9.6.1 Calibration Methods

A calibration involves measuring the conformance to or discrepancy from a specification for an instrument and an adjustment of the instrument to conform to the specification. In this sense, other than directional alignment checks, a true calibration of the upper-air instruments described in this document is difficult. Due to differences in measurement techniques and sources of meteorological variability, direct comparison with data from other measurement platforms is not adequate for a calibration. Instead, a calibration of these sensors consists of test signals and diagnostic checks that are used to verify that the electronics and individual components of a system are working properly. Results from these calibrations should not be used to adjust any data. All calibrations should be documented in the station log.

System calibration and diagnostic checks be performed at six month intervals, or in accordance with the manufacturer's recommendations, whichever is more frequent. The alignment of remote sensing antennas, referenced to true north, should be verified at six month intervals. Generic guidance and definitions of terms related to calibrations is provided in Section 8.3.

Radiosonde Sounding Systems For radiosonde sounding systems, the primary calibration that is required is to obtain an accurate surface pressure reading using a barometer that is regularly calibrated and periodically audited. This pressure reading is used to determine if an offset needs to be applied to the radiosonde pressure data. If an offset is needed, the data systems of the commercially available instruments will make the adjustment automatically. It is also useful to obtain surface readings of temperature and atmospheric moisture using a psychrometer or similar instrument. These data can be used to provide a reality check on the radiosonde measurements. This check can be performed using data from a nearby tower. A more robust check can be made by placing the sonde in a ventilated chamber and taking readings that are then compared to temperature and moisture measurements made in the chamber using independent sensors. The alignment of the theodolite should be validated against the reference marker that was installed at the time of system setup.

Sodar Recent advances in instrumentation for auditing of sodar instruments [104] have led to the development of a transponder that can simulate a variety of acoustic Doppler shifted signals on certain sodars. This instrument can be used to verify the calibration of the sodar's total system electronics and, in turn, validate the overall system operation in terms of wind speed and altitude calculations. However, such a check should not be considered a “true” calibration of the system since it does not consider other factors that can affect data recovery. These factors include the system signal-to-noise ratio, receiver amplification levels, antenna speaker element performance, beam steering and beam forming for phased-array systems, and overall system electronic noise.

Radar Wind Profilers and RASS A transponding system for radar does not yet exist, but the feasibility of such a system is being explored. Therefore, there is no simple means at present of verifying the accuracy of the Doppler shifted signals in the field other than to perform a comparison with some other measurement system, as described later in this section. Instead, calibrations of radar wind profiler and RASS systems are performed and checked at the system component level. These checks should be performed in accordance with the manufacturer's recommendations. Like some sodar systems, the radar systems use both software and hardware diagnostics to check the system components.

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