W O R L D M E T E O R O L O G I C A L O R G A N I Z A T I O N COMMISSION FOR INSTRUMENTS AND METHODS OF OBSERVATION

Transcription

1 W O R L D M E T E O R O L O G I C A L O R G A N I Z A T I O N COMMISSION FOR INSTRUMENTS AND METHODS OF OBSERVATION WORKING GROUP on GROUND-BASED UPPER-AIR OBSERVING SYSTEMS New Delhi, India 6-10 December 1999 FINAL REPORT

2 AGENDA 1. ORGANISATION OF THE MEETING 1.1 Opening of the meeting 1.2 Working arrangements 1.3 Adoption of the agenda 2. INTRODUCTION 2.1 Progress report of the chairman 2.2 Working procedures for the intersessional period 2.3 Evaluation of working progress 2.4 Participation of manufacturers at meetings 2.5 National progress reports 2.6 Y2K status 3. MATTERS RELATED TO RADIOSONDE OPERATIONS 3.1 Report of the Rapporteur on Radiosonde Compatibility Monitoring 3.2 Quality assurance of the radiosonde observing system Quality assurance during the production Laboratory evaluations and calibrations Training of operators Pre-flight quality checks Results from national / bilateral comparisons 3.3 Review of data quality, including relative humidity measurements 3.4 Status of GPS windfinding systems 3.5 Coding issues including the input of surface observations and a plan for moving from TEMP code to BUFR 3.6 Testing output from algorithms using standard data sets 3.7 Review of automatic radiosonde launch systems 3.8 Recommendations from the Expert Meeting on Operational Issues for Radiosonde Applications in the Tropics and Sub-tropics 3.9 Priorities for future radiosonde developments 4. MATTERS RELATED TO GROUND-BASED REMOTE SENSING SYSTEMS, INCLUDING REPORTS OF THE RELATED RAPPORTEURS 5. MATTERS RELATED TO THE COMPATIBILITY OF GROUND-BASED OBSERVATIONS WITH OTHER OBSERVING SYSTEMS, INCLUDING REPORTS OF THE RELATED RAPPORTEURS 6. RADIO-FREQUENCY ALLOCATIONS FOR GROUND-BASED OBSERVING SYSTEMS 7. PREPARATION OF WORK PLANS AND TARGETS 8. ANY OTHER BUSINESS 9. CLOSURE OF MEETING ***************

3 GENERAL SUMMARY OF THE WORK OF THE SESSION 1. ORGANIZATION OF THE SESSION 1.1 Opening of the session The session of the Working Group on Ground-based Upper-air Observing Systems (further called Working Group) was held at the Hotel Samrat in New Delhi, India, from 6 to 10 December 1999 on kind invitation of the India Meteorological Department (IMD). The lists of participants and their addresses are attached as Appendices A and B to this report. The meeting was opened on Monday, 6 December 1999 at a.m. by Dr R.R. Kelkar, Director-General of Meteorology of IMD and Permanent Representative of India with WMO. He welcomed the participants and noted IMD s pleasure in hosting the session. He referred to the importance of considerations and exchange of experience in the field of upper-air in-situ and remote measurements and briefly informed on the work done by India in both areas. Dr Kelkar underlined that the conventional radiosonde measurements will remain, for many years to come, the backbone of the upper-air observation network. In this regard, he draw attention to the extended network of upper-air stations operated by IMD and to the fact that approximately 30,000 radiosondes were manufactured per annum within an IMD owned factory established for this purpose. He reiterated the importance of continued operation of radiosonde stations notwithstanding cost pressures although there were possible alternatives including profilers, Doppler radar, and satellite systems which might be considered as suitable means for replacement of in-situ observations in the future. He urged the Working Group to seek solutions for overcoming some difficult problems in radiosonde operation in India, mainly related to improving the quality of observations. He offered any support and hospitality to participants and wished them every success within the session and a nice stay in New Delhi. Dr S.K. Srivastava, President of CIMO and Additional Director-General of Meteorology within IMD, emphasised in his statement the need for a strong observing system and for high quality observations as a requirement for all applications. He underlined that homogeneity of data was a critical issue, which needed serious considerations of all concerned but especially of CIMO experts. He underlined that IMD had learned the importance of standardisation and calibration through own experience and will support all efforts in this regard. He urged participants to focus their efforts on solutions and recommendations in this regard and offered his total support in his position in IMD as well as President of CIMO. Dr J. Nash (UK), Chairman of the Working Group, thanked Dr Kelkar personally and the IMD for hosting the meeting. He saw it as an opportunity to bolster collaboration between countries around the world and especially as a significant contribution to the Global Observing System GOS. He suggested that quality of work and operations was important for consideration at the session. He hoped that experts from India, who attend the session either as member of the WG or as observers, could discuss matters of their concern and interact with the Working Group. Dr Nash sought bridge building between many countries to work effectively as a group to develop systems for the future. He welcomed the representatives of manufacturers and saw their participation as valuable. Co-operation between National Meteorological Hydrological Services (NMHSs) and manufacturers was essential and processes need to be considered to develop and make that interaction effective. He underlined in this regard that data accuracy was important since for example study of long-term changes in climate research required highly accurate data. He expressed a general sentiment that constructive criticism was very important to help improve systems. He finally referred briefly to some interesting topics of the Agenda for consideration. He invited experts to express their personal views and inform on their countries requirements. Mr K. Schulze welcomed as representative of the WMO Secretariat the participants in New Delhi and conveyed the best wishes and the gratitude from Professor G.O.P. Obasi, Secretary General of WMO, to the delegates. He expressed his gratitude to India for its invitation to organise the session. He appreciated the efforts of Members of WMO to enable experts to participate and also that of several manufacturers of upper-air equipment for attendance of their representatives. The main purpose of inviting the latter is that Members requirements needed to be communicated effectively to manufacturers through WMO bodies, such as this Working Group. CIMO was a good

4 - 2 - vehicle for example to promote this important interaction. Mr Schulze emphasised that work of CIMO was the basis for quality of observations - many programmes and data users relied critically on the data quality. He, however, expressed his concern that the number of instrument experts still available for this important work seemed to be decreasing and no matter what the future of CIMO might be the work on instruments and quality has to be continued by WMO through experts work. Finally, he underlined that the most important objective of this Working Group session was to precisely define its work programme based on the terms of reference given by CIMO-XII and to allocate the tasks for the next two years to the experts concerned in noting that CIMO will meet in He wished the session every success in its work. 1.2 Adoption of the agenda The Chairman briefly introduced the Provisional Agenda and invited the participants for providing comments. Dr Griersmith raised the issue of Y2K problem, which was not yet reflected within the Provisional Agenda, and it was agreed it would briefly be addressed under Agenda Item 2.6. The session then adopted the Agenda as basis for its work with the understanding that it could be amended during the session, if necessary. The final agenda can be found in front of this report. 1.3 Working arrangements for the session The session determined its working hours and the participants were informed on the local arrangements necessary for carrying out the session. English was selected the working language for the session. 2. INTRODUCTION 2.1 Progress report of the chairman The chairman underlined that this first session of the Working Group after already more than one year since the CIMO XII passed is of high importance for accomplishing the work. Dr Nash highlighted in his progress report on the work done so far especially issues of radio frequency allocation and his own work in that area: this could have a big potential impact on future operations of Services (e.g. on upper-air observations). He underlined that several Member countries have not yet fully recognised the real potential impact of it. Several radio frequency bands are threatened, such as the MHz band where there was tremendous pressures for its application for mobile satellite services (MSS). Dr Nash underlined that another area of relevance to the Working Group was the ongoing studies on the upper-air component of the GOS, which require close collaboration with CBS. There was a strong view that a CBS representative preferably from one of CBS s Open Programme Area Groups (OPAG) should take part in the further activities of the Working Group. It was noted that Dr Griersmith, Bureau of Meteorology of Australia (BOM), agreed to communicate relevant matters to Mr A. Sharp (also BOM) who was involved in the work of CBS related to representing upper-air systems in the study related to the redesign of the GOS. Finally to this agenda item, the chairman expressed his appreciation to members of the Working Group and especially to the rapporteurs for the valuable contributions already provided. 2.2 Working procedures for the intersessional period The chairman drew the attention to the fact that the establishment of effective communication mechanism for exchanging information between the chairman and members of the Working Group is the most important measure besides the allocation of tasks to experts for getting the comprehensive work done best. In this connection it has to be taken into account that no further session of the Working Group can be convened prior to CIMO-XIII due to serious financial constraints while latter is planned to be held in autumn In referring to Document 9 (Draft Work Plan for the Working Group), Dr Nash outlined some of the key issues pertaining to the primary task, and underlined that by end of 2001 major information and work results must be prepared to meet the needs for submission of documents for CIMO-XIII. He specifically highlighted in this regard that one overall aim of the work was to get the quality of radiosonde or ground-based upper-air data improved globally to better meet the increased

5 - 3 - requirements of users. For achieving this objective best, new concerted efforts needed to be made. In further considering how the Working Group could conduct its business within the intersessional period, the meeting was informed that the holding of expert meetings with limited participation might still be a possible and suitable means for tackling current problems. However, such a meeting could be organised only if budgetary savings could be made available in WMO and when a strong justification can be given. An example in this regard was the recently organised Expert Meeting on Radiosonde Applications in the Tropics held in Geneva. The meeting noted that there was a strong requirement for better interaction with manufacturers of radiosondes at a time when many new products were being introduced, and when many changes were taking place in ground systems and operational procedures. A meeting once every four or five year with the relevant WMO experts seemed an inadequate method of interaction. Dr Nash indicated that a good collaboration of the members of the Working Group, which includes all rapporteurs, with the chairman on one hand as well as between the experts on the other hand, is an essential precondition for the success of the work. This includes the regular exchange of information, the submission of outlines for contributions, and of reports for further considerations and improvement. It was informed that the concise documents, which have to be prepared for submission to CIMO-XIII, could reflect the working results of the group in a very limited manner only due to the significant limitation in words permitted for these contributions 1. Furthermore it was noted with concern that only CIMO members and potential delegates of CIMO XIII would have access to these documents which do not have any official status after the commission session, since their contents will not be reflected in its Final Report. In considering this, it was agreed that in addition to these CIMO-XIII documents more comprehensive reports should be prepared, which would reflect valuable output of the work in greater detail. They should be related to specific fields of interest as requested within the terms of reference of the Working Group or related to actual issues of common concern. These reports could be published within WMO s Instruments and Observing Methods Report series (IOM series) and should, in addition, be put on WMO s Web Home Page for general access. Furthermore, it should be considered to also publish them after an editing process in an appropriate scientific journal, if their content is of more general interest. Related to this, the Working Group considered whether some of the documents prepared for the WG session might already be suitable for this purpose after a review, such as Document 16 prepared by Mr Mannoji on GPS derived precipitable water content. (See relevant decisions of the Working Group as reflected further down in this report.) Mr Mannoji: Preparation of a IOM publication on GPS derived water vapour content M1 In this regard the WG also noted with interest that there were already several nationally prepared reports of common interest available which contain valuable information, such as on radiosonde comparisons carried out nationally. If, for various reasons, these reports cannot be published by WMO, it was proposed to create a Hyperlink from WMO s Web page to such reports, which have to be put on the related national Web page for making them accessible to a wider public. 2.3 Evaluation of working progress Based on the currently available working results and objectives, consideration were made towards enhancing the effectiveness of the work of the Working Group in view of the capacities and capabilities of their individual members. In this regard, Dr Nash suggested a target of improving compatibility of radiosonde data and integration with other observing systems such as aircraft and remote sensing. Related to this, he mentioned his concern that there were problems in getting the final report completed for the last WG established at CIMO-XI which was also chaired by him. because of a poor flow of information from WG members. This problem has to be avoided in the working mechanism used in the intersessional period between the Commission sessions. 1 The strong limitation of words is caused by significantly reducing the high translation costs because all documents for commission sessions have to be made available in English, French, Spanish, Russian, Arabic, and Chinese languages.

6 Participation of manufacturers at sessions The Working Group highly appreciated that representatives of 7 manufacturers 2 of upper-air equipment attended the session during the first three days. The chairman invited reports and comments from these manufacturers. Mr Hoerhammer (Vaisala, Finland) cited two key issues, namely there was a need to improve communication of manufacturing requirements and feedback was required especially on the operation of GPS radiosondes related to wind data. He was concerned that the manufacturers could obtain access to all TEMP reports from their systems. He underlined that the ECMWF quality monitoring system has potential to give data on performance of radiosondes. He further underlined that about 75-80% of TEMP reports is already available on the Web. He noted with appreciation the approach of the NWS of the USA, as reflected in Appendix F of Doc. 3. Dr Kats (KOMET, Russian Federation) informed that KOMET was established on the basis of Central Aerological Observatory (CAO) in 1996 by Roshydromet mainly for producing radiosondes to be applied within the Russian upper-air network mainly equipped with modernised AVK ground stations. Specialists of KOMET still continued research activities in upper-air measurements, including calibration and testing, especially for humidity measurements. Mr Curran (Sippican/VIZ, USA) raised several issues. User requirements were often defined by each client, and so varied to an unnecessary extent, which could lead to an increase in the cost of the product. He underscored that user behaviour was sometimes very problematic. He urged CIMO to continue defining uniform requirements for general application as far as needed. He underlined that feedback to manufacturers was important and the already established good relation with CIMO s upper-air experts should be enhanced. In partly responding, Mr Bower drew attention to an ensuing discussion by suggesting that standard for contracts and warranty compliance conditions would help as already applied by some Services. Mr Naaman (Metatron, Israel) highlighted the information contained in Document 6 on the actual performance of operational radiosonde stations in specifically noting that GPS radiosonde performance was not yet satisfactory. He underlined that more attention of Members should be directed to the application radiotheodolites as a viable alternative within some climatic regions 3. Mr Parini (InterMet Systems, USA) said the minimum requirement for radiosonde systems should be articulated as part of a basic system requirement. If users requirements increase, the recommendations should be specified so that manufacturers meet those standards. Mr Pepin (Geolink, France) informed that they had a new generation radiosonde system was available for testing. Resulting from related discussion on standards and quality including the need expressed by representatives of manufacturers it was agreed that interface standards for various components should be better articulated. At a specific short side meeting with the representatives of manufacturers 4 which was held on invitation of and chaired by the Dr Srivastava, President of CIMO, with attendance of Mr D.C. Schiessl, Director for Basic Systems within the WMO Secretariat, the outcomes of the collaboration between WMO and manufacturers was reviewed and proposals developed on how to proceed best in this regard. The concise report of this meeting was prepared separately and was made available to those concerned. 2 See List of Attendance as provided in Appendix A 3 This issue will be reconsidered later in the agenda. 4 Convened on Wednesday, , in the afternoon.

7 National progress reports All members of the Working Group provided reports on issues related to the national upper-airobserving network more comprehensively reflected in the documents submitted to the session but concisely summarised below, as follows: CHINA The upper-air network of CMA operates 89 stations, with most of them using a secondary wind finding radar at 403 MHz. Because this radar sounding system was developed in the 1960s already, the technique does not provide measurements of fully satisfactory accuracy. In order to solve this problem, CMA began in 1990 to develop a new upper-air sounding system based on two different radar types. One was working in the C-band but due to serious frequency interference related to the radiosonde, it could not be introduced for operational use. Therefore a secondary radar system working in the L-Band (1680 MHz) was developed and prototypes are now under test with the objective to begin the operational introduction within the next 2 to 3 years. USA The National Weather Service of the USA (NWS) supports 102 rawinsonde sites situated in the contiguous United States and Alaska, the Pacific Islands, as well as in the Caribbean Co-operative Hurricane Upper-air Network. Sixteen sites are designated as part of the GCOS Upper-air Network (GUAN). The NWS uses radiosondes manufactured by Vaisala and Sippican Meteorological System Group (formerly VIZ). Radiosondes from both radiosonde vendors are deficient in the measurements of relative humidity (RH). Vaisala has changed the desiccant used in packaging to reduce outgassing contamination and the subsequent RH dry bias. Sippican has continued efforts to improve the manufacturing process and has introduced additional calibration coefficients to improve sensor accuracy. Operational practices, such as radiosonde pre-flight checking procedures, disposition of faulty radiosondes, and operation data quality assurance activities were outlined. The NWS reported on Y2K readiness activities and on the status of the U.S. LORAN-C network. Furthermore, the session was informed on procurement activities for a new radiosonde system to replace its Radio-direction Finding System (over 40-years old). The new system will be a 1680 MHz radiosonde system. The system and GPS radiosondes are scheduled for initial implementation in late 2001 and implementation should conclude by The US National Centres for Environmental Prediction will receive high resolution 1 to 2 second data, with associated latitude, longitude, time, and height data for application in forecast models. South Africa South Africa operates 12 upper-air stations, including remote sites on Marion Island and Gough Island. All these stations previously used the OMEGA system, but since the termination of this system, they have all changed to GPS radiosondes, (Vaisala RS80-15G) with the DigiCora ground equipment. A large number of ascents produced no winds or had significant gaps in the wind profile. The radiosonde manufacturer sent a representative to South Africa, and conducted tests to determine the cause of these failures. Results showed that the majority of the radiosondes failed due to the so-called Intermediate Frequency (IF) drift problem. Some flights failed due to poor launching techniques. Vaisala has provided new software to address the IF drift problem, as well as is prepared making changes to some of the electronic components of the radiosondes. It was also recommended that all radiosondes be ground-tested to ensure that sufficient satellites are received by the radiosonde and that operators be instructed to use the proper launch technique. The introduction of these changes have improved the quality of the data, although the radiosondes in use are not yet of the new type (post-may 1999). A number of radiosondes are being rejected at ground level, and the manufacturer has undertaken to replace radiosondes, which are unable to receive signals before launch. Due to the high cost of the GPS radiosondes, which is further aggravated by foreign currency exchange rates, as well as budgetary constraints, South Africa has been forced to

8 - 6 - drastically reduce the number of GPS radiosondes used, and to make use of the cheaper PTU only radiosonde combined with winds obtained by optical theodolite. Egypt The Egyptian Meteorological Authority (EMA) operates 4 upper-air stations. Three of them have been performing observations twice daily for more than 40 years while the fourth station was established in Since 1995 all stations were equipped with radiotheodolite systems (1680 MHz). The TEMP as well as CLIMAT TEMP messages are automatically generated and transmitted to the HQ in Cairo for further checking and quality control before feeding them into the GTS. All systems are tested and proved to be Y2K compliant. EMA is planning to enhance its upper-air network by another two similar systems up to India India Meteorological Department is operating a network of 34 RS/RW and one RS station. Presently a IMD-MK III radiosonde is used, equipped with an aneroid-based baroswitch, rod thermistor, and LiCl hygristor. The PTU data are recorded by strip chart recorder. To derive winds, tracking of the radiosonde is being done by a mix of radars (with attached target), automatic, and manual tracking radiotheodolites. A new Radiosonde IMD-MK IV has been planned using an electronic switch and is equipped with an improved aneroid sensor, rod thermistor, and carbon hygristor. All the existing ground equipment is being upgraded for automatic direct ingest of PTU and Azimuth and Elevation data by a PC. Seven old WBRT ground equipment are being replaced by the new automatic tracking radiotheodolites operated at 1680 MHz. Vigorous online and offline quality checks are planned for further improvement of data quality. One calibration system has been installed for individual calibration of thermistors. Carbon hygristors are being produced on a limited scale. Due to the requirement of low limiting angles of >7 degrees elevation for getting wind data of suitable quality, 1680 MHz shall continue to be used at seven stations along with 401 MHz for remaining stations. Germany The radiosonde network of Germany consists of 11 upper-air stations, from which 3 are operated by the Military Meteorological Service. One site is designated as GUAN station. The stations perform 2-4 ascents per day and 10 of them fly the Vaisala RS while one station is using the radiosonde produced by GRAW. The winds are derived from radar tracking. For environmental protection purposes 12 additional stations are utilised during smog weather situations. 4 ASAP Containers are shipped onboard of vessels. After the termination of the OMEGA system, they have changed to the Vaisala RS80-15G radiosonde. The reliability of the GPS module of the RS 80 radiosonde has been poor for radiosondes produced before May Due to humidity problems of the reference oscillator, many radiosondes failed the ground check or loose wind raw data for periods greater than 4 minutes. Due to this effect about, 1/3 of the ASAP wind data get lost. Vaisala has identified the problem and started to replace the faulty modules. The ground based upper-air network will be reorganised during the years The future network will consist of 3 Vaisala Autosonde systems, 7 GPS radiosonde stations, 4 wind profiler radar and AMDAR data from about 60 Lufthansa aircraft. Two of the three Vaisala Autosonde systems were in acceptance testing and operation will start in January The third system will be delivered in spring In 2001 the German Meteorological Service (DWD) and the Military Meteorological Service will change the wind finding from tracking radar to GPS. The tender for the ground equipment and the radiosondes will be launched in Australia Australia operates 38 radiosonde stations, from which 12 are using the RS80G GPS and 26 the RS80 radiosonde (both Vaisala) with winds being derived from radar tracking. Whilst the radar tracking technique is more cost effective and presently more reliable than GPS for wind finding, a number of stations are not suitable for radar tracking. These include the remote Antarctic stations and 8 Autosonde stations.

9 - 7 - Between 1995 and 1997, Australia performed a number of comparison trials between GPS and radar tracking systems. After the resolution of wind accuracy problems in the GPS system discovered in the early trials, it proved possible to achieve a RMS difference between the two systems of less than 1 ms -1. Another problem experienced in these trials was a lack of reliability. A number of soundings had significant periods of missing wind data and some radiosondes failed to produce any winds. This problem still exists albeit to a lesser extent. In August 1999, Australia was experiencing 7% loss of data at Autosonde sites and 15% loss of data at Antarctic stations. The radiosondes in Antarctica are months old by August because they can only be shipped during the supply voyages in the southern summer. It is likely that the immature radiosonde design in this early production stage is the cause of the higher data loss rate. The overall quality of radiosonde sensors has declined in recent years. Accuracy and reliability problems in the PTU parameters are often more common in specific batches of radiosondes. It is also noted that care needs to be taken to ensure that ground systems are set up properly and launch procedures are very important to minimise data loss. Russian Federation The Russian upper-air network consisted of 117 upper-air stations which were daily performing 133 launches in 1999, compared to 128 stations with 185 launches in This reduction was caused by serious budgetary constraints. 19 Meteorite (operated with MARZ radiosondes) and 98 AVK (with MRZ radiosondes) radar systems were used. At two stations a pilot trial with the newly developed radiosonde RF-95 was carried out using Vaisala probe sets. By the end of 1999, the first prototype of the new commercial upper-air radar MARL-A, applying a phased array antennae with electronic scanning beam, was intended be established which can be operated with both MRZ and RF-95 type radiosondes (all 1680 MHz). Taking into account the great number of Meteorite and AVK radars, which had been operationally applied in the network for quite a long time, a comprehensive program for upgrading these radars was started. It included several measures, such as replacing old computers by Pentium-type PCs and the transceiving microwave unit by a solid-state module, improving the aerial system, in particular, by introducing antennae with electronic beam scanning, removing energy-intensive units, etc. Japan JMA operated 18 upper-air stations with each 2 complete soundings per day (00 and 12 UTC), and at 16 stations in addition rawin observation twice a day (06 and 18 UTC). At all stations equipment produced by Meisei (Japan) was applied. The wind data are derived from a balloon tracking radar. Upper-air observation could also be made, if required, at five ships owned by JMA. The Meteorological Research Institute (MRI) of JMA operated one radiosonde system for research purposes while these observations could be reported to JMA for operational use (such as NWP) with TEMP MOBIL, if necessary. The systems operated by MRI and at ships applied GPS radiosondes. According to the GCOS recommendation, high altitude observation up to 5 hpa were started operationally in July 1999 carried out once a day at 12 UTC at Tsukuba (GUAN station). Furthermore, JMA operated one rocket observational station with launches of rocketsondes once a week up to 60 km height, as well as measurement of PT and wind at descent down to 20 km. United Kingdom The Met. Office(UK) operated 11 upper-air stations, 3 of which are autosonde systems from Vaisala. A re-evaluation of requirements is being done and it is expected that two manned stations plus four Autosondes will remain in the UK and four manned systems at overseas locations. From the network, 5 stations are operated within GUAN as GCOS stations, which included among others Gibraltar, St. Helena, and Falkland Islands. So far, the Vaisala PC CORA system was mainly applied which was about 10 years old. Tests were ongoing with the new generation Vaisala Digicora 3 which has been chosen as the next UK operational system. Dr Graw (Germany) and Sippican (USA) systems were also purchased

10 - 8 - to provide GPS height measurements for radiosonde testing. It was planned that the new network would have a high level of automation while the GUAN stations were not intended to be automated especially because of the need for using larger balloons to reach the 5 hpa level. 2.6 Y2K status The session briefly reviewed the status of the Y2K compliance related to the upper-air equipment operated by NMHSs, which were represented at the Working Group meeting. The representatives of manufacturers had also been invited to providing information relevant their equipment. Resulting from adequate tests carried out by the experts of Services concerned it was found that the upper-air equipment operational applied was compliant with the Y2K problem. Manufacturers also confirmed that their equipment would cope with the Y2K needs, i.e. they provided the users concerned with the required information and /or software for upgrading their systems. It was hoped by the session that the staff concerned would follow these instructions provided as well as would timely install the required software as appropriate. However, there was no guarantee on the full compliance if users apply PCs for date evaluation or TEMP generation, which were individually procured. In any case, it was noted with appreciation that least the main manufacturer intended to provide through a Help desk at the critical period over from the to an uninterrupted service. 3. MATTERS RELATED TO RADIOSONDE OPERATIONS 3.1 Report of the Rapporteur on Radiosonde Compatibility Monitoring Mr Elms, Rapporteur on Radiosonde Compatibility Monitoring provided a report on the results of the work carried out so far. He provided a quarterly statistics of the mean differences and standard deviations of the differences between 100 hpa geopotential height observations (OB) and First Guess (FG) Fields for 00 and 12 UTC for all land radiosonde stations 5. These summaries, from Q1 (January to March) 1998 to Q4 (October to December) 1999 were generated from monthly data produced by the European Centre for Medium Range Weather Forecasts (ECMWF) in Reading (UK). In comparing these data with the 3 years (1995 to 1997 inclusive) data displayed in the previous Rapporteur s report 6, two notable contrasts were highlighted:- i) There has been an overall 30% reduction in the number of Russian Federation stations making observations during the last 2 years. ii) There is evidence of a systematic difference between the Vaisala RS80 (OB-FG) biases evaluated for daytime measurements at European and USA stations. Related to the latter, daytime data for approximately 35 RS80 stations in the USA showed an average 3-monthly bias in the 100hPa geopotential height of about 25m lower than those in Europe. This corresponds to RS80 mean temperatures from the surface to approximately 16km being systematically about 0.4 C colder in USA than in Europe in relation to the model field. These systematic differences have been present in the ECMWF monitoring data since the last quarter of 1997 and are confirmed by similar biases when the UK Met Office Model is used as the First Guess comparison field. The Working Group invited the Rapporteur to further investigate the reasons for these anomalies. 3.2 Quality assurance of the radiosonde observing system The meeting considered in depth matters related to the quality assurance of upper-air observations. In this regard, the chairman alerted participants attention to the draft task list at Document 10 as related to quality assurance issues. Discussion and agreement was reached on 5 see Document 29 6 Oakley, T - Instruments and Observing Methods Report No. 72 (WMO-TD-No. 886/1998

11 - 9 - who might contribute to various activities as it is finally reflected within the commonly agreed workplan of the WG (see Appendix E). In further discussing the issue, members were invited to report on their overall experience already obtained in this field of serious concern. Dr Dibbern reported that within the DWD of Germany a quality assurance concept for upper-air observations had already successfully been set up for which the following 4 steps are applied: Formal checks of the software applied at the receiving station are carried out. The observer can interact with the system, i.e. if there is a need, he can change the profiles before the TEMP message will be distributed. With a time delay of 1 or 2 days a comparison will be carried out with neighbouring stations and with the numerical models. The last step of quality assurance especially used for research applications is to carry out homogeneity tests to be done for selected data sets. The WG noted with appreciation this approach and invited the experts nominated for further dealing with this matter to take advantage of the experience gained by Germany when developing relevant recommendations Quality assurance during the production The meeting considered the basic issue related to the quality assurance, namely the quality assurance procedures applied to the production of radiosondes and related equipment. Operational experience showed that the radiosondes of various manufacturers were subject to variation in performance because of production difficulties, particularly with relative humidity and pressure sensors, and in some cases the ASICs used with GPS windfinding systems. In some national designs, quality problems with the radiofrequency transmitters were also critical. The meeting decided to develop relevant proposals and recommendations in close collaboration with manufacturers concerned Laboratory evaluations and calibrations Resulting from an intensive discussion on this matter, the WG underlined the high value of the tests to be carried out by Member countries in getting information on the performance and homogeneity of sensors resulting from operational production. It agreed that based on the review of the results of laboratory tests already carried out nationally or under the auspices of WMO/CIMO, recommendations for laboratory evaluations and calibrations should be developed with the objective to improve the quality of observations. There was concern that the maintenance of the standards relied on a very few countries and research institutes continuing to support the necessary testing. These activities could easily die out as Members rationalise support for upper air measurements, and it appears necessary that WMO + Members develop strategic plans to ensure quality is sustained Training of operators Considering the urgent need for highly qualified staff to operate the upper-air systems properly and to widely guarantee accurate and reliable data, the meeting discussed measures on how to improve the knowledge and skills of experts concerned. It underlined the importance of in-situ and of more basic training of operators which should primarily be carried out by NMHSs nationally and, as far as possible, organised by WMO on regional levels. The Working Group agreed that a basic programme for training events should be developed to assist Members in their efforts to organise training. In addition, specific more detailed training material and guidelines should be prepared which could generally be applied by lecturers for facilitating their preparation of training workshops. The session further agreed that experts of the WG should be consulted in preparation of WMO workshops either in supporting the selection of suitable lecturers or assists themselves in this regard Pre-flight quality checks The Working Group noted the importance of applying pre-flight checks to ensure the reliability and quality of observations. It considered the need for reviewing already existing guidelines in adapting them to the latest requirements as caused by various factors, such as the application of new radiosondes or radiosonde systems. This, for example, recently happened when the new

12 GPS-based systems were introduced into operation. An example of specific guidelines developed for the application of Vaisala GPS radiosondes can be found attached as Appendix F. The WG agreed that further work has to be done in this regard and agreed on the responsibility for this task as reflected in the Workplan (Appendix E) Results from national / bilateral comparisons Experts were invited to report on results obtained from national or multilateral radiosonde intercomparisons with the objective to derive conclusions on how to proceed best in this regard by the Working Group. Dr Dibbern presented results obtained from a GPS radiosonde intercomparison carried out at the Lindenberg Observatory in Germany 7 at which radiosondes of 3 manufacturers have been compared, namely: BBL Elektronik (Germany) Dr. Graw Messgeräte (Germany) Vaisala (Finland) The main objective of the comparison was to evaluate the accuracy and smoothing of wind observations as well as the precision of the temperature and humidity measurements. The obtained data were compared against the equipment operationally applied in the DWD, i.e. the Vaisala RS-80 radiosonde and tracking radar from Gematronik (Germany). In noting that the radiosonde of BBL was still under development, the main purpose of this participation was to detect software errors. The 3 manufacturers concerned were directly involved in this test. The results of this test are reflected in Document 15 which provides useful general information on the performance of radiosondes involved and, in addition, detailed information on the reasons for some deficiencies in wind, temperature, and humidity data. The WG discussed this matter in detail and agreed that further activities should be undertaken for getting information on the results of nationally or bilaterally held intercomparisons. It was noted that these tests were generally serving specific purposes determined by the national needs. Because these comparisons were not organised according to the rules adopted by WMO/CIMO for carrying out these tests and evaluating their results, namely by an International Organising Committee, which has to be set up by the president of CIMO, their final results cannot be published by WMO. However considering that the results, as reflected within the final reports, could be very useful for experts and Services, it was proposed to make them accessible through WMO s/cimo s Web page by providing a hyperlink to the related report which has to be put on the national Web page of the Member country concerned. The meeting agreed that Mr Elms as Rapporteur on Radiosonde Compatibility Monitoring should undertake necessary activities for getting the related information from NMHSs and inform WMO on it. J. Elms collects information on national RSO tests M Review of data quality, including relative humidity measurements The WG considered the experience generally obtained from upper-air observations, the results of the previously carried out series of WMO radiosonde intercomparisons which included the most recent WMO Radiosonde Humidity Sensor Intercomparison as well as of tests carried out nationally or bilaterally. This included some detailed information provided by Dr Griersmith on national comparisons between surface relative humidity and RS-80A Vaisala radiosonde measurements at release. There was an apparent dry bias, but because the study is correlative, no definitive causal conclusions were available. There was a possibility that the actual surface observations are not totally accurate and/or representative of the radiosonde release point. In addition to this, Mr Elms provided results of an UK comparison study of Vaisala RS-80 radiosondes equipped with H-humicaps and VIZ radiosondes investigating the dry bias in relative humidity for conditions when the radiosondes were observed to pass through clouds. The Chairman noted that the raw data of both studies might be considered for further evaluations. Mr Hoerhammer informed that Vaisala had already implemented a number of amendments for improving the humidity 7 See also Doc. 15

13 measurements. He also underscored that there were quite a few sites available worldwide attempting to understand the upper-air humidity measurements properly. Mr Mannoji informed the session on results of a comparison between GPS precipitable water (TPW) data and humidity data from radiosonde (see Document 27). A radiosonde station in Thailand as part of the GAME IOP was used. TPW showed diurnal variation with a minimum at 06 UTC. Just after launch radiosondes often showed a big drop (approx. 30%) in RH. Comparison between GPS TPW versus radiosonde TPW generally showed a dry bias. Comparison between radiosonde and NWP model data also showed a dry bias of about 15% while no explanation was available yet. Resulting from the discussion it was considered the possibility that the type of radiosonde (AIR) used was not very suitable (i.e. the RH sensor positioning might not be optimal, with significant solar heating in daylight). The session was also briefed on the results of a test that was undertaken by the UK Met. Office in May 1999 for validation of the AMSU-B (Advanced Microwave Sounding Unit), a passive sounding sensor provided by the UK Met. Office which was flown on the NOAA-15 satellite. Four radiosonde types were compared which also including an expensive chilled mirror sensor Snow White produced by Meteolabor Switzerland. In considering the results of the previous WMO intercomparisons as well those contained in the above reports given, it was agreed that the quality of measurements obtained from different types of radiosondes is not yet fully satisfactory. It needs further efforts of the WG to evaluate the quality of temperature especially of humidity measurements while especially the characteristics of the various GPS windfinding systems needs further considerations. The WG agreed that further tests should be carried out in this regard with the objective to derive guidelines and recommendations for application by Members as well as manufacturers aimed towards an improvement of the reliability of the systems, as well as the accuracy and homogeneity of the data obtained. Measures and responsibilities in this regard are reflected within the Workplan, which is attached as Appendix E. 3.4 Status of GPS windfinding systems The WG recognised that since the operation of the Omega windfinding system was ceased late in 1998, nearly 200 GPS-based systems were operated by more than 50 Member countries which is nearly ¼ of the upper-air stations of the GOS which reported regularly. Most of these systems didn t work in a fully satisfactory fashion yet, with the reliability of wind measurements poorer than expected 8. It was noted with appreciation that several improvements were already introduced by Vaisala, which was so far the main provider of these GPS systems for NMHSs. These improvements had led to some improvement in availability of winds in some countries, but there were still areas with outstanding problems. Information was also given on the results of related WMO questionnaire sent to all Members operating GPS radiosondes. WMO received 27 responses, which contained data on their operational performance obtained in the first half of It showed an overall high failure rate of up to 20% when summarising the various reasons for not coping the needs. In underlining the usefulness of distributing queries for getting information on the performance of GPS-based radiosonde systems, it was agreed that with support of Mr Elms, Rapporteur on Radiosonde Compatibility Monitoring, a next similar questionnaire should be distributed in the second half of 2000 for getting actual information on its improved performance. J. Elms: Preparation of a questionnaire M3 Participants were invited to report on the experience obtained within their Services regarding the GPS performance. The WG noted with appreciation the results of the GPS radiosonde intercomparison carried out in Germany as introduced by Dr Dibbern (see also Section above). The main conclusion of the test done for 3 types of radiosondes was that all of them were generally suitable for operational use regarding the upper-wind measurements while there was still a strong need to significantly improving the humidity measurements in the upper atmosphere for some of them. 8 See also Doc. 6

14 The WG discussed in more detail the not yet satisfactory reliability of GPS radiosondes which also included the rejection of radiosondes when there was not lock achieved to the satellites at the ground-check or when there was, in as it happened in some cases, a re-launch needed if the first launch failed or was aborted. It was noted in this regard that the quality requirements for GCOS / GUAN upper-air stations of the GOS are especially high. There was a concern expressed regarding these stations that specifically some of them may erode in performance through lack of funding or using up supplies of radiosondes. In this connection also matters on the partly unreliable application of GPS radiosondes within ASAP at ships were discussed 9. Because most of the GPS radiosondes operationally applied by NMHSs have so far been provided by Vaisala, the session noted with appreciation that they had admitted the shortfalls and already implemented several significant improvements which led to an increased reliability of the radiosondes and especially to a better quality of wind observations. The session agreed in this connection that it would be useful to establish and keep up-to-date list of Members network managers for ground-based upper-air observing systems. For getting the information on contact persons and to establishing a database of focal point people with the intention making it available on WMO/CIMO s Web page, a letter should be drafted and send to NMHSs. It is expected that this would improve communications substantially. J. Elms: Drafting a letter and collecting the information M4 10 There was also a discussion on the access and use of GPS satellites especially noting some of the problems reported at higher latitudes. Mr Curran (Sippican) informed there were a number of Internet sites, which could indicate how many satellites could be available for use at a particular time and location. This was useful to check whether all available satellites were being used when GPS radiosondes were checked prior to launch or in flight. This was recognised a good diagnostic tool to assist in determining whether there are problems with set-up or with satellite availability. Mr Curran agreed to prepare some documentation on this matter. Since this information was already provided prior this report was finalised, it can be found attached as Appendix G. T. Curran: GPS Sat. Inf. M5 (already done) The WG drew attention to the great value of the Catalogue of Radiosondes and Upper-air Windfinding Systems in Use, which will regularly be updated 11 and is accessible through WMO s Web page (URL-Address: In referring to Section above, the WG agreed that the reliability of GPS radiosonde systems and the quality of their measurements should be monitored further. It was confirmed that some smaller size WMO radiosonde intercomparisons in climatic regions of most concern should be planned. Specific attention should be directed to tropical areas first, where both the GPS performance and the relative humidity sensor performance should be compared. The WG noted with appreciation that Brazil was considering hosting such a test still in The WG invited its chairman to assist the preparation of this and the following test by close collaboration with the WMO Secretariat. Furthermore, in noting his extended experience in preparing WMO and national tests, Dr Nash was invited to support the selection of suitable experts for serving in the International Organizing Committee (IOC) to be set-up by the president of CIMO and to chair it. J. Nash & WMO: IOC GPS-RSO M4 (already done) Finally related to this agenda item, the WG briefly discussed the process of procurement of equipment and radiosondes. It was also noted with concern that there was an apparent lack of options in the available commercial upper-air observing systems. The WG and WMO were aware that individual countries approaches especially in procurement varied widely, and this made collaboration in procurement difficult. It was agreed that the WMO Secretariat had a role for 9 See also Doc It is proposed to attach it to SGs letter for reviewing the GPS sonde performance which is normally be sent to Services concerned only while it could simply copied to all PRs. It should be distributed in September 2000 for getting information from July 1999 to June It should have a similar text and questionnaire attached as done last year. 11 The so far latest issue is dated Nov./Dec. 1999