raphy from manned spaceflight and Earth Resource Survey Satellites. 9. Because the full rawind capability from ship platforms is a critical observational requirement, the United States and other countries should make urgent efforts toward further development and trial of upper-air wind- xii

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measuring systems based on navigational aids. These results should be made generally available as soon as possible. 10. Studies on the problem of estimating rainfall rate from a combination of radar data, enhanced satellite photographs, and rain gauge records should be undertaken. The feasibility of making these observations from geosta- tionary altitudes should be critically examined. 11. Tests of the GATE andFGGE data systems should be conducted well before the operational phases of these programs to ensure that the systems will support the planned objectives. xiii

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Contents 1. INTRODUCTION 1.1 The GARP Tropical Subprogram 1 1.2 GARP Experiments 1 1.3 Characteristics of the Atlantic Experiment 2. SCIENTIFIC OBJECTIVES 2.1 Properties of the Observed Fields 5 2.2 Interrelations between the Observed Fields 9 2.3 Concluding Remarks 12 3. OBSERVATIONAL STRATEGY 13 3.1 Definition of the Synoptic-Scale Fields 13 3.2 Determination of the Bulk Properties of Convective Ensembles 14 3.3 Intensive Sampling of Convective Ensembles 15 3.4 Other Observations 16 3.5 An Overview 17 xv

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DATA ACQUISITION 19 4.1 Platforms 19 4.2 Radiosonde Observations 21 4.3 Radar 21 5. DATA PROCESSING 22 5.1 Real-Time Requirements 22 5.2 Constant-Lag Time Requirements 22 6. MANAGEMENT CONSIDERATIONS 23 6.1 Relation of the U.S. Plan to the International Plan 23 6.2 Toward a Detailed U.S. Plan 23 6.3 Communications 24 6.4 Field Operations 25 6.5 Data Processing 25 XVI