WORLD METEOROLOGICAL ORGANIZATION OPERATIONAL HYDROLOGY REPORT No. 46 PRECIPITATION ESTIMATION AND FORECASTING By C. G. Collier WMO–No. 887 Secretariat of the World Meteorological Organization – Geneva – Switzerland THE WORLD METEOROLOGICAL ORGANIZATION The World Meteorological Organization (WMO), of which 185* States and Territories are Members, is a specialized agency of the United Nations. The purposes of the Organization are: (a) To facilitate worldwide cooperation in the establishment of networks of stations for the making of meteorological observations as well as hydrological and other geophysical observations related to meteorology, and to promote the establishment and maintenance of centres charged with the provision of meteorological and related services; (b) To promote the establishment and maintenance of systems for the rapid exchange of meteorological and related information; (c) To promote standardization of meteorological and related observations and to ensure the uniform publication of observations and statistics; (d) To further the application of meteorology to aviation, shipping, water problems, agriculture and other human activi- ties; (e) To promote activities in operational hydrology and to further close cooperation between Meteorological and Hydrological Services; and (f) To encourage research and training in meteorology and, as appropriate, in related fields and to assist in coordinating the international aspects of such research and training. (Convention of the World Meteorological Organization, Article 2) The Organization consists of the following: The World Meteorological Congress, the supreme body of the Organization, brings together the delegates of Members once every four years to determine general policies for the fulfilment of the purposes of the Organization, to approve long-term plans, to authorize maximum expenditures for the following financial period, to adopt Technical Regulations relating to international meteorological and operational hydrological practice, to elect the President and Vice-Presidents of the Organization and members of the Executive Council and to appoint the Secretary-General; The Executive Council, composed of 36 directors of national Meteorological or Hydrometeorological Services, meets at least once a year to review the activities of the Organization and to implement the programmes approved by Congress; The six regional associations (Africa, Asia, South America, North and Central America, South-West Pacific and Europe), composed of Members, coordinate meteorological and related activities within their respective Regions; The eight technical commissions, composed of experts designated by Members, study matters within their specific areas of competence (technical commissions have been established for basic systems, instruments and methods of observa- tion, atmospheric sciences, aeronautical meteorology, agricultural meteorology, marine meteorology, hydrology, and climatology); The Secretariat, headed by the Secretary-General, serves as the administrative, documentation and information centre of the Organization. It prepares, edits, produces and distributes the publications of the Organization, carries out the duties specified in the Convention and other Basic Documents and provides secretariat support to the work of the constituent bodies of WMO described above. ________ * On 31 December 1999. WORLD METEOROLOGICAL ORGANIZATION OPERATIONAL HYDROLOGY REPORT No. 46 PRECIPITATION ESTIMATION AND FORECASTING By C. G. Collier WMO–No. 887 Secretariat of the World Meteorological Organization – Geneva – Switzerland 2000 ACKNOWLEDGEMENTS The author has drawn freely on a wide range of literature and acknowledges the contributions of the authors of many papers, the results and analysis from which have been referenced only via specialist reviews. He has also used material he has published else- where, but has drawn together and edited for this review. A particular source is Collier, C. G., 1996: Applications of Weather Radar Systems: A Guide to Uses of Radar Data in Meteorology and Hydrology, Second edition, Praxis Ltd., Chichester and John Wiley. Cover photo: Dr Kevin Tilford University of Salford © 2000, World Meteorological Organization ISBN: 92-63-10887-0 NOTE The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Secretariat of the World Meteorological Organization concerning the legal status of any country, territory, city or area, or of its authorities, or concerning the delimitation of its frontiers or boundaries. CONTENTS Page Foreword . vii Summary (English, French, Russian and Spanish) . ix CHAPTER 1 — INTRODUCTION . 1 1.1 Background . 1 1.2 Structure of document . 1 CHAPTER 2 — MEASUREMENTS . 2 2.1 Point and areal measurements of precipitation . 2 2.2 Hydrological requirements for measurement: resolution, precision and accuracy . 2 CHAPTER 3 — POINT MEASUREMENTS USING GAUGES . 4 3.1 Siting . 4 3.2 Raingauge types . 4 3.3 Errors in measurement . 4 3.4 Areal measurements of rainfall derived from raingauge networks . 5 3.5 Global Precipitation Climatology Centre (GPCC) . 6 CHAPTER 4 — POINT MEASUREMENTS OF SNOWFALL AND HAIL . 7 4.1 Techniques . 7 4.1.1 Snowfall . 7 4.1.2 Hail . 7 4.2 Difficulties and errors . 7 CHAPTER 5 — GROUND-BASED RADAR MEASUREMENTS . 9 5.1 Outline of techniques . 9 5.2 Single polarization reflectivity measurements . 9 5.2.1 Problems arising from the characteristics of the radar and of the radar site . 9 5.2.2 Problems arising from the characteristics of the precipitation . 10 5.2.3 Z:R relationships . 12 5.2.4 Raingauge adjustment procedures . 12 5.2.5 Range and bright-band corrections . 15 5.2.6 Area-average method . 16 5.2.7 Measurements of snowfall . 16 5.2.8 Measurements of hail . 17 5.2.9 Summary of accuracy of single polarization radar measurements of precipitation . 20 5.2.10 Examples of operational systems . 21 5.3 Multi-parameter techniques . 21 5.3.1 Basic theory . 21 5.3.2 Accuracy of measurements . 21 5.3.3 Other possible polarization techniques . 22 5.4 Doppler radar . 23 5.4.1 Theoretical framework . 23 5.4.2 Contribution of Doppler radar to precipitation measurements . 24 5.5 Frequency diversity . 24 CHAPTER 6 — SATELLITE MEASUREMENTS . 26 6.1 Introduction . 26 6.2 Use of visible and infrared techniques . 26 6.2.1 Cloud indexing methods . 26 6.2.2 Life-history methods . ..