WORLD METEOROLOGICAL ORGANIZATION INSTRUMENTS AND OBSERVING METHODS REPORT No. 50 WMO /TD-No. 541 1993 FOREWORD Instrument developers are constantly striving to improve measurement systems to meet the desire of national meteorological services to achieve greater precision and accuracy and to satisfy performance requirements most efficiently and reliably. The introduction of the resulting new equipment or procedures means making changes, normally improvements, in meteorological observing systems. These changes, however, can pose problems to the climatologist who would like a set of long-term homogeneous observations of high quality. The changes in instmments and observing practices can lead to changes in the quality or error characteristics of the data, which could be misintetpreted as a change in climate if not properly recognized. To alleviate the danger of interpreting inhomogeneous upper-air data records, CIMO-X tasked a Rapporteur to u~dertake a study of "Historical Changes in Radiosonde Instruments and Practices." This report is the result of that effort. The report summarizes historical infonnation from the 1930's to the early 1990's. However, instmments and observing practices will continue to change and, as we continue to be interested in detecting changes in our atmosphere, the effects of the changes in the measurement equipmeqt and procedures will need to be understood. Therefore, each National meteorological service is strongly encouraged to maintain complete and detailed records of changes made in upper-air and other meteorological observing systems and to endeavor to determine the effects of those changes on the data record. I would like to thank Dr. Dian Gaffen, the Rapporteur who has prepared this report, and the National Oceanic and Atmospheric Administration (NOAA) of the USA for its support of this undertaking. I also thank those meteorological services who participated in this study and especially the national experts whose efforts in compiling their information will surely be appreciated by the readers of this report. (Jaan Kruus) President of CIMO FORWARD Instrument developers are constantly striving to improve measurement systems to meet the desire of national meteorological services to achieve greater precision and accuracy and to satisfy performance requirements most efficiently and reliably. The introduction of the resulting new equipment or procedures means making changes, normally improvements, in meteorological observing systems. These changes, however, can pose problems to the climatologist who would like a set of long-term homogeneous observations of high quality. The changes in instruments and observing practices can lead to changes in the quality or error characteristics of the data, which could be misinterpreted as a change in climate if not properly recognized. To alleviate the danger of interpreting inhomogeneous upper-air data records, CIM:O-X tasked a Rapporteur to undertake a study of "Historical Changes in Radiosonde Instruments and Practices." This report is the result of that effort. The report summarizes historical information from the 1930's to the early 1990's. However, instruments and observing practices will continue to change and, as we continue to be interested in detecting changes in our atmosphere, the effects of the changes in the measurement equipment and procedures will need to be understood. Therefore, each National meteorological service is strongly encouraged to maintain complete and detailed records of changes made in upper-air and other meteorological observing systems and to endeavor to determine the effects of those changes on the data record. I would like to thank Dr. Dian Gaffen, the Rapporteur who has prepared this report, and the National Oceanic and Atmospheric Administration (NOAA) of the USA for its support of this undertaking. I also thank those meteorological services who participated in this study and especially the national experts whose efforts in compiling their information will surely be appreciated by the readers of this report. (Jaan Kruus) President of CIM:O SUM:MARY Because of the possibility that climate changes due to increases in atmospheric greenhouse gases and other pollutants or due to natural processes may influence human well-being, interest is growing in the analysis of historical meteorological data to detect climate variations and trends. Radiosonde data, which currently have the longest record available with global coverage, can be used to determine changes in upper-air temperature and humidity. However, interpreting archived radiosonde data properly requires know ledge of changes in instruments and observing practices that may influence the data record. Because documentation of such changes was not readily available to the scientific community, WMO conducted a survey of Members on historical changes in radiosonde instruments and practices. Responses were received from 49 nations. The focus of the survey and of the responses was national changes affecting temperature and humidity measurements, although some information on station histories and wind and pressure measurements was also collected. On the basis of the survey results, this report presents chronological summaries of instrument changes and changes in observing and data reporting practices for each nation. For some nations, lists of relevant publications are given; bibliographies of general references on upper-air measurement systems are also included. The survey included a question about techniques used to convert measured relative humidity and temperature to dewpoint depression. A comparison of 26 different techniques reported shows the heterogeneity in the global data due simply to this post-measurement treatment of the data. The different techniques can introduce a bias in the data that is generally smaller in magnitude than typical measurement errors and than differences in measurements among contemporary instruments. However, an important inhomogeneity results from the use of temperature, humidity, and pressure cutoffs in the methods, which affect the data from cold, dry regions most severely. This survey was conducted to facilitate the interpretation of global radiosonde data archives by climate researchers. It is hoped that the information collected will allow easier identification of changes in upper-air temperature and humidity data that are related to changes in instruments or observing practices, and, possibly, adjustment of historical data to account for changes in biases in the record. ill CONTENTS Summary . ill 1. Introduction . 1 2. Survey Procedures . 2 3. Summary of Survey Results . 3 3 .1. Radiosonde Types . 4 3.2. Summary of Historical Changes by Nation ....................... 12 General Format ........................................ 13 Afghanistan . ·. 14 Argentina . 15 Australia ............................................ 16 Bahamas ............................................ 18 Belgium ............................................ 19 Belize ............................................. 21 Botswana . 22 Brazil .............................................. 23 Bulgaria . 24 Canada ............................................. 25 Cape Verde . 28 Chile .............................................. 29 China .............................................. 30 Costa Rica . 31 Cuba .............................................. 32 Czechoslovakia . 33 Denmark ............................................ 35 Finland ............................................. 37 France ............................................. 39 Germany ............................................ 40 Greece ............................................. 42 Guinea .............................................. 43 Hong Kong .......................................... 44 Iceland ............................................. 46 India .............................................. 47 Indonesia . 49 Ireland ............................................. 50 Israel .............................................. 51 Malaysia . 52 Netherlands . 53 New Zealand . 55 Pakistan . 58 iv Philippines .......................................... 59 Poland, Republic of ..................................... 60 Republic of Korea . 62 Romania ............................................ 63 Saudi Arabia . 64 Seychelles . 65 Singapore . 66 Solomon Islands . 68 South Africa . 69 Sudan ............................................. 70 Switzerland .......................................... 71 Thailand . ............ 74 Tunisia ................... ~ ......................... 75 Union of Soviet Socialist Republics ........................... 76 United Kingdom of Great Britain and Northern Ireland ............... 80 United Republic of Tanzania . 84 United States of America .................................. 85 VietNam, Socialist Republic of ....................· .......... 91 4. Dewpoint Algorithm Comparison .................................. 92 5. Conclusions . 104 6. Bibliographies . 105 6.1. Bibliography on Dewpoint Depression Computations . 105 6.2. Bibliography on Radiosonde Performance and Comparisons ............ 106 6.3. Bibliography on the Use of Radiosonde Data for Climate Research ....... 109 6.4. Bibliography of Other Relevant References ...................... 109 7. Acknowledgments . 111 8. Appendices Appendix 1. Preliminary Questionnaire ........................... 112 Appendix 2. Detailed Questionnaire on Historical Changes in Radiosonde Instruments and Practices .................................... 113 Appendix