9.2.2 Performance Characteristics of Radiosonde Sounding Systems

Radiosonde sounding systems are the most widely used upper-air instruments. The wind and thermodynamic data provided by these systems are critical to the numerical weather prediction (NWP) and forecasting programs conducted by all countries that provide such services. Thus, the performance characteristics of radiosondes and the relative accuracy of radiosonde winds have been the subject of a great deal of scrutiny over the last few decades. The World Meteorological Organization (WMO) and national weather agencies such as the U.S. NWS and British Meteorological Office have all sanctioned a number of inter-comparison studies to determine the performance characteristics of radiosonde systems (references [9], [75], and [77]). Inter-comparison and performance evaluation studies have also been conducted by independent researchers who have been interested in determining the accuracy of radiosonde wind and/or thermodynamic measurements for meeting specific research objectives (see reference [81] for a recent summary of some of these studies, especially those related to boundary-layer measurements). Some references are also provided in Table 9-4. Radiosonde systems will continue to be an important source of upper-air data for the foreseeable future, and efforts to characterize and improve radiosonde sounding system performance specifications continues [79].

Performance tests of radiosonde systems have involved “flying” multiple radiosondes on the same balloon, and/or obtaining independent tracking information using high-precision tracking radars [79]. Such tests do not provide information on absolute accuracy of either the radiosondes or the tracking systems. Rather, they provide measures of the relative differences between comparable instrument systems, e.g., of temperature or relative humidity measured by different radiosondes flown at the same time and winds measured by radio the odolites or NAVAID systems. The NWS and WMO perform such tests to quantify the functional precision of the instruments, which is defined as the rms of the differences between the measurements, that is, if the differences have a Gaussian distribution then 67 percent of the differences would lie within the range specified by the functional precision. The functional precision is thus similar to the comparability statistic defined by Equation 9-2. Performance specifications for radiosonde systems are summarized in Table 9-1, the performance specifications are based on manufacturer's specifications and inter-comparison tests described in references [77] and [79].

Errors and uncertainties encountered in radiosonde measurements, particularly errors in temperature and moisture, can occur at higher altitudes (e.g., beginning in the upper-troposphere), and are caused by factors such as exposure to solar radiation, sensor heating, and time lag. Data collected at lower altitudes (e.g., below about 10 km) do not tend to display such errors. Likewise, the relative accuracy of upper-air winds measured by radiosondes tends to decrease with increasing altitude. This is due in part to many weather services using radio theodolite sounding systems, where errors in tracking angles (especially elevation) become more troublesome as the balloon approaches the horizon and the antenna reaches its tracking limit. At altitudes below about 10 km, radiosonde winds tend to show good agreement with other independent upper-air measurements [79]. As noted earlier in this document, there are circumstances under which data resolution within the lowest few hundred meters can be compromised.

9. UPPER-AIR MONITORING 
9.1 Fundamentals  
      9.1.1 Upper-Air Meteorological Variables  
     9.1.2 Radiosonde Sounding System  
     9.1.3 Doppler Sodar 
     9.1.4 Radar Wind Profiler 
     9.1.5 RASS  
 9.2 Performance Characteristics  
     9.2.1 Definition of Performance Specifications  
     9.2.2 Performance Characteristics of Radiosonde Sounding Systems 
     9.2.3 Performance Characteristics of Remote Sensing Systems  
 9.3 Monitoring Objectives and Goals  
     9.3.1 Data Quality Objectives  
 9.4 Siting and Exposure
 9.5 Installation and Acceptance Testing 
 9.6 Quality Assurance and Quality Control 
     9.6.1 Calibration Methods  
     9.6.2 System and Performance Audits  
     9.6.3 Standard Operating Procedures 
     9.6.4 Operational Checks and Preventive Maintenance  
     9.6.5 Corrective Action and Reporting  
     9.6.6 Common Problems Encountered in Upper-Air Data Collection 
 9.7 Data Processing and Management (DP&M) 
      9.7.1 Overview of Data Products  
     9.7.2 Steps in DP&M 
     9.7.3 Data Archiving  
 9.8 Recommendations for Upper-Air Data Collection