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