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1 Introduction 1.1 The GARP Tropical Subprogram InGARP publication No. 4 (Sec. 1.1.1) the aims of the GARP Tropical Sub- program are summarized as they relate to the main objectives of GARP. The GARP Tropical Sub-programme is primarily concerned with the problems related to the energy-exchange processes between the various scales of atmospheric motions in the tropical atmosphere. It is hoped that an understanding of these processes will lead to find- ing ways of representing them in terms of parameters defined by the large-scale variables. Hence, the basic purpose of the GARP Tropical Sub-programme is to study those phys- ical problems in tropical meteorology, solutions to which are deemed to be essential for the development of adequate numerical models of the large-scale atmospheric circulation. A number of tropical meteorologists have emphasized that the problems of forecasting in the tropics deserve attention in their own right and not only in- sofar as they relate to the problem of middle-latitude forecasting. Fortunately, there is no need to debate this issue, since regardless of the motivation, an improved understanding of scale interaction processes in the tropics will serve both ends. Knowledge of tropical motion systems will serve to improve the accuracy and usefulness of numerical prediction models for the tropics as well as for middle latitudes. 1.2 GARP Experiments GARP Report No. 3 (Sec. 1.2.1) defines GARP Experiments as "consisting of large observational programs designed to determine the behavior of the whole 1

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atmosphere or some part of it relevant to the particular sub-program." This definition specifically excludes field programs of a purely exploratory nature, since each experiment should be designed to accomplish a specified set of sci- entific objectives. While it is recognized that it may be desirable to formulate these objectives in terms of one or more specific hypotheses to be tested, as is often the case in laboratory experiments, a field program of the extent here contemplated could not be justified if classification of tropical motion sys- tems and preliminary physical hypothesis-making had not progressed to a point where a high probability of defining the essential physical mechanisms did not exist. 1.3 Characteristics of the Atlantic Experiment The factors that determine the nature of the G ARP tropical experiments are mentioned in GARP Publication No. 4 (Sec. 1.1.2b): In the design of each experiment a delicate balance must be reached between scientific requirements based on theoretical considerations and previous observational findings, on the one hand, and the practical constraints imposed by the availability of technical means and financial resources, on the other. In the following paragraphs we will give a broad outline of the scientific re- quirements and the practical constraints that relate to this experiment. Chap- ter 3 outlines an observational strategy that reflects the desired balance. 1.3.1 SCIENTIFIC REQUIREMENTS There is a broad consensus among meteorologists in this country that the experiment should focus upon the interaction between synoptic disturbances and organized, deep cumulus con- vection* and the mechanism responsible for the Intertropical Convergence Zone. Chapter 2 considers the question of scientific objectives in some detail. *In the terminology of GARP Report No. 4, it is desired to study the interaction between Scale A and Scales C and D. This represents a departure from the experimental strategy adopted in GARP Report No. 4 (Sec. 3.1) and subsequent JOC reports. In these reports there is considerable emphasis on the "cloud cluster" scale of 100-1000 km (Scale B). Two types of experiment are defined: Type I, which deals with the interac- tions between Scales A and B, and Type II, which deals with the interactions between Scales B and C, including the collective effect of Scale D. It is firmly held that the cloud- cluster scale can only be defined in terms of the other scales (i.e., the cloud cluster is the rain area of synoptic-scale disturbances, and it is the envelope of active cumulus convec- tion). It does not represent a scale on which atmospheric instabilities occur in the same sense that the convective and synoptic scales do. Thus, no particular significance is attached to the interaction between Scale B and the other scales. Type I and Type II experiments cannot be wholly separated.

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1.3.2 PRACTICAL CONSTRAINTS The experiment is planned to take place in the tropical north Atlantic during the summer of 1974, over a period of about three months. The region of the experiment will be under surveil- lance by one or more geostationary satellites plus a number of polar-orbiting satellites. At the Brussels meeting in March 1970, the participating nations tentatively pledged a total of between 15 and 24 ships for the experiment, most of which would be fully equipped for meteorological and oceanographic observations. Four of these were pledged by the United States for full-time use and a fifth for part-time use. The United States also pledged the use of three research aircraft. However, two of these would be available only when not needed for hurricane reconnaissance work. The Soviet Union pledged one or two specially equipped long-range aircraft, and the United Kingdom pledged one aircraft specially equipped for cloud-physics investigations. Canada indi- cated that it might be able to provide one specially equipped aircraft. Fur- ther details regarding special facilities available for the experiment are given in the Brussels Report as summarized in the GARP Special Report No. 1.