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
EXPERT MEETING
on
OPERATIONAL ISSUES FOR RADIOSONDE APPLICATIONS IN THE TROPICS AND SUB-TROPICS
Geneva, Switzerland
18 - 22 October 1999
FINAL REPORT
AGENDA
1. ORGANIZATION OF THE SESSION
1.1 Opening of the session
1.2 Working arrangements for the session
1.3 Adoption of the agenda
2. PURPOSE AND SCOPE OF THE MEETING
3. REPORTS
4. RADIOSONDE SYSTEMS AND SENSORS OPERATIONALLY APPLIED
4.1 Performance of GPS radiosonde systems
4.2 Performance of radiotheodolites
4.3 Performance of temperature and relative humidity sensors
5. REVIEW OF OPERATIONAL PROCEDURES AND REPORTING PRACTICES
6. PREPARATION OF GUIDELINES, RECOMMENDATIONS, AND ACTIONS
7. ANY OTHER BUSINESS
8. CLOSURE OF THE MEETING
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KS [C:\My Documents\WG-EXPGR\E-UAM-1\E-UAM-1-Rep.doc] Version of 25 November 1999
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GENERAL SUMMARY OF THE WORK OF THE SESSION
1. ORGANIZATION OF THE SESSION
1.1 Opening of the session
The Expert Meeting on Operational Issues for Radiosonde Applications in the Tropics and Sub-tropics (Expert Meeting) was held at the Headquarters of the World Meteorological Organization (WMO) in Geneva from 18 to 22October 1999. The lists of participants and their addresses are attached as AppendicesA and B to this report.
Dr A.S. Zaitsev, Assistant Secretary-General, WMO Secretariat, opened the Expert Meeting on behalf of Prof. G.O.P. Obasi, Secretary-General of WMO, on Monday, 18 October 1999, at 10.00h. He welcomed the delegates and was pleased that WMO was able to host this meeting in its new Headquarters Building. Dr Zaitsev underlined that the conventional radiosonde measurements will remain, for many years to come, the backbone of the upper-air observation network. He noted with appreciation that the operational performance and reliability of the newly developed GPS-based wind-finding systems implemented due to the urgent need for replacing the OMEGA-based systems, will be discussed. He was pleased that specific attention would be given to all issues on upper-air measurements related to the tropics and sub-tropics, which are in several respects areas of WMO’s concern. He underlined the importance of the results of several radiosonde intercomparisons carried out under the auspices of CIMO in the past 15 years with the objective to obtain information on the performance characteristics of the various types of operationally used radiosondes and their sensors. He was of the opinion that new tests might be justified for getting a clearer picture on the present situation, especially related to the application of the GPS-based radiosonde system. He noted that the recommendations to be developed by the Expert Meeting will be appreciated by Meteorological Services and by manufacturers of these systems in assisting them in their efforts to detect the cause of the problems and to improve the systems. He assured the meeting the full support of the Secretariat in its work and wished the participants a successful meeting and a pleasant stay in Geneva.
Dr J. Nash (UK) welcomed the experts in his capacity as the chairman of the Expert Meeting. He was pleased to note that the experts invited could arrange for their participation, which clearly demonstrated the great interest of Meteorological Services in this important field of common concern. He was convinced that this meeting would also provide some significant input for the work of the CIMO Working Group on Ground-based Upper-air Observing Systems. Finally, he underlined that there would be a fruitful and effective discussion and exchange of experience on matters of upper-air observations in the tropics and sub-tropics.
1.2 Adoption of the agenda
The proposed Provisional Agenda was adopted for the work of the session 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 arrangements necessary for carrying out the session. English was selected the working language for the session.
2. PURPOSE AND SCOPE OF THE MEETING
The Expert Meeting was designed to review the present status of radiosonde operations in the tropics and sub-tropics. It would focus special attention to problems in operational use and for climate monitoring and develop proposals and guidelines on how best to proceed. The main objective of the meeting was, however, to develop recommendations on operational practices and on radiosonde comparisons. The aim was to prepare relevant submissions for consideration and approval to CIMO-XIII and also to prepare for a possible tropical radiosonde intercomparison.
Of specific attention would be the:
§ Preparation of guidance material for radiosonde operators or system managers.
§ Activities related to the organisation of tests of systems, possibly by WMO Intercomparisons with presentation of the results in reports published by WMO.
§ Development of recommendations addressed to manufacturers, e.g. on the type of systems available for tropical work in future, and also on problems with the current systems, including issues related to the updating of system software.
§ Organisation of regional training events.
3. REPORTS
3.1 National reports
The delegates were invited to present concise reports on the performance of the upper-air observing systems and especially related to the functioning of the GPS equipment related to the Service or organization concerned. Most experts had already submitted reports prior to the meeting which were reproduced in documents prepared for facilitating the discussion. Oral presentations were given by all experts which can briefly be summarised as following:
· Brazil
The Brazilian radiosonde network consists of 26 upper-air stations, from which 14 are operated by the Brazilian Air Force and 12 are by the National Meteorological Institute (INMET). The Brazilian Air Force Network is using 10 VIZ-W 9000 and 4 Vaisala DigiCora systems most of them performing GPS measurements.
The main problems occur with GPS since anomalous strong winds have been observed. Also very strong interference patterns due to the proximity of airport surveillance radars have been explored. Both problems have already been reported to Vaisala and the wind intensity measurement has been improved. However the number of missing data increased after the correction procedure, unfortunately. The interference due to the radar is still occurring.
There is a need for setting up various quality procedures to the Brazilian upper-air network, including the organisation of a data checking procedure for the all stations as well as to make some field experiments aiming an intercomparison with other operational systems.
· Kenya
Kenya Meteorological Department started the operation of GPS radiosondes in September1998 in applying Vaisala systems. Concerning problems with missing and inaccurate data it was found from the two consignments of GPS radiosondes procured, that there was a significant problem with the pressure sensor as well as related to gaps in the upper-wind observations which led to a rejection of a number of GPS radiosondes. However, a reduction of the number of radiosondes rejected was noticed in the second consignment of GPS radiosondes.
It is probably important to mention that the staff operating the three upper-air stations have received on the job training on operational aspect of radiosondes only. In order to improve capacity building and institutional strengthening, a formal training of observers and system managers is highly desirable. In this regard, WMO is invited to explore the possibility of supporting such a training programme.
· Australia
Australia operates 38 radiosonde stations, from which 12 are using the RS80G GPS and 26the Vaisala 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.
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 sondes 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 10-20 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 sondes. 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.
· 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 sondes, (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 sonde manufacturer sent a representative to South Africa, and conducted tests to determine the cause of these failures. Results showed that the majority of the sondes 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 Intermediate Frequency (IF) drift problem, as well as making changes to some of the electronic components of the radiosondes. It was also recommended that all sondes be ground-tested to ensure that sufficient satellites are received by the sonde, 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 sondes in use are not yet of the new type (post-May 1999). Anumber of sondes are being rejected at ground level, and the manufacturer has undertaken to replace sondes, which are unable to receive signals before launch.
Due to the high cost of the GPS sondes, which is further aggravated by foreign currency exchange rates, as well as budgetary constraints, South Africa has been forced to drastically reduce the number of GPS sondes used, and to make use of the cheaper PTU only sonde combined with winds obtained by optical theodolite.
· 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, this technique does not provide a 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 interferences related to the radiosonde this system could not be introduced for operational use. Therefore a secondary radar system working in the LBand (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.
· United Kingdom
The UK has operated 3 GPS radiosonde stations since October 1997, two at remote locations in the South Atlantic. Failure rates in flight have remained at about 10 percent throughout this period, compared to about 1% for Loran-C windfinding systems operated in the UK. Software updates from Vaisala have been implemented during 1999, but difficulties have been experienced by Met. Office technical staff when attempting to implement the software revisions. The main impact of the latest software revision 8303 appears to be in decreasing the gap between launch and the first reported winds.
However, unacceptable gaps in measurements continue on at least 10 per cent of operational flights. In 1999, UK introduced an ASAP system with GPS-based windfinding. The initial performance of this system has been poorer than at the remote sites. Probably, because the operators were not able to check the faulty radiosondes before launch.
· USA (NWS)
The NWS uses radiosondes from SIPPICAN INC (formerly VIZ) and Vaisala. These radiosondes are used to support 102 sites located in the Pacific Islands, the Caribbean Cooperative Hurricane Upper-air Stations Network, the contiguous United States, and Alaska. These sites cover regions from the tropics to Arctic. Vaisala provides radiosondes for 60 sites and SIPPICAN INC (VIZ) for 42 stations. Radiosondes from both manufacturers are deficient in the measurement of relative humidity. Improved manufacturing processes and the introduction of additional calibration coefficients have improved the SIPPICAN (VIZ) humidity element performance. Vaisala radiosondes continue to exhibit a dry bias in relative humidity measurements. Changes in the desiccant used in radiosonde packaging reduced the out gassing contamination to the humicap sensor. However, a dry bias continues. VIZ Mark II Microsondes are deficient in temperature measurements in the stratosphere. This radiosonde lacks a radiation correction to compensate for long and short wave radiation effects on the temperature sensor in the stratosphere.
Operational practices covering radiosonde checks prior to launch, the replacement of faulty radiosondes, and data quality evaluation procedures based on monthly statistical data on individual upper-air station flight performance were presented. Issues evaluated are timeliness of reports, minimum altitude performance, percent of soundings reaching 50hPa, percentage reaching 20hPa and percentage of temperature and wind levels rejected.
Primary operational radiosonde problems addressed the supply of radiosondes by the vendors and ground equipment maintenance of an over 40 years old systems. Radiosonde inventory levels of VIZ radiosondes have become critically low after the move of the VIZ production facility to Mexico.
Issues of national concern are matters related to the Year 2000 problem (Y2K), the possible termination of the US Loran-C Navigation system in 2000, because the NWS supports 5 Loran-C sites, and the NWS activities in replacing the existing radiosonde network with a 1680 MHz GPS-based radiosonde system.